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1、CIRCULARITY CONCEPTSIN WOOD CONSTRUCTIONGeneva,2023CIRCULARITY CONCEPTSIN WOOD CONSTRUCTIONGENEVA TIMBER AND FOREST STUDY PAPER 95ECE/TIM/DP/95 Forestry and Timber Section,Geneva,SwitzerlandCircularity concepts in wood constructionCOPYRIGHT AND DISCLAIMERCopyright 2023 United Nations and the Food an
2、d Agriculture Organization of the United Nations.All rights reserved worldwide.The designations employed in UNECE and FAO publications,which are in conformity with United Nations practice,and the presentation of material therein do not imply the expression of any opinion whatsoever on the part of th
3、e United Nations Economic Commission for Europe(UNECE)or the Food and Agriculture Organization of the United Nations(FAO)concerning the legal status of any country;area or territory or of its authorities or concerning the delimitation of its frontiers.The responsibility for opinions expressed in stu
4、dies and other contributions rests solely with their authors,and publication does not constitute an endorsement by UNECE or FAO of the opinions expressed.Reference to names of firms and commercial products and processes,whether or not these have been patented,does not imply their endorsement by UNEC
5、E or FAO,and any failure to mention a particular firm,commercial product or process is not a sign of disapproval.This work is co-published by the United Nations(UNECE)and FAO.Photo credits:Cover:Brudder Productions,courtesy ;KK Law,courtesy of ;Depositphotos.pp.9-11,17,30,35:Depositphotos;p.16,Brudd
6、er Productions,courtesy ;Paul Raftery;p.22,Wikimedia Commons.ABSTRACTWhen it comes to sustainability and circularity,wood as a natural raw material has several advantages over other building materials.As a bio-based resource,it has considerable benefits concerning greenhouse gas emissions,carbon-sto
7、ring,thermal insulation as well as human health and well-being compared to other construction materials.New types of wood products,being the result of extensive research,enable the extensive use of wood in tall buildings.At the same time,innovative wood products provide less manufacturing waste,low
8、carbon-emission alternatives and store massive quantities of carbon while new technologies speed construction processes,promote energy efficiency and minimize waste.This study examines the benefits of wood as a construction material and discusses practices applied in the wood construction sector fro
9、m the perspective of circularity,sustainability and climate change mitigation.It analyses how circularity concepts can be applied in the construction industry using different construction methods and at different stages of value chains.The study describes how different construction techniques and pr
10、actices contribute to the renewal and sustainability of construction value chains.The analysis is supported by examples of good practice in UNECE member States.ECE/TIM/DP/95UNITED NATIONS PUBLICATIONSales No.:E.23.II.E.4ISBN:978-92-1-117328-4e-ISBN:978-92-1-002417-4ISSN:1020-7228eISSN:2412-1304iiiAC
11、KNOWLEDGEMENTSACKNOWLEDGEMENTSThe Joint UNECE/FAO Forestry and Timber Section expresses its deep gratitude to everyone who contributed to this study.The study is the result of a cooperative effort involving a network of authors,reviewers,editors,the UNECE/FAO Team of Specialists on Sustainable Fores
12、t Products,and a team of people working in the Joint UNECE/FAO Forestry and Timber Section in Geneva and at FAO in Rome.The Joint UNECE/FAO Forestry and Timber Section acknowledges the authors who wrote the chapters and,in so doing,shared their expertise and knowledge.The authors are:Jim BowyerKatie
13、 FernholzAlicja KacprzakThe study also benefited greatly from the in-kind contribution to the publication by a number of experts who volunteered their time and expertise to provide information free of charge.Jana BaumgartnerKate HendryRodney McPheeMichal SynekAdam CorneilNikolay IvanovMichael Plesch
14、emit TurhanGlenda Garcia-SantosSylvain LabeClaudiane PomelonJeremy WallBranko GlavonjicDaniel McKendryMaria SokolenkoMagdalena WolickaWithout the help of all these experts,it would not have been possible to produce this publication.The project was managed by Alicja Kacprzak,at the Joint UNECE/FAO Fo
15、restry and Timber Section.The publication was reviewed at UNECE by Paola Deda,Liliana Annovazzi-Jakab and Florian Steierer and at FAO by Sven Walter and Ekrem Yazici.The editorial work was done by Ian Silver.ivCircularity concepts in wood constructionCONTENTSGlossary.viAbbreviations.ixExecutive Summ
16、ary.xCHAPTER 1 SETTING THE STAGE FOR CIRCULARITY IN WOOD CONSTRUCTION.1 1.1 Understanding Circularity and Sustainability.1 1.2 Circularity and Sustainability in Wood Construction.2 1.3 Background and Objectives of the Study .5 1.4 Scope and Limitations.6 1.5 Methods and Data Sources.6 1.6 Structure
17、of the Study.6CHAPTER 2 THE ROLE OF WOOD CONSTRUCTION IN A CIRCULAR ECONOMY.9 2.1 History of Wood Use in Construction.9 2.2 Traditional Construction Methods .10 2.3 Benefits of Wood Use in Construction.122.3.1 Produced by Solar Energy.122.3.2 Largely Composed of Captured and Stored Carbon.122.3.3 Re
18、newable.132.3.4 Strong,Yet Light Weight.132.3.5 A Natural Thermal Insulator.132.3.6 Recyclable.132.3.7 Aesthetically Pleasing and Beneficial to Human Health.13 2.4 Circularity and Sustainability of Wood Use in Comparison to Other Construction Materials.142.4.1 Relative Impacts of Building Materials.
19、142.4.2 Relative Impacts of Building Structures .152.4.3 GHG Emissions and Climate Change.172.4.4 Energy Efficiency.192.4.5 Fire Performance .202.4.6 The Durability of Wood Structures.21CHAPTER 3 CIRCULARITY AND SUSTAINABILITY IN WOOD CONSTRUCTION PRACTICES.25 3.1 The Circularity of Wood Material.25
20、 3.2 Sustainability of Wood Material.26 3.3 The Circularity of Modern Construction Methods .273.3.1 Circularity and Construction Techniques .303.3.2 Opportunities for Greater Use of Wood in Buildings.333.3.3 Opportunities for Greater Application of Innovative Building Methods.33 3.4 Retrofitting,Dec
21、onstruction and Demolition.333.4.1 Wood in the Waste Stream.343.4.2 Potential for Deconstruction and Cascading Use of Wood.343.4.3 Benefits of Retrofit and Deconstruction.343.4.4 Barriers to Greater Levels of Cascading Use of Wood .353.4.5 Design for Disassembly.35CONTENTSvLIST OF TABLES Table 1 Emb
22、odied CO2e in Common Construction Materials.14 Table 2 Thermal Conductivity of Selected Construction Materials.19 Table 3 Building Types and Construction Methods.27LIST OF FIGURES FIGURE 1 Biological and Technical Cycle in a Circular Economy Model by the Ellen MacArthur Foundation.2 FIGURE 2 Circula
23、r Economy in the Wood Construction Sector.3 FIGURE 3 Circularity and the 9Rs.4 FIGURE 4 Wood Life Cycle in the Circular Economy.5 FIGURE 5 On-site Construction Typical of the United States of America and Canada.9 FIGURE 6 Cross-Laminated Timber(CLT).10 FIGURE 7 Mortise and Tenon Connection.11 FIGURE
24、 8 Brock Commons,University of British Columbia,Canada.16 FIGURE 9 John Hope Gateway,Edinburgh,United Kingdom of Great Britain and Northern Ireland.16 FIGURE 10 Terminal 2 Gardermoen Airport,Oslo,Norway.17 FIGURE 11 Butler building,Minneapolis,United States of America.22 FIGURE 12 Circularity Consid
25、erations at the End of Building Life.25 FIGURE 13 Typical Variation in Strength of Wood of a Single Species.28 FIGURE 14 Laminated Veneer Lumber-Large Timber from Small Trees.29 FIGURE 15 Modular Mass Timber Construction.30 FIGURE 16 Percentage of Housing Units in the United States of America by Num
26、ber of Storeys,2020.33 FIGURE 17 Percentage of Commercial Buildings in the United States of America by Number of Storeys,2020.33 FIGURE 18 Percentage of Floor Area in Commercial Buildings in the United Stated of America by Number of Storeys,2020.33 FIGURE 19 Deconstruction for Building Materials Rec
27、overy.35CHAPTER 4 EXAMPLES OF GOOD PRACTICE.37 4.1 Austria-Policy Supporting Wood Construction.38 4.2 Austria-Wood-based Building in Vienna.40 4.3 Canada-Unbuilders-a deconstruction and salvaging company.42 4.4 Czechia-Sustainable Procurement Law.43 4.5 Poland-Wood Promotion Centre in Jata.44 4.6 Po
28、land-Office Building of the Posk Forest District.46 4.7 Russian Federation-Low-carbon construction material:Segezha Sokol CLT.48 4.8 Serbia-CLT modular building system Koralevi.50 4.9 Trkiye-Public-private project promoting the Use of Wood.51 4.10 United Kingdom of Great Britain and Northern Ireland
29、-Material Consideration:Library of Sustainable BuildingMaterials.53CHAPTER 5 CONCLUSIONS.55REFERENCES.57viCircularity concepts in wood constructionGLOSSARYAcidification Refers to the potential acidification of soils and water due to the release of gases such as nitrogen oxides and sulphur oxides.Bal
30、loon frame construction An early form of construction in the United States of America and Canada in which vertical members are cut to the full height of walls,from sill to roof line.The fact that wall supports were the full height of walls marks the key difference between balloon frame construction
31、and platform framing which came later.Balloon framing Refers to balloon frame construction.Bio-based Refers to commercial or industrial products that are composed in whole,or significantly,of biological products or renewable domestic agricultural or forestry materials.Bioeconomy This term refers to
32、the share of the economy based on products,services and processes derived from biological resources(e.g.,plants and microorganisms).Cascading use of wood(Cascaded use)Cascaded use refers to the use of material resources in such a way as to create the most economic value over multiple lifetimes with
33、energy recovery as the last option and only after all potential for higher-value products and services has been exhausted.Circular economy A circular economy is one in which materials and products are kept in circulation for as long as possible.Such an economy uses a systems-focused approach and inv
34、olves industrial processes and economic activities that are restorative or regenerative by design.This enables resources used in such processes and activities to maintain their highest value for as long as possible and aims to eliminate waste through the superior design of materials,products and sys
35、tems(including business models).Circularity Refers to the use of materials and products in alignment with the principles of a circular economy.Carbon dioxide equivalent(CO2e)A term used in reference to greenhouse gas emissions,based on the reality that other compounds have greater climate warming po
36、tential than CO2 and calculated by weighting the volumes of various gases emitted with their potency as a greenhouse gas.Cross-laminated timber(CLT)This term is used synonymously with mass timber.Both terms refer to large structural panels made by assembling layers of dimension lumber that are glued
37、 or nailed together,or connected by dowels,with the grain of alternate layers laid at right angles to one another,much like the veneers of plywood.Deconstruction Refers to the selective dismantlement of building components,specifically for reuse,repurposing,recycling and waste management.It differs
38、from demolition where a site is cleared of its building by the most expedient means.Demolition A description of the action taken at the end of a buildings useful life with little or no regard for the potential reuse,recovery,repurposing or recycling of its components.Embodied carbon The sum of CO2 e
39、quivalent emissions associated with materials and construction processes throughout the whole lifecycle of a product,building or piece of infrastructure,including the emissions resulting from the manufacturing of component materials(material extraction,transport to manufacturer,manufacturing),the tr
40、ansportation of those materials to a manufacturing or work site as well as all activities involved in secondary manufacturing,assembly or construction.Embodied energy The sum of energy consumption associated with materials and construction processes throughout the whole lifecycle of a product,buildi
41、ng or piece of infrastructure,including emissions resulting from the manufacturing of component materials(material extraction,transport to manufacturer,manufacturing),the transportation of those materials to a manufacturing or work site as well as all activities involved in secondary manufacturing,a
42、ssembly or construction.Engineered wood Refers to products made of wood elements that have been reformed using adhesives or other means of assembly to create a more useful product.Eutrophication A process involving a transfer of nitrogen and/or phosphor-containing compounds into fresh or ocean water
43、 ecosystems that can result in accumulating and decaying mats of algae which consume dissolved oxygen from the water and cause death of fish and other aquatic organisms.Freshwater ecotoxicity An indicator of the potential impact on freshwater organisms of toxic substances released into the environme
44、nt.viiGLOSSARYGlulam A stress-rated engineered wood beam composed of wood laminations,or lams,that are bonded together with durable,moisture-resistant adhesives.The grain of the laminations runs parallel with the length of the member.Glulam has versatile forms,ranging from simple,straight beams to c
45、omplex,curved members.Half-timbered construction A construction where timber provides the structural frame of a building while the spaces between the frames are filled with plaster,brick or wattle and daub.Laminated veneer lumber(LVL)LVL is made from multiple layers of softwood veneer aligned such t
46、hat the gain directions of all layers are parallel and aligned with the long axis of the lumber.LVL is a part of a family of products,namely structural composite lumber(SCL),that are made of dried and graded wood veneers,strands or flakes that are layered upon one another and bonded together with a
47、moisture resistant adhesive into large blocks known as billets.Other products in this group include oriented strand lumber(OSL)and parallel strand lumber(PSL).Life cycle assessment(LCA)Refers to an environmental accounting and management approach that systematically considers all aspects of resource
48、 use and environmental releases associated with a product or industrial system from the defined beginning and ending points.Assessment through an entire life cycle considers the environmental impacts resulting from the procurement of raw materials as well as the production and distribution of energy
49、,transportation,manufacturing,assembly and product use through to the end of its useful life.Light frame construction Refers to typical platform frame construction wherein wall sections are constructed with evenly spaced(usually 40 to 50 cm)vertical elements that are affixed to upper and lower horiz
50、ontal elements.Mass timber A term used synonymously with cross-laminated timber or CLT.Both terms refer to large structural panels made by assembling layers of dimension lumber that are glued or nailed together,or connected by dowels,with the grain of alternate layers laid at right angles to one ano
51、ther,much like the veneers of plywood.Mass timber construction Construction in which mass timber panels are the predominant or major building material,typically in combination with other engineered wood beams or columns such as glulam,parallel strand lumber and laminated veneer lumber.Modular constr
52、uction A type of construction involving the assembly of building components almost entirely in a factory to manufacture separate,three-dimensional,box-like modules,including attached walls,floor,ceiling,wiring,plumbing and interior fixtures.These modules are then transported to a building site where
53、 they are positioned on a previously constructed foundation system and interconnected to create a finished structure.Oriented strand board(OSB)Refers to an engineered wood panel that is manufactured from thin,narrow wood strands 100 to 150 mm in length that are arranged in cross-oriented layers and
54、bonded by waterproof heat-cured adhesives.Its strength and performance characteristics are similar to that of construction-grade plywood.Oriented strand lumber(OSL)Refers to a product made from thin,narrow wood strands 100 to 150 mm in length that are arranged with the long axis of the strands paral
55、lel to the long axis of the lumber.The thin stands are combined with a structural adhesive before being oriented and formed into a large mat or billet and pressed.The resulting billet is then sawn into sizes similar or identical to that of construction lumber or timbers.Oriented strand lumber has hi
56、gh strength,stiffness and dimensional stability with consistent and predictable mechanical properties.Oven dry weight Refers to the weight of wood achieved through drying of wood to a constant weight in a ventilated oven at a temperature generally above the boiling point of water(103+/-2C).Panelized
57、 construction A type of construction involving off-site prefabrication of building components,ranging from simple framed and sheathed wall and roof sections delivered to a building site with pre-cut window and door openings,to prefabricated floor systems,roof trusses and finished wall and roof secti
58、ons that incorporate windows,doors and both exterior and interior finishes.Site delivery follows the on-site construction of a foundation system and is followed by the assembly of the prefabricated sections.Parallel strand lumber(PSL)Similar to laminated strand lumber(LSL)and oriented strand lumber(
59、OSL),PSL is made from flaked wood strands that are arranged parallel to the longitudinal axis of the member and have a length-to-thickness ratio of approximately 300.The wood strands used in PSL are longer than those used to manufacture LSL and OSL.Combined with an exterior waterproof phenol-formald
60、ehyde adhesive,the strands are oriented and formed into a large billet then pressed together and cured using microwave radiation.The resulting billet is then sawn into sizes similar or identical to that of construction lumber or timbers.PSL has high strength,stiffness and dimensional stability with
61、consistent and predictable mechanical properties.viiiCircularity concepts in wood constructionParticleboard Refers to a panel product made by compressing thin shavings,flakes or slivers of wood while simultaneously bonding them with an adhesive.Many types of particleboards can differ greatly concern
62、ing the size and geometry of particles,the amount of adhesive used and the density to which panels are pressed,all of which can have a significant impact on its strength,other properties and potential uses.Platform frame construction In platform frame construction,the exterior,and some interior wall
63、s,carry the load of the upper levels and roof.Vertical wall elements are the height of one level of the building and capped by a floor system,with each successive floor created by the addition of another platform,another set of walls and so on.Wall sections are constructed with evenly spaced vertica
64、l elements that are affixed to upper and lower horizontal elements.Post and beam construction Refers to a type of construction where a building is supported by a structural frame with wall elements typically being non-loadbearing.Primary energy Refers to energy in the form that it is first accounted
65、 for in a statistical energy balance,before any transformation to secondary or tertiary forms of energy.For example,coal can be converted into synthetic gas,which can then be converted into electricity;in this case,coal is primary energy,synthetic gas is secondary energy and electricity is tertiary
66、energy.Structural composite lumber(SCL)Refers to a family of products that are made of dried and graded wood veneers,strands or flakes that are layered upon one another and bonded together with a moisture resistant adhesive into large blocks known as billets.Other products in this group include lami
67、nated veneer lumber(LVL),oriented strand lumber(OSL)and parallel strand lumber(PSL).Timber construction Often used synonymously with wood construction.Timber frame construction Refers to a construction method where the structural frame,consisting of both horizontal and vertical members,is made of la
68、rge cross-section wood members(timbers),with connections made using notching,mortise and tenon joints and/or wood pegs.Useful life Refers to the amount of time an asset is expected to be functional and fit-for-purpose.With regard to a building,useful life can be defined as the number of years before
69、 the building deteriorates to the point that it is no longer safe or desirable for continued use,the point at which it no longer meets existing code requirements and would be too costly to bring up to code,the point in time at which other uses for the building site are more financially viable than k
70、eeping the existing building in place,and so on.Wattle and daub Refers to infill in the walls of buildings where a woven lattice of wood strips called wattle is thickly covered with a sticky material usually made of a combination of wet soil,clay,sand,animal dung and straw.Used mainly prior to the 1
71、7th century in parts of Europe.Waferboard Refers to a type of structural panel made from thin wood wafers roughly square in shape bonded together with waterproof phenolic resin under extreme heat and pressure that briefly appeared in commercial markets prior to replacement by oriented strand board(O
72、SB).ixABBREVIATIONSABBREVIATIONSCADcomputer aided designCDWconstruction and demolition wasteCLTcross-laminated timberCOFFICommittee on Forests and the Forest IndustryCO2carbon dioxideCO2ecarbon dioxide equivalentEFCEuropean Forestry CommissionEUEuropean UnionFAOFood and Agriculture Organization of t
73、he United NationsGEF-7Global Environment Facility,seventh replenishmentGHGgreenhouse gasesGlulamglue laminated timberHVACheating,ventilation and air conditioningIEAInternational Energy AgencyISOInternational Organization for StandardizationLCAlife cycle assessmentLSLlaminated strand lumberLVLlaminat
74、ed veneer lumbermmmillimetresMSWmunicipal solid wasteNHSNational Health Service of the United Kingdom of Great Britain and Northern Ireland NLTnail laminated timberOSBoriented strand boardOSLoriented strand lumberPSLparallel strand lumberSCLstructural composite lumberSDGSustainable Development Goals
75、SFMsustainable forest managementUBCUniversity of British ColumbiaUNECEUnited Nations Economic Commission for EuropeUSEPAUnited States Environmental Protection Agency3Dthree dimensionsxCircularity concepts in wood constructionEXECUTIVE SUMMARYWhen it comes to sustainability and circularity,wood as a
76、natural raw material has several advantages over other building materials.The natural cycle of wood begins in forests as trees grow,with solar energy and carbon dioxide(CO2)as the key inputs for wood formation.The cycle continues with harvesting from sustainably managed forests with the wood being u
77、sed to produce a broad range of products.When used in industry in a cascaded way,wood circulates in the technical cycle where it can be recovered either at the end of its first useful life or in the form of residues or by-products from production processes.Wood used in construction can be applied in
78、 diverse functions,as parts of buildings(e.g.,for structural frames,decking,flooring,wall and roof sheathing,window frames,doors and more)or at different stages of construction processes(e.g.,for foundation formwork supports and scaffolding).Whether or not a practice is sustainable rests on three pi
79、llars:environmental protection,economic viability and social equity.Wood fares well in all these categories.The fact that wood is renewable and can be converted into useful products using relatively little fossil energy makes it less environmentally detrimental than materials such as steel,masonry a
80、nd reinforced concrete.These aspects,of course,only translate to environmental advantage if wood is produced in a sustainably managed forest or plantation.Furthermore,wood has an advantage in that third-party oversight of forest management is widely practiced via forest certification programmes that
81、 have been in place for almost three decades.These programmes provide for rigorous evaluation of all aspects of forest management,including impacts on soil health,water quality,fish and wildlife habitats,rare and endangered flora and fauna,cultural and historical sites,among others.Doing so results
82、in a means of ensuring attention to important issues while producing sustainable volumes of wood and other products and services.The programmes also provide a social context for wood production,bringing to the fore common social concerns and allowing an external overview of industry practices.In man
83、y parts of the UNECE region,wood dwellings account for only 10 to 11 percent,or less,of new construction while limitations on building with wood,including limits on construction height,also exist in many places.The new types of wood products that have enabled wood to replace steel and reinforced con
84、crete in tall buildings are all the result of extensive research over many years and are the result of focused attention on obtaining greater uniformity of properties than exhibited by solid wood.The cumulative result of many decades of research-and more than a century since the issuance of a German
85、 patent for glue laminated timber(glulam)-mass timber buildings today contribute to circularity and environmental sustainability while also providing a highly engineered and high-performance material for construction.Mass timber allows for the beneficial use of renewable resources that can be fashio
86、ned into useful products with less manufacturing waste than previous forms of structural wood products,provides low carbon-emission alternatives to reinforced concrete and steel while also storing massive quantities of carbon for as long as they remain in existence.Innovative wood construction metho
87、ds have been developed with economic pragmatism in mind,intuitively applying sustainability and circularity principles at the same time.New technologies incorporating a high degree of prefabrication are employed that speed construction processes,provide for precision sizing of modules and connection
88、s-thereby promoting the energy efficiency of completed buildings,greatly reducing waste and protecting prefabricated modules from the effects of weather.Wood use in construction is more circular and sustainable than the use of other common building materials.Wood has inherent advantages and provides
89、 multiple benefits because it is a natural material,can be fashioned into a diverse array of building components with minimal climate impact and can be incorporated into buildings which have lower lifecycle energy consumption and CO2 equivalent1(CO2e)emissions than non-wood structures.The substituti
90、on of wood for concrete or steel in construction results in reduced embodied CO2 emissions.Significant additional carbon storage could occur within the built environment with greater use of wood in construction,with the caveat that the wood does not go to landfill following demolition or deconstruct
91、ion.Wood use in the construction sector results in lower use of fossil fuel energy and lower embodied fossil energy in the built environment.The reduced greenhouse gas emissions and use of renewable bioenergy in wood-product manufacturing contribute to circularity and sustainability.1 A number of co
92、mpounds are classed as GHGs.Although CO2 is predominant in terms of volume,other compounds,such as methane,nitrous oxide and fluorocarbons,though emitted in lesser quantities than CO2,are far more potent.Methane and nitrous oxide,for example,have 28 and 265 times the warming potential as CO2 over a
93、100-year time horizon.In calculating the potential greenhouse effect of emissions from an industrial operation or other activity,the volumes of various gases emitted are weighted by their potency as a GHG to calculate carbon dioxide equivalent emissions,expressed as CO2e.xiEXECUTIVE SUMMARYAlthough
94、wood use in construction offers substantial sustainability and circularity benefits,additional innovation is needed.Currently,waste from building deconstruction is not being recovered effectively.Designing for building adaptability,disassembly and effective material recovery would improve the circul
95、arity of wood in the construction sector.The data suggests that there is considerable room for improvement in wood recovery and recycling at buildings end of life.The greatest opportunity for improved circularity of wood in existing buildings is in the recovery,reuse and/or recycling of building dem
96、olition waste.However,for an overall transition of the wood construction sector to a more circular model,a systemic approach is needed to enhance increased integration across and along value chains.Such an approach should move away from business-as-usual towards more cross-cutting collaboration amon
97、g different actors within and outside the construction sector.Increased collaboration of building research organizations,designers,architects,urban planners,engineers,municipality actors and legislators would contribute to achieving greater sustainability and circularity at different stages of const
98、ruction value chains.CHAPTER 1 Setting the stage for circularity in wood construction 1.1 Understanding Circularity and SustainabilityMany of the global priorities embedded in the 2030 Agenda for Sustainable Development2 and the Sustainable Development Goals(SDGs)relate to forests and forestry,fores
99、t-based industries and bioenergy.SDG 15 Life on Land directly refers to the need for the sustainable use of ecosystems,the sustainable management of forests and the reversing of land degradation and biodiversity loss.SDG 13 is dedicated to Climate Action and cannot be achieved without resilient fore
100、sts and responsible forestry practices,while SDG 6 clearly mentions the need to protect and restore water-related ecosystems,including forests,wetlands,rivers and lakes.Existing linear production and consumption patterns,based on make,use,dispose models,are no longer sustainable and many key economi
101、c sectors and industries,including those using forest-based products,such as construction,furniture manufacturing and the pulp and paper industry,significantly contribute to pollution and waste generation.SDG 12 calls for responsible production and consumption and refers to circularity principles as
102、 well as the sustainable use of natural resources.It points out the need to increase resource efficiency,promote sustainable lifestyles,produce more with less and decouple economic growth from environmental degradation in the long-term.The achievement of many of these objectives in the context of th
103、e increasing use of forest resources and growing environmental challenges,linked with greenhouse gas(GHG)emissions and waste generation,requires the application of production and consumption models based on the sustainable use of natural resources and the regeneration of biological systems.Although
104、the term circular economy does not appear in the 2030 Agenda for Sustainable Development,circular economy practices can contribute to achieving several SDGs.A study by Schroeder Anggraeni and Weber 2019 noted that the strongest relationship exists between circular economy and SDG 6(Clean Water and S
105、anitation),SDG 7(Affordable Clean Energy),SDG 8(Decent Work and Economic Growth),SDG 12(Responsible Production and Consumption)and SDG 15(Life on Land)(UNECE/FAO 2022).2 https:/www.un.org/sustainabledevelopment/development-agenda/3 Claude Durocher,unpublished study.State of the Global Forest Bioecon
106、omy.A transition towards a sustainable,bio-based,circular economy at the global level is often perceived as a way to achieve an economic model which can increase sustainability at the environmental,economic and social levels while at the same time reducing the global economys dependence on non-renew
107、able resources in the long term.Different circular economy models coexist in the policy space and in research with a number of concepts that have been developed earlier or simultaneously.The origins of circularity itself are older and more diverse than it is commonly perceived and are rooted in ecol
108、ogical and environmental economics as well as industrial ecology(UNECE/FAO,2022).Concepts regularly referenced today,such as circular economy,green economy,bioeconomy and sustainable economy,all differ but are consistent with each other since they all aim at the synchronized optimization of ecologic
109、al economic and social objectives at different levels(personal communication Durocher,2021)3 in the same way as the 2030 Agenda for Sustainable Development does.In this study,the concept of a circular economy used is based on the model of the Ellen MacArthur Foundation(Figure 1),as described in UNEC
110、E/FAO(2022),and takes into consideration its modifications by Oneil and Russel(2020)presented below.The Ellen MacArthur Foundation model distinguishes between technical(blue)and biological(green)cycles.This interpretation of circularity involves materials of biological origin that can return to the
111、biosphere in the form of nutrients while technical materials that cannot biodegrade can still circulate in closed loops thanks to circular practices.Oneil and Russel(2020)applied the Ellen MacArthur Foundation model for the use of wood in construction,furniture manufacturing and bioenergy production
112、 to illustrate the flow of wood from the biological to the technical cycle and back as well as within the technical cycle(Figure 2).This modification of the Ellen MacArthur Foundation model acknowledged that wood begins its life cycle as a renewable resource(green cycle)and then crosses over into th
113、e technical(blue)cycle,where it splits into two distinct streams:1)solid and engineered wood circulating in the technical(blue)cycle and 2)by-products and residues crossing back to biological(green)cycle.In both cases wood continues its lifecycle in a cascaded way until it is recovered for bioenergy
114、 at the end Circularity concepts in wood construction2of its life,at which point CO2 is released to the atmosphere and made available for trees to begin a new cycle(Oneil and Russel.2020).Based on this model,this study also assumes that emissions associated with resource extraction and waste managem
115、ent linked to the use of non-renewable materials will decrease with a measured optimization of resource extraction and a steady replacement of non-renewable materials by renewable resources in the long term.In doing so,a new economic model will not only be circular but also bio-based and more sustai
116、nable.In this study,the circularity and sustainability practices are understood by the application of the 9R approach(Figure 3)at different stages of construction value chains,as presented in UNECE/FAO(2022).This was done with the recognition that the focus is on analysing industry practice,without
117、considering forests and forest operations,to which a separate study will be dedicated.While the 9R model will be the basis for consideration of circularity and sustainability in the wood construction sector,it is understood that many of these R-approaches should be seen differently when applied to m
118、any technical materials.This is because once wood is transformed,it spans through several reuse,recovery and recycling processes in a cascaded way before it is shredded or incinerated for energy production.This allows it to feed back into the biological cycle of wood growth before it is ready to be
119、used by the technical cycle again.In contrast,many technical materials,such as steel,aluminium and glass used in different elements of buildings,once they enter the technical cycle can be recycled and transformed into materials similar to their original form without leaving the technical cycle.1.2 C
120、ircularity and Sustainability in Wood ConstructionConstruction is a complex undertaking due to the diversity of materials,methods and products used as well as the combination thereof.Wood has been used in building for centuries and it is still one of the most widely used materials,however,with the a
121、ppearance of engineered wood products,interest in the use of wood as a construction material is growing.FIGURE 1 Biological and Technical Cycle in a Circular Economy Model by the Ellen MacArthur Foundation3CHAPTER 1-Setting the stage for circularity in wood construction FIGURE 2 Circular Economy in
122、the Wood Construction SectorThe concept of a circular economy is already familiar to some in the construction industry although its exact meaning is still vague.Circularity approaches are traditionally present in wood construction through practices such as rebuilding from used lumber recovered from
123、old wood buildings(Antikainen et al.,2017).More recently,environmental arguments favour a shift to the use of more wood in construction based on advances in wood-based building materials and construction techniques while seeking gains in resource efficiency objectives as well as minimization of prod
124、uction waste both off site and at the construction site.Together with the sectors increased adoption of engineered wood,attention is being given to waste reduction in the construction process,including the development of modular prefabricated construction techniques which ease disassembly at the end
125、-of-life stage.Recently,attention has also been given to increased use of sustainable material and product,including cross laminated timber(CLT)and engineered wood timbers in building construction.CLT was invented in Europe and was the key development which allowed high-rise(tall wood)construction w
126、hile also stimulating increasing interest in prefabricated residential and non-residential buildings.Related to this,a high degree of customization and application of wood for nearly any building part,including load-bearing structures,is transforming the wood construction sector and is contributing
127、to material efficiency(Verkek et al.,2022).Applying circularity approaches to construction value chains through innovative design,regular maintenance,adaptive reuse,refurbishment,repair,recovery and recycling can help to recapture some of the value of the built environment,including wood buildings(D
128、elphi Group,2021).However,wood can be considered a renewable material only when it is sourced from sustainably managed forests.Combined with sustainable spatial planning and eco-design,it is a durable,Source:Oneil and Russel,2020Circularity concepts in wood construction4 FIGURE 3 Circularity and the
129、 9RsSource:UNECE/FAO,20225CHAPTER 1-Setting the stage for circularity in wood constructionreusable and recyclable resource,fitting the principles of a circular economy,contributing to the sustainable use of natural resources and the mitigation of climate change.Being a natural,renewable,biodegradabl
130、e and bio-based raw material,wood has the potential to play a central role in a transition to a sustainable,bio-based,circular economy.In addition,wood is also a readily available construction material that is strong and durable in relation to its weight and economically competitive in many parts of
131、 the world.After the adaptation of the Ellen MacArthur Foundation circular economy model to represent wood flows,Oneil and Russel built on the same model to present a lifecycle of solid wood products destined for building construction(Figure 4).The model shows the life cycle of products coming from
132、working forests and flowing to construction uses through cascaded value retention processes in the technical(blue)cycle until their end of life and then into the biological(green)cycle at the end of their useful life(Oneil and Russel,2020).This model illustrates a more inclusive view of a circular e
133、conomy,compared to existing circular economy models.It takes into consideration energy recovery and the CO2 absorption by forests,highly relevant for circularity in the forest sector because of woods characteristics as a material that is suited to cascaded use,with bioenergy production coming at its
134、 end of life and the emissions returning to forests to initiate a new cycle.4 (ECE/TIM/2021/2 FO:EFC/2021/2)1.3 Background and Objectives of the Study This study aims to provide a comprehensive overview of how circularity concepts and sustainability practices,based on the models presented above,can
135、be applied in wood construction.The work on the study resulted from a mandate given by the Committee on Forests and the Forest Industry(COFFI)of the United Nations Economic Commission for Europe and the European Forestry Commission(EFC)of the Food and Agriculture Organization of the United Nations.D
136、uring their Joint Session in November 2021,COFFI and EFC requested UNECE and FAO to“(a)prepare a series of studies further reviewing the application of circular models in specific forest-based industries,including through the identification of case studies and best practice,and(b)to take into consid
137、eration the whole forest-based value chain and bring attention to the circular nature of wood as a renewable resource and the role of sustainable forest management”4.The focus of the studies was identified through consultations with the UNECE/FAO Team of Specialists on Sustainable Forest Products be
138、tween April and June 2022 and validated by the Joint UNECE/FAO Working Party on Forests Management,Economics and Statistics during its session in June 2022.The series will include the following studies:Universal preconditions of circularity in forest-based industries FIGURE 4 Wood Life Cycle in the
139、Circular EconomySource:Oneil and Russel,2020Circularity concepts in wood construction6 Circularity concepts in the wood construction sector as an example of a long-lived products value chain Circularity concepts in the pulp and paper industry as an example of a group of commodities with a short life
140、 span.The studies build on the previous UNECE/FAO study Circularity concepts in forest-based industries(2022)and aim to present a more detailed insight into the circularity issues in forest-based value chains.They contribute to the research and guidance for policymaking activities of the UNECE/FAO I
141、ntegrated Programme of Work 2022-2025,implemented by the Joint UNECE/FAO Forestry and Timber Section in Geneva.1.4 Scope and LimitationsThis study examines the benefits of wood use in construction as a bio-based material,compared to other construction materials,from the perspective of circularity,su
142、stainability and climate change mitigation.It considers circularity practices at different stages of construction value chains,including retrofitting and deconstruction,as well as end-of-life solutions.Different construction types(residential,industrial,commercial,civil engineering)and construction
143、methods are analysed to provide evidence of how construction design,planning and practices contribute to circularity and how circularity concepts can be further promoted in the construction industry.This study,as part of a series,focuses on the use of wood in construction as an example of a long-liv
144、ed wood-based products value chain where analysis is supported with examples of good practice in the construction sector.Building on existing circular economy models,the focus of this study is on analysing circularity in an industry context rather than the optimal use of forest resources for constru
145、ction.This limitation was adopted as the implications of circular approaches on forest health and the sustainability of wood provision,in particular the balance between the use of wood and other forest ecosystem services.This balance will be given due attention in a separate study of the series.Whil
146、e this study presents the current industry context and points out opportunities and challenges in a transition to a more sustainable and circular economy,it is important to note that circularity does not always equate to environmental sustainability or climate neutrality.Therefore,an effort has been
147、 made to recognize that successful implementation of circularity principles in the wood construction sector should also take into consideration a variety of aspects,such as the impact on the environment and human health as well as their practicality and economic feasibility,often not included in the
148、oretical models.Consequently,the objective of this study is to understand how wood flows in the construction sector and how it contributes to the renewal and sustainability of construction value chains.1.5 Methods and Data SourcesEvidence and information reviewed in this study come mainly from desk
149、research,a review of the scientific literature and subject matter knowledge.Additional information has been provided from government information sources and partnering organizations,including invited case studies and examples of good practice.1.6 Structure of the StudyChapter 1 sets out the context
150、and the objectives of the study.It defines a circular economy as referenced in this study and how it applies to the forest sector,in particular to wood construction.Chapter 2 describes the benefits of wood use in the construction sector.It examines the benefits of wood,as a bio-based material and co
151、mpared to other construction materials,on GHG emissions,carbon storage,thermal insulation as well as human health and well-being.It takes into consideration the perspective of circularity,sustainability and climate change mitigation.Chapter 3 discusses the circularity practices applied in the wood c
152、onstruction sector.It describes how wood flows in the construction sector as a material and how different construction techniques and practices contribute to the renewal and sustainability of construction value chains.Chapter 4 provides construction project experiences and case studies in the UNECE
153、region in which the principles of circularity and sustainability were implemented in the wood construction sector.Chapter 5 presents the studys conclusions and recommendations.CHAPTER 2 The role of wood construction in a circular economy 2.1 History of Wood Use in ConstructionWood is widely availabl
154、e,relatively abundant worldwide,renewable,light weight yet strong,readily fashioned into useful products and aesthetically pleasing.Wood is today,and has been for many centuries,the predominant material used in the construction of homes(i.e.,single-family residences)and some types of commercial buil
155、dings,including low-rise buildings,in many regions of the world.As reported by Cabral and Blanchet,2021,wood buildings account for 90 percent of single-family homes in Canada and the United States of America,45 to 70 percent in parts of Europe and 45 percent in Japan.Such use of wood in construction
156、 is likely to continue due to its basic characteristics,its availability and the growing interest in circularity and sustainability.The type of wood construction used varies widely by region.Wood post and beam construction is,for example,typical in Japan along with the prefabrication of building com
157、ponents common.In post and beam construction,timber is the main structural material,with wall elements typically non-loadbearing.In contrast,single-family homes and other low-rise residential structures in the United States of America and Canada are commonly platform frame construction built of wood
158、(sometimes referred to as light frame construction).Using this method,exterior and some interior walls carry the load of the upper levels and roof with wall sections usually constructed with evenly spaced(usually 40 to 50 cm)vertical elements that are affixed to upper and lower horizontal elements(F
159、igure 5).Much of the component assembly typically occurs on-site,the exception being prefabricated floor and roof trusses.In Northern Europe,homes are also constructed predominantly of wood,although wall assemblies tend to be more robust than in Canada and the United States of America,with the vast
160、majority of homes constructed off-site to some degree,including sectionalized and modular components(Hedges and LaVardera,2017).In many other parts of Europe and especially Southern Europe,wood construction is less common.In Germany and the United Kingdom of Great Britain and Northern Ireland,for ex
161、ample,wood-dwelling units account for 10 to 11 percent of new construction,in Italy and France it is 7 percent and 4 percent respectively while in Spain and other parts of Europe 2 to 3 percent(Hildebrandt,Hagemann and Thrn,2017).Across Europe,local regulations and other considerations have generall
162、y not constrained the height of wood structures.An exception is Germany where federal rules have limited the construction of wood-framed houses to a height such that the flooring of the upper level that contains a living space be no more than 13 metres above the ground level.Other jurisdictions in E
163、urope have required the installation of sprinkler systems for wood buildings taller than several storeys(Mahapatra,K.and Gustavsson,L,2009).In Canada and the United States of America,codes for many years specified maximum building heights of no more than 4 storeys.This limit was increased in recent
164、decades to 6 storeys in some jurisdictions,particularly with the advent of podium slab construction wherein wood construction rises above one or two storeys of reinforced concrete.Nevertheless,the allowable height of wood structures has historically been effectively almost universally limited due to
165、 safety concerns in the event of a building fire.The development of a number of new types of wood-based mass timber products has created opportunities for wood construction at greater heights while meeting other objectives,including addressing safety requirements.Over the past four decades,innovatio
166、n in wood products has led to unprecedented changes in the possibilities for wood use in construction,particularly regarding its use in the construction of tall buildings.The 2021 edition of the US-based International Building Code and the 2022 edition of the Canadian National Building Code have bot
167、h adopted new provisions allowing mass timber structures as high as 18 storeys in the United States of America and 12 storeys in Canada.These new products also allow improved utilization of varied wood species,sizes and grades to contribute to less waste and greater circularity.FIGURE 5 On-site Cons
168、truction Typical of the United States of America and CanadaSource:DepositphotosCircularity concepts in wood construction10The use of these products contributes to sustainability goals through their market-based support for forest management and investments in forest-based businesses and green jobs.M
169、any of these new products are structural wood composites,produced by assembling small wood pieces and particles or larger wood members,into much larger products with the capacity to be used in new ways as structural components of buildings.To create some of todays innovative wood construction produc
170、ts,relatively small pieces of wood are glued together,with the grain of the pieces running parallel to one another to form large wood beams and columns.These kinds of products and techniques have been used for over 100 years to construct spectacular roof supports for church buildings and other types
171、 of structures.In the mid-to late 20th century,wood scientists began to experiment with ways to create large structural wood members from relatively small trees.With initial work done primarily in the United States of America and Canada,a number of new products were introduced,including various form
172、s of structural composite lumber(SCL)created from multiple layers of veneer referred to as laminated veneer lumber(LVL).Other products were formed from thousands of thin strands of wood compressed into large members,such as oriented strand lumber(OSL)and parallel strand lumber(PSL),which can be made
173、 to virtually any size and eliminate the problem of large variations in wood strength due to the natural features of solid wood.By eliminating or dispersing knots,holes,slope of grain and other limiting factors,these new products offered uniform strength and other properties that established wood,fo
174、r the first time,as an engineering material with predictable features and comparable applications to steel and structural concrete.As noted previously,the development that served to change the potential use of wood in tall buildings had its beginnings in Europe,namely the introduction of CLT or mass
175、 timber.First used for roof systems in Germany in the early 1970s,then further developed in Germany,Austria and Switzerland during the 1990s(Karacabeyli and Douglas,2013).This product is made of a number of layers of lumber,glued together with the grain of alternate layers laid at right angles to on
176、e another,much like the veneers of plywood.Panels made from CLT today can be as large as 0.5 x 6 x 18 metres and offer many advantages as load-bearing components that provide building stability,fire resistance,long-term carbon storage and renewability.A related product,made by nailing wood component
177、s together,is marketed as nail laminated timber(NLT).Engineers and architects soon discovered that through the use of CLT and NLT panels,in combination with large-engineered wood columns and beams,wood buildings could be constructed to previously unfeasible heights.Within a period of less than two d
178、ecades,mass timber buildings have transformed the skylines of cities around the world.Such structures have appeared in Canada,Norway,Sweden,the United Kingdom of Great Britain and Northern Ireland as well as the United States of America(Verkek et al.,2021),the Russian Federation and elswhere.Mass ti
179、mber construction,which typically involves the use of CLT in combination with other structural wood composites,is increasingly finding application in large-scale structures,including multi-storey residential buildings,industrial and commercial structures as well as in the construction of civil engin
180、eering works.The transformation of construction techniques to incorporate greater use of wood offers opportunities to enhance circularity and sustainability throughout the built environment and related industries.2.2 Traditional Construction Methods Wood structures have been built for thousands of y
181、ears,with one of the first documented examples being Europes Neolithic longhouse,a freestanding timber building constructed between 5000 and 6000 BCE(Cochran,n.d.).In many parts of FIGURE 6 Cross-Laminated Timber(CLT)Source:Depositphotos11CHAPTER 2-The role of wood construction in a circular economy
182、the world where timber was abundant,early inhabitants used wood in many forms to build simple structures,including log buildings for shelter.Over time,log construction became more sophisticated as logs were flattened on two sides or fashioned into square timbers to improve the continuity of wall sur
183、faces and provide greater protection from wind and rain as well as heat loss.At some point,timbers began to be formed into structural frames,incorporating both horizontal and vertical members,with connections made using notching,mortise and tenon joints as well as wood pegs(Figure 7).Timber framing
184、later spread to Central Europe and soon thereafter to northern regions of the continent(Cochran n.d.).In its early form,timber frame construction is described as half-timbered as timber provided the structural frame of the building while the spaces between the frames were filled with plaster,brick o
185、r wattle and daub.The half-timbered method of construction was common in parts of Europe until the 17th century in modern-day France,Germany and the United Kingdom of Great Britain and Northern Ireland in particular.After 1400,many European houses were made of masonry on the first floor thereby prov
186、iding greater protection from bands of marauders-with half-timber construction above(Chisholm,1911).By the early 1600s,several factors contributed to a shift away from wood construction and the use of wattle and daub in much of Europe.A major factor was that there was a lack of established sustainab
187、le forestry practice and an overuse of wood for a myriad of purposes,including the production of charcoal,home heating and building construction,which led to a strain on the supplies of timber.Another factor was changing fashion,which led to an imitation of Mediterranean construction and the increas
188、ed use of clay and stone in construction(WoodMasters,2015).Through succeeding centuries and up to the present day,sustainable forestry practices have become widespread,timber supplies have recovered,and timber framing has continued to be the most common form of wood construction in Central Europe.Mo
189、rtise and tenon joints have been replaced with various types of metal connectors while what was wattle and daub infill is now non-load bearing wall framing sections which are anchored to vertical elements of the timber frame.Many houses also continue to be built in the half-timber style wherein a fr
190、ame of wood timbers provides the structural strength and infill consists of non-wood materials such as stone,concrete block or brick.Another form of construction involves stacking squared timbers to create walls(sometimes both interior and exterior).Roof structures are wood and wide overhangs common
191、.In northern Europe,post and beam construction is common,with heavy timbers supporting the building;heavy wood framing or timbers typically fill spaces between timbers,with extensive attention to tight construction and insulation.As Europeans began migrating to North America in the early 1600s,they
192、brought with them knowledge of construction methods from their home countries.German settlers in what would become the State of Pennsylvania often constructed precision-built log homes(Youngquist and Fleischer,1977)and Scandinavians,for the most part,also opted for log buildings(Carlsen 2008).Those
193、from other areas and especially from todays United Kingdom of Great Britain and Northern Ireland,built half-timber structures using wattle and daub as infill.But those in half-timber houses in the northern reaches of the Americas soon found that the wall construction methods that had proven adequate
194、 in the United Kingdom of Great Britain and Northern Ireland and southern France did not provide sufficient protection from the cold in their new location.Consequently,timber frame buildings began to be sheathed with solid wood siding of oak or pine,sometimes with narrow strips of wood underneath(Yo
195、ungquist and Fleischer,1977).Log and half-frame construction remained predominant in the eastern half of North America through the early 1800s,when dramatic change came about due to three developments,which in combination effectively changed everything(Carlsen 2008).First,automation brought the mass
196、 production of nails,which previously had to be hammered out on a forge one-by-one.Thus,whereas nails had previously been relatively scarce and expensive,they became plentiful and inexpensive.At about the same time,a number of sawmills converted to steam power rather than waterpower,meaning that the
197、se mills no longer needed to be located near rivers and could instead be situated more closely to where lumber was needed.The result was an increase in the number and availability of sawmills which could quickly convert logs into long,narrow pieces of lumber often referred to as dimension lumber.Tha
198、t development,in turn,led to the introduction in 1830 of a FIGURE 7 Mortise and Tenon ConnectionSource:DepositphotosCircularity concepts in wood construction12building technique known as balloon frame construction,a form of timber frame building(Carlsen 2008).Similar to platform frame construction i
199、n common use today,balloon frame construction involved using nails in the assembly of wall sections composed of vertical members connected at each end by top and bottom plates.Wall sections were assembled on the ground,with these walls subsequently raised into place by a team of workers.Each vertica
200、l member was cut to the full height of the wall,from the sill to the roof line.As houses were commonly built to two storeys,this meant that vertical members were approximately six to nine meters in length.The fact that wall supports were the full height of walls marks the key difference between ball
201、oon frame construction and platform framing,another construction method that came later.Balloon framing allowed rigid construction of buildings involving relatively few people and,because of these efficiencies in labour and materials,it became the predominant form of home construction in the United
202、States of America and remained so well into the 20th century(Carlsen 2008).By the 1940s,platform framing-a refinement of timber frame construction-had largely displaced balloon framing.Using this method,buildings were constructed one floor at a time.After putting in foundations and floor platforms,w
203、alls were assembled as before but wall supports were cut to the height of the one floor being added.This was followed by the addition of another platform and another set of walls,and so on.While fire concerns limited wood building height to only 2-3 storeys,from an engineering perspective,platform f
204、raming allowed for much taller buildings than balloon framing.Soon after the shift to platform frame construction,softwood plywood came onto the market,allowing for the rapid sheathing of exterior walls which were then covered by siding or other weather-resistant materials(Carlsen 2008).Other than a
205、 few changes designed to enhance energy efficiency and the prefabrication of some building elements,such as roof and floor trusses,this form of construction is representative of most current homebuilding in the United States of America and Canada,including multi-storey and multi-family construction.
206、In conclusion,despite examples of deforestation and forest degradation in some parts of the world over centuries,traditional wood construction in many regions followed circular and sustainable approaches in the light of todays concepts and definitions.In many areas,it was based on local sourcing of
207、raw materials,whereas buildings were repaired,refurbished and reused for different purposes over decades.2.3 Benefits of Wood Use in ConstructionResponsible wood use in construction is more circular and sustainable than use of other common building materials.Wood has inherent advantages and provides
208、 multiple benefits because it is a natural material,is renewable and can be fashioned into useful building components with minimal climate impact.Also,it can be incorporated into buildings that have lower lifecycle energy consumption and lower CO2e emissions than non-wood structures.Where wood is pr
209、oduced in sustainably managed forests or plantations,there are many environmental advantages of wood as a construction material.Below are key characteristics of wood.2.3.1 Produced by Solar EnergyWood is a material produced by the process of photosynthesis;a process driven by solar energy.The natura
210、l,solar energy-based process of tree growth provides an extraordinary sustainability advantage for wood.As a consequence of trees utilizing freely available,zero-impact energy,the additional energy used,in particular the fossil energy,in processing wood into building components is typically signific
211、antly lower than for other major construction materials.Additionally,in many cases,the utilization of solar energy in the forest is supplemented by the use of renewable biomass energy in wood processing facilities,further adding to this reduced use of fossil-fuel-derived energy.As an added bonus,and
212、 one which no other building material can duplicate,the natural process of photosynthesis which results in wood production is accompanied by the production and release of oxygen.2.3.2 Largely Composed of Captured and Stored CarbonThe fact that wood is produced as a result of photosynthesis translate
213、s to another key advantage:growing trees capture CO2 from the air,sequestering much of that carbon in the form of wood.Among all species of wood found in the world,carbon composes an average of one-half of its dry weight.When trees are subsequently harvested to produce wood products,the carbon withi
214、n their wood continues to be sequestered in the products made from it for as long as those products last,which,in the case of buildings,can be 100 years or more.Therefore,when wood is used in construction,specific buildings and even entire neighborhoods become additional carbon storage pools alongsi
215、de forests and grasslands.In the United States of America,where wood is predominant in homebuilding,the quantity of CO2e represented by the carbon contained in wood in use in 2020 was estimated at 1.5trillion tonnes,or over 10 percent of the quantity contained in above-ground forest biomass,a figure
216、 that is increasing annually at a rate of about 20 million tonnes(USEPA,2022b).The reality is that wood stores a great deal of carbon,the magnitude of that storage is also exemplified by Sherrill and Bratkovich(2018),who determined that a single white oak dining room table with ten chairs sequesters
217、 approximately 331 kg of CO2e.By combining carbon capture during tree growth with the effect of delayed emissions due to carbon storage in wood products,the use of wood brings immediate 13CHAPTER 2-The role of wood construction in a circular economybenefits and contributes to long-term and extended
218、climate mitigation goals consistent with circular economy principles.2.3.3 RenewableA key advantage of wood in comparison to other materials commonly used in building construction is that wood is renewable.If forests and plantations from which wood is obtained are sustainably managed and responsibly
219、 harvested,the availability of wood for human use can be sustained for the long term without sacrificing other critical values and amenities provided by forests.Wood is the major construction material that provides multiple sustainability benefits throughout its production cycle as the use of wood r
220、equires the continuous growth of trees and the support of associated biodiversity.2.3.4 Strong,Yet Light WeightIt has long been known that wood has a high strength-to-weight ratio.In the emerging era of mass timber and tall wood buildings,this reality has come into focus for many in the building des
221、ign and engineering community.What this means is that wood buildings of comparable strength to buildings constructed of alternative materials weigh considerably less.One study,which compared a multistorey mass timber building with an otherwise identical reinforced concrete building,determined that t
222、he mass timber building weighed 67 percent of the reinforced concrete equivalent(Chen et al.,2020).As a result,buildings of great height that incorporate large amounts of wood can be built with less massive foundations,footings and pilings than would otherwise be required(Gosselin et al.,2017).Wood
223、is also a natural choice when additional storeys are desired on an existing building in which the foundation and footings are not sufficiently robust for the addition of functionally equivalent upper floors built of steel or reinforced concrete.This advantage can result in a tangible reduction in th
224、e use of energy-intensive building materials,concrete in particular.A reduced reliance on concrete can positively influence the carbon footprint of a building and contribute to the circularity and sustainability of the construction sector.2.3.5 A Natural Thermal InsulatorWood and products made from
225、it provide natural protection from heat transfer and loss.This is because wood fibres are hollow,creating air pockets which serve to protect against heat transfer.Although building standards require greater protection from heat loss than wood alone can provide,the quantity of additional and often en
226、ergy-intensive insulation needed in wood exterior walls is generally significantly less than in concrete walls or those framed in steel.The use of wood and the benefits of its natural insulating properties can contribute to reduced use of other insulation materials that have associated climate impac
227、ts.The natural thermal insulation attribute of wood contributes to its value in reducing the environmental impacts of the built environment.There are also innovative opportunities for the development of insulation materials made from wood and wood fibre.Future development of wood-fibre insulation ca
228、n further advance circularity and sustainability in the construction sector.2.3.6 RecyclableAt the end of the useful life of a structure made wholly or partially of wood,building components may be recovered for reuse and recycling.While the reuse and recycling of wood at end of life is currently rel
229、atively rare in developed countries,considerable potential exists for such processes in a circular economy.Although reuse and recycling are also possible for many other categories of materials,wood has the advantage of also storing carbon and energy throughout its life.The potential for energy recov
230、ery from wood which,for one reason or another,cannot be reused or recycled,adds another dimension to its end-of-life possibilities.Combustion with energy recovery is commonly practiced today,although there again exists great potential for expansion of end-of-life conversion to energy.The many altern
231、atives for what can be done with wood after its first useful application offers circularity and sustainability benefits that are suitable for diverse situations,including where renewable energy generation is a priority or where the avoidance of waste and the reuse of materials is essential.2.3.7 Aes
232、thetically Pleasing and Beneficial to Human HealthAs reported by Lowe(2020),many studies have found positive effects on human health and well-being from exposure to wood.Findings include those that have documented increases in human comfort(i.e.satisfaction with room conditions such as lighting,nois
233、e and temperature)when in spaces with extensive wood surfaces as compared to spaces containing no visible wood(Watchman,Potvin and Demers,2017).Positive effects on human health have also been documented where,for example,research has reported that the presence of visual wood surfaces in a room lower
234、ed the activation of the sympathetic nervous system,a mechanism which is responsible for physiological stress responses in humans(Fell,2010).Kotradyova et al.,2019,similarly found that the inclusion of wood materials in medical facilities has a“regenerative and positive impact on the human nervous s
235、ystem”,citing a range of factors from appealing aesthetics to contact comfort and acoustics.Circularity and sustainability objectives are often focused on the resiliency of the economy and reduced impacts on the natural environment however,human emotional,mental and physical health aspects are also
236、important for sustainability and can be supported with wood in construction.Circularity concepts in wood construction14 2.4 Circularity and Sustainability of Wood Use in Comparison to Other Construction MaterialsThe discussion that follows focuses on the quantity of energy used and CO2e emissions ge
237、nerated in the process of producing building components and constructing various types of buildings using wood,steel and concrete.Comparisons can be challenging as modern buildings are virtually never constructed of only one material.Instead,builders and designers tend to use various materials in va
238、rious proportions to take maximum advantage of the unique properties and construction benefits of each material.This is often true of both structural and non-structural elements.For this discussion and the several case studies referenced herein,the type of building is determined by the predominant m
239、aterial used to construct the load-bearing frame.2.4.1 Relative Impacts of Building MaterialsExamples provided below are based on extensive analyses involving the application of LCA,a science-based tool specifically designed to allow the determination of multiple specific environmental impact indica
240、tors and interrelationships.With roots in the 1970s,but increasingly employed in the 21st century,LCA provides a mechanism for systematically evaluating environmental impacts linked to a product,from raw material procurement,transport,manufacturing,use and maintenance through to end-of-life treatmen
241、t,e.g.,re-use,recycling or disposal to landfill.The use of LCA is beneficial in the evaluation of products that are as small as a pencil or as large as a tall building.Application of LCA yields definitive information regarding such indicators as impact on climate change,water use,acidification,eutro
242、phication,fresh water eco-toxicity,particulate emissions,ozone depletion,fossil fuel depletion,human toxicity and more.Throughout this chapter,LCA-based findings are frequently referenced in discussions of how enhanced and optimized use of wood in construction can help to reduce GHG emissions and cl
243、imate change.The application of LCA is also a key strategy for supporting circularity and sustainability because LCA findings inform actions that improve the outcomes of material use and recovery.As noted previously,there are three primary structural materials used in construction:steel,steel-reinfo
244、rced concrete and wood.Energy consumption and CO2 emissions linked to the production of various materials(commonly referred to as embodied energy and embodied carbon,respectively),as determined by LCA,on both a mass and volume basis,are shown in Table 1.Although this data is specific to the United K
245、ingdom of Great Britain and Northern Ireland,values are comparable to those of other European countries.In view of this,and although various materials are not used in equal mass and volume when constructing functionally equivalent buildings or components,the figures nonetheless provide insights into
246、 the relative impacts of key structural materials.Table 1 shows the embodied carbon associated with the production of various materials.To apply this information effectively in the quantification of construction impacts,it is important to know how materials are being utilized in a building.These com
247、parisons are discussed in detail in the next sections.MaterialkgCO2e/kg 1/kgCO2e/m3 2/kgCO2e/kg 1/kgCO2e/m3 2/Carbon(CO2e)stored in material not includedCarbon(CO2e)stored in material includedSteel reinforced concrete0.149354-Precast concrete0.172409-Precast concrete beams and columns0.194-0.249462-
248、593-Hollow core,reinforced concrete for flooring applications 3/-373-Steel(structural)1.2109,498-Softwood lumber0.263126-1.29-619Laminated veneer lumber(LVL)0.504242-1.25-600Glue laminated timber(glulam)0.512246-0.90-432Cross laminated timber(CLT)0.437210-1.20-576 TABLE 1 Embodied CO2e in Common Con
249、struction Materials 1/Source:Jones,C.and Hammond,G.2019.Inventory of Carbon and Energy(ICE)Database,v.3.0.Bath University/Circular Ecology.Value for hollow core reinforced concrete adapted from ICE data per m2.2/Conversion from kg/kg to kg/m3 based on a concrete mass of 2,380 kg/m3;a steel mass of 7
250、,850 kg/m3 for structural steel;and for wood products of specific gravity of 0.48.Structural steel is assumed to be comprised of 85 percent recycled material.15CHAPTER 2-The role of wood construction in a circular economy2.4.1.1 SteelThe environmental impact of steel construction materials is highly
251、 dependent upon their recycled content.The energy consumed in making steel is considerably greater if it is produced from iron ore versus from steel recovered from recycling.The impacts of producing new steel can be nearly 4 times greater than producing the equivalent quantity from fully recycled st
252、eel.Impacts also vary depending upon the types and quantities of metals used in creating different alloys of steel.Whether compared on a weight or volume basis,the production of steel requires greater energy consumption and results in greater CO2e emissions than the production of steel-reinforced co
253、ncrete.However,steel does not substitute for concrete on a kilogram to kilogram or m3 to m3 basis.The greater ability to span long distances without intermediate support(as required in concrete construction),coupled with requirements for substantially smaller beam dimensions than when designing in c
254、oncrete,give steel an environmental advantage.When functionally equivalent structures of steel and concrete are compared,results almost always show lower embodied energy and CO2e emissions for steel structures.Conversely,comparisons of structural steel with structural composites,such as LVL and glul
255、am,show similar embodied energy on a weight basis but vastly lower emissions for the composites on a volume basis.Having said that,the weight and volume of functionally equivalent wood and structural steel are quite different;in this context,both the embodied energy and emissions linked to the produ
256、ction of engineered wood are consistently lower than for structural steel.The recycled content of steel is dependent upon the intended use of a specific steel product.The degree to which recycled content steel can be incorporated into new steel products is limited by the extent to which alloying met
257、als are present.Because current technology does not result in the complete removal of all alloying metals,recovered steel becomes increasingly contaminated each time it is recycled.One consequence is that the recycled content of thin steel used in making such things as wall framing and auto bodies i
258、s,by necessity,quite low(and thus the embodied energy is high).The recycled content of large cross-section structural steel components is thus far not constrained,although it will likely become so at some point in the future.2.4.1.2 ConcreteThe environmental impacts linked to the production of concr
259、ete depend upon its desired strength which is,in turn,determined by the water-to-cement ratio.The greater the quantity of cement,the greater the impact and adding reinforcing steel to structural concrete further adds to the overall impact.Comparisons of embodied CO2e emissions in reinforced concrete
260、 and various wood products(Table 1)indicate that on a kilogram to kilogram,or cubic metre to cubic metre basis,concrete is a lower impact material than wood.However,because of high strength-to-weight ratios,building modules made of wood are both less massive and far lighter than functionally equival
261、ent concrete modules.On both measures,wood consistently outperforms both structural and non-structural concrete,often by a substantial margin.2.4.1.3 WoodLumber has the lowest environmental impact and offers the greatest contribution to the sustainability of any structural wood product.Lumber produc
262、tion is highly technical,engineered and exacting,using sophisticated scanning and computer control technology but does not entail an energy-intensive manufacturing process.Lumber production involves only sawing logs and then trimming and shaping the pieces produced before undergoing a drying process
263、.Engineered wood products,such as LVL,involve first cutting solid wood into thin veneers and then recombining these veneers using adhesive,or in case of glulam,end-jointing lumber made into longer laminations and face glueing laminations to create glulam.This process increases the magnitude of the e
264、mbodied energy and with these additional processing steps,combined with the use of adhesives,these materials have associated impacts that are higher than that of lumber(Table 1).For example,the production of CLT results in a significantly greater impact per kilogram or cubic metre than lumber alone,
265、even though the product is composed largely of layers of lumber.The difference is due to the use of resin and/or large metal fasters to bind components together.Of the three primary structural materials(concrete,steel and wood),wood is the only one that is composed of substantial quantities of carbo
266、n.About one-half the oven dry weight of wood is carbon,and wood continues to store that carbon as long as it exists,therefore also throughout the life of the building or building component made of it.Wood is the only principal building material that stores substantial carbon and,as noted previously,
267、significantly less carbon is emitted in the manufacture of wood building materials than available alternatives.While some types of steel are classified as high-carbon products these contain smaller quantities of carbon than wood.2.4.2 Relative Impacts of Building Structures The differences shown in
268、Table 1 are large as a result of comparisons based on weight and volume.However,when material use is viewed in the context of an actual building,the significance of these differences becomes clearer.What follows are three examples of wood(or largely wood)buildings that have been constructed in recen
269、t years.These examples illustrate that in real-life situations the use of wood in construction compares favourably to available alternatives and contributes significantly to circularity and sustainability.Circularity concepts in wood construction162.4.2.1 Wood Construction on the RiseBrock CommonsBr
270、ock Commons is an 18-storey student residence built on the campus of the University of British Columbia(UBC)in Canada.The 54-metre-tall Brock Commons residential complex includes housing for 404 students,assembly spaces,and units that each serve four student rooms(two per floor)which contain a pass-
271、through kitchen,bathroom and bedroom,assembly and study rooms as well as a student study-social lounge,in addition to mechanical spaces.This building is a hybrid structure composed of a combination of mass timber(CLT and glulam),structural steel and reinforced concrete.As detailed by Pilon et al.,20
272、17 and Pilon,Teshnizi and Lopez,2018,the wood in the building(2,233 cubic meters of CLT and glulam)contains 1,753 tonnes of CO2 that will be stored throughout the life of the structure and potentially beyond,depending upon the fate of the materials at the end of the buildings life.In addition,the ex
273、tensive use of wood in the structure rather than steel and concrete avoided 679 tonnes of CO2e emissions,meaning the total carbon benefit of this building equates to 2,432 tonnes of CO2e.Expressed differently,the carbon savings from the selection of wood rather than more concrete and steel are equiv
274、alent to not driving a typical passenger vehicle in Canada 18,713 km.John Hope Gateway Entrance to Royal Botanical Gardens,EdinburghIn this project,CLT panels supported by a diagonal lattice of 117 exposed tapered glulam beams were used to create a dramatic effect above a restaurant and other areas
275、associated with the John Hope Gateway Entrance to the Royal Botanical Gardens in Edinburgh,Scotland.The CLT forms a single horizontal timber plane that is accentuated by the supporting glulam beams that are used in conjunction with slender steel columns.A total of 674 cubic metres of wood were incor
276、porated into this structure,resulting in the long-term sequestration of 366 tonnes of CO2e.An additional 142 tonnes of CO2e emissions were avoided as a result of the selection of wood rather than steel or reinforced concrete for the roof of the building.Roof Beams Gardermoen Airport Terminal Buildin
277、g,OsloIn designing the roof structure for an addition to the Gardermoen airport terminal in Oslo,Norway,a question arose as to what material to use for the roof-support beams:steel or glulam timbers.An assessment of the impacts of using steel versus glue-laminated spruce beams found that the manufac
278、turing of steel beams for that project would have FIGURE 8 Brock Commons,University of British Columbia,CanadaSource:Brudder Productions,courtesy FIGURE 9 John Hope Gateway,Edinburgh,United Kingdom of Great Britain and Northern IrelandSource:Paul Raftery17CHAPTER 2-The role of wood construction in a
279、 circular economyrequired 2 to 3 times more energy and 6 to 12 times more energy from fossil fuels than would functionally equivalent glulam beams.Analysts noted that if virgin rather than recycled steel were used,the differences indicated above would be substantially greater.In the most likely scen
280、ario,manufacturing steel beams for this project was estimated to result in fivefold more GHG emissions than if glulam was used.The structure was subsequently built using spruce glulam beams(Petersen and Solberg,2002).These examples have highlighted the GHG emissions savings when using CLT and engine
281、ered wood compared to other construction materials.Further examples illustrate that any wood structure exhibits a similar carbon advantage over structures constructed of alternative materials.2.4.3 GHG Emissions and Climate ChangeAs the above examples have illustrated,the substitution of wood for re
282、inforced concrete or steel in construction results in reduced CO2 emissions.In creating a building,the mass of material used to construct it varies considerably depending upon the materials used.For example,the weight of functionally equivalent structural framing will vary greatly depending on wheth
283、er it is made of concrete,steel or wood.Concrete structures weigh more than steel and far more than wood,this difference in weight has a direct bearing on embodied carbon and the overall environmental impact(Table 1).The International Organization for Standardization(ISO)compliant comparative LCAs h
284、ave consistently shown there are lower climate impacts from wood buildings than those constructed of concrete and/or steel.The brief summaries that follow address research findings that have considered energy consumption from the point of raw material extraction(or recovery and recycling of raw mate
285、rials if applicable)through to the completion of various building projects.A Dutch study of four house types modelled with increasing quantities of wood used in construction found that a 12percent reduction of CO2emission related to material use for residential buildings would be possible in the nea
286、r term through increased wood use in residential buildings.(Goverse et al.,2001).A comprehensive assessment of single-family residential homes in two regions of the United States of America(Lippke et al.,2004)showed CO2 emissions from raw material procurement through to completion of a finished wood
287、 structure to be 31 percent lower than for that structure to be made of concrete and 26 percent lower than if it was made of steel.Because all of the structures analysed had concrete foundations,the relative emissions noted above were affected by the influence of emissions linked to this use of conc
288、rete.Analysis of only the above-ground portions of these structures,hence eliminating the concrete foundation element,showed CO2e emissions differences between wood and concrete,and wood and steel,to be 80 percent and 33 percent,respectively.A Swedish study of concrete and wood-framed buildings(Gust
289、avsson,Pingoud and Sathre,2006)found higher energy and CO2 balances for concrete structures(with differences in the range of 30 to130 kg CO2 per m2 of floor area)and concluded that reducing the proportion of concrete building materials relative to wood building materials would be an effective means
290、of reducing fossil fuel use and CO2 emissions.A study in the United States of America in which six commercial buildings having different functionalities,material systems and building techniques were redesigned through modelling to determine the impact on climate change potential(global warming poten
291、tial)of substituting wood for steel and concrete in construction.The study found an average reduction in climate change potential due to wood substitution of 60 percent across all building types examined(Milaj et al.,2017).A Swedish analysis of a number of life-cycle studies of multi-storey CLT buil
292、dings(Younis and Dodoo,2022)compared to equivalent structures made of alternative materials found notable savings in GHG emissions associated with the use of CLT.Reported emission reductions associated with CLT construction averaged 40 percent,primarily in comparison to concrete buildings and where
293、the differences were greatest when carbon sequestration was considered in the analysis.A Canadian assessment of the relative environmental impacts of a mid-rise office building constructed with structural concrete as opposed to CLT and engineered wood determined that the global warming potential of
294、the concrete design was almost four times greater than the CLT/engineered wood design(Robertson,Lam and Cole,2012.)FIGURE 10 Terminal 2 Gardermoen Airport,Oslo,NorwaySource:DepositphotosCircularity concepts in wood construction18 A study compared conventional buildings of 8,12 and 18 storeys,constru
295、cted with concrete and steel with otherwise identical buildings constructed of mass timber in three regions of the United States of America.The analysis found that in all regions and building heights,embodied carbon in mass timber buildings was 22 percent to 50 percent lower than in otherwise identi
296、cal steel-reinforced concrete buildings(Puettmann et al.,2021).Furthermore,in all of the mass timber buildings studied,carbon storage was determined to be greater than the carbon released in the process of product manufacture(including both fossil and biogenic carbon).In other words,the net global w
297、arming potential of the structure itself at the end of building life was a net negative.The study concluded the carbon storage benefit of mass timber construction more than offset GHG emissions from manufacturing.A Norwegian study involving a comparative LCA of structural frames of timber,steel and
298、reinforced concrete for commercial structures found net negative climate change potential for timber framing,as defined in terms of CO2e emissions per m2 of building.The net negative climate change potential of timber frames was compared to significant emissions for steel and reinforced concrete fra
299、mes with the margin of difference being considerable.The difference in GHG emissions between wood and steel,and wood and concrete widened as the designed length of span increased(Hegeir et al.,2022).Practical limits prevent more examples of consistent and replicable research findings from being give
300、n.However,as seen from the above examples,buildings constructed of wood in any form consistently show lower embodied energy and CO2 emissions than buildings constructed from concrete or steel.This is particularly the case when analysis factors out the confounding effects of common concrete foundatio
301、ns.Based on a large body of scientific studies,it is clear that the more wood is substituted for steel or concrete in creating a structure,the lower the impact on the climate and the greater potential for circularity and sustainability benefits.2.4.3.1 Carbon StorageAs noted previously,about 50 perc
302、ent of the oven dry weight of wood is composed of carbon that was captured from the atmosphere in the process of tree growth.This sets wood apart from other construction materials that contain little or no carbon and are not derived from a natural and renewable growth process.For example,even high-c
303、arbon steel beams and columns contain only 0.6 percent to 2 percent carbon as a percentage of their total weight.In the case of concrete,the production of which involves massive releases of CO2,the finished product contains virtually no carbon,although 5 chemical reaction of CO2carbon is slowly rega
304、ined through carbonation5 over the life of concrete products.Consequently,the increased use of wood in construction could substantially increase the volume of carbon stored in buildings.An example of this carbon storage potential is provided by a study conducted by thePotsdam Institute for Climate I
305、mpact Research in Germany and as reported in the journal Nature Sustainability(Churkina et al.,2020).This study examined four possible scenarios of timber use in buildings over the succeeding 30 years with results compared to what was described as“business as usual:0.5 percent of buildings construct
306、ed of wood,with the vast majority remaining constructed of concrete and steel”.For comparison,scenarios were developed in which 10 percent,50 percent and 90percent of new buildings were of wood construction.Results showed the potential for as much as 55 million tonnes of additional carbon storage in
307、 buildings across Europe per year.This result corresponded to the scenario where 90percent of new buildings were made of wood,however,55 million tonnes is an amount equal to about half of Europes cement industrys annual CO2e emissions.Among the studys conclusions was that the carbon storage capacity
308、 of buildings is far more determined by the number and the volume of wood elements used in the structural and non-structural components than by building type,size or the species of wood used.This conclusion suggests that in any kind of building,a reasonable carbon strategy is to incorporate as much
309、wood as possible as a replacement for steel or concrete.2.4.3.2 Building Lifecycle EmissionsThe energy and emissions embodied at the construction stage of a building are viewed as increasingly important.In view of consistently lower embodied GHG emissions of wood structures,the climate advantages of
310、 wood construction are widely recognized today by architects and engineers and are increasingly considered in building design.The embodied emissions advantage of wood,combined with carbon storage within the material itself,translates to lower emissions for wood structures throughout a given building
311、s life(Chen et al.,2020;Duan,Huang and Zhang,2022).The improvement in building life cycle emissions between mass timber and reinforced concrete has generally been found to be 20 to 35 percent(Durlinger,Crossin and Wong,2013;Jayalath et al.,2020).A comparison of building life cycle emissions of mass
312、timber and steel structures of 5 and 12 storeys determined that there were 31 to 41 percent lower GHG emissions for mass timber structures(Allan and Philips,2021).Given that operational energy consumption within a building tends to be quite similar regardless of the primary building material employe
313、d,significantly lower embodied fossil energy and associated lower 19CHAPTER 2-The role of wood construction in a circular economyGHG emissions at the point of construction completion lead to superior climate performance throughout the life of a building.Therefore,wood use in the construction sector
314、results in lower use of fossil fuel energy and lower embodied fossil energy in the built environment,thus contributing to its sustainability.2.4.4 Energy EfficiencyThe energy efficiency of a building is defined by two primary factors:embodied energy and operating energy.As previously indicated,embod
315、ied energy is the sum of all energy expended in the production(raw material extraction through to finished product),transport and on-site assembly of building materials into a completed structure.Operational energy is all the energy expended thereafter to heat,cool,maintain and otherwise occupy and
316、operate the building.2.4.4.1 Operational EnergyEnergy efficiency codes and standards for buildings require design for comparable performance regardless of the primary building material used.The operational energy consumption of buildings constructed predominantly of wood is often equivalent to the o
317、perational energy consumption of buildings constructed of alternative materials;however,wood buildings can require less insulation to attain the required energy performance due to the lower thermal conductivity of wood and wood building materials compared to concrete or steel.Table 2 shows the therm
318、al conductivity of common construction materials.The right-hand column illustrates the thickness of each material that would be needed to provide the same insulation value as 25mm thick softwood construction lumber a common material with the greatest inherent thermal resistance.The conductivity valu
319、e for softwood lumber also applies to wood construction materials such as CLT,LVL,glulam and plywood.The thermal conductivity of composite wood products such as LSL is about 8 percent higher than that of solid softwood(Tripathi and Rice 2017).Structural components of buildings(and metals in particul
320、ar)are not commonly directly exposed to outdoor environments.Nonetheless,structural materials can serve as a conduit of heat transfer across a building envelope and bridging between the interior and exterior of the building.This can lead to heat loss in winter and heat gain in summer.For high-conduc
321、tivity materials,such as steel,added insulation is needed to obtain comparable energy efficiency to buildings characterized by materials of lower thermal conductivity.The addition of insulation increases the embodied energy and carbon impacts of building with non-wood materials.That wood buildings r
322、equire less in the way of added insulation than buildings constructed of alternative materials is one reason why wood buildings are associated with lower embodied energy than other types of buildings.The embodied energy difference is often substantial,as described in the following discussions.2.4.4.
323、2 Embodied Energy and Associated EmissionsAs demonstrated by the many studies cited previously under the heading“GHG Emissions and Climate Change,”climate-warming emissions linked to mass timber buildings have been consistently found to be lower than for functionally equivalent buildings constructed
324、 of steel and concrete.Many other similar studies have confirmed these findings.Most of these same studies have also found,however,that embodied energy associated with wood buildings is greater than for structures constructed of alternative materials when all energy sources are treated the same.The
325、higher embodied energy findings are due to the use of renewable woody fuel for energy generation during wood product manufacturing,which is less efficient than energy generation from fossil fuels that typically fuel steel and concrete manufacturing.Total primary energy requirements for the creation
326、of wood buildings,and mass timber buildings in particular,are typically higher than for buildings constructed of concrete and/or steel(Liang et al.,2020;Felmer et al.,2022;Duan,Huang,and Zhang,2022).The Duan et al.study,which involved an extensive review of LCAs of mass timber construction,found tha
327、t the averagereported embodied energyof mass timber buildings upon completion of building construction was,on average,23 percent higher than for equivalent reinforced concrete buildings.However,embodied GHG emissions of reinforced concrete buildings were more than 42 percent higher than for mass tim
328、ber.MaterialAverage Conductivity (W/m K)*Relative Thickness for Equal Thermal Resistance of 22mm Softwood Construction LumberSoftwood Construction Lumber0.1-0.141Aerated Concrete0.161.3Concrete(light)2.04.8Concrete(limestone)1.29.6Concrete 0.616Carbon Steel60480Aluminium1801,440 TABLE 2 Thermal Cond
329、uctivity of Selected Construction Materials*The lower the conductivity value,the greater the resistance to heat transmission or lossSource:Straube,J.2016.Heat Flow Basics for Architectural Calculations.Circularity concepts in wood construction20The reason for the apparent anomaly is that fossil emis
330、sions associated with the production of wood building components and subsequent construction are significantly lower than for buildings constructed of alternative materials.For the most part,steel and concrete manufacturing currently rely on fossil fuels for the process thermal and electric energy n
331、eeds.Wood product manufacturing includes the utilization of the byproducts of sawmilling(such as bark,trimmings and chips)to generate renewable bioenergy.The question then arises,how much difference does this make when considering lifecycle emissions of a building when considering construction as we
332、ll as heating/cooling cycles and building operation through to the end of the useful life of the structure?For buildings constructed prior to the implementation of strict energy codes in the 1980s,the answer to this question usually was“not much”.In older buildings embodied energy commonly accounts
333、for only a small fraction(10 to 20 percent)of total energy consumed throughout the life of a building(Dimoudi and Tompa,2008;Ramesh,Prakash and Shukla,2010).However,as building energy efficiency has increased,as measured by the consumption of operational energy,embodied energy has become much more significant.Today,the embodied energy of buildings accounts for a much greater portion of the total e