SPECIFICATION GUIDANCE

Maximize Recycled Content 

In general, the greater the recycled content of steel, the lesser its embodied carbon. Specify minimum recycled content percentages to be required for all structural steel used on a firm’s projects. Consider whether these minimum percentages will apply to each piece of steel, or on average for the entire project. 

Note – average percentages are more commonly used and often easier to achieve. Above 75% recycled content is typical for hot rolled sections. For HSS and plate, it may be more useful to specify EAF production rather than recycled content.

Recycled content requirements may also be applied to metals in other specification sections, including steel decking, steel joist framing, and cold-formed steel. Cold-formed steel and decking are often formed from steel produced in BOFs.

Lower Embodied Carbon Steel Products

Many structural steel, steel deck, and cold-formed steel producers have published product-specific EPDs, so it is becoming increasingly viable to directly specify a maximum GWP for these components. The EC3 tool provides access to a wide variety of producer EPDs, and can help project teams set appropriate thresholds depending on their region.

Note – engineers should exercise caution when interpreting EC3’s Boxplot Diagram results for steel. The diagram aggregates many EPDs together that have differing and non-comparable features that affect embodied carbon, and therefore it is not an adequate representation of the steel market at this time. For example, EPDs are lumped together regardless of whether or not they include the effects of fabrication or galvanizing, whether or not the LCA accounting was developed according to a consistent Product Category Rule (PCR), whether or not the LCA modules included are consistent, and whether or not the product fits the category as defined in EC3. Engineers should confirm the EPD assumptions and be specific about parameters when specifying maximum GWP. 

Fabrication is an often overlooked portion of the fabricated structural steel package, but it accounts for nearly 10% of the total cradle-to-fabricator-gate GWP. At the end of 2023, AISC launched a Sustainability Partner Program (aisc.org/partnerprogram) for their fabricator membership that gets fabricators educated on their own footprint, commits them to reporting environmental data, and has them create sustainability goals that make an embodied carbon difference. You care that subcontractors are doing their part, so ask your contractor if the fabricator on your project has joined AISC’s free program. And ideally, consider requiring Sustainable Steel Fabricators in your specification.

Steel Resources:

• Model LCA Specifications: (Source: Carbon Leadership Forum)
• Carbon Impacts of Steel: (Source: Carbon Smart Materials Palette)
• More than Recycled Content: The Sustainable Characteristics of Structural Steel  (Source: American Institute of Steel Construction)
• Making your steel specification more sustainable: (Source: The Institution of Structural Engineers)
• How Clean is the U.S. Steel Industry (Source: Global Efficiency Intelligence)

Concrete Strength

Over-specifying concrete strength results in increasing embodied carbon. Specify only the strength that is actually required in the design of members. If durability provisions in the code require a higher strength, consider taking advantage of that in design.

Requiring concrete to achieve its design strength at an early age also increases the embodied carbon. Specify 56-day (or longer) strength, unless high early strength is needed, such as possibly for post-tensioning. If specifying high early strength, also take full advantage of the resulting 28-day or 56-day strength for design. Concrete continues to gain strength well beyond 28 days, especially when including fly ash. Specifying longer cure times can allow less over-design of the concrete mix by giving it more time to come up to strength.  Specifying the use of concrete maturity sensors is also recommended.

Reducing the cement content as much as possible will have a significant effect on lowering embodied carbon.  Mixes with less water will meet higher strengths with less cement.  Choose an appropriate prescription for the total amount of water in the mix, with provisions for use of plasticizers or water reducers for meeting slump/workability requirements. Additionally, designers may specify a higher allowable water / cement ratio to reduce unnecessary cement in the mix.

Air content also impacts the ability to achieve design strength – in general, higher cementitious materials content will be required for air-entrained concrete.

Specialty Services & Technologies 

There are a growing number of technologies and services that can reduce the embodied carbon of concrete.  Many of these technologies require changes to material specifications before they can be included in a project.  It is recommended that structural firms work with these technology producers to draft specifications and insert language so that these can be more readily specified on a project.

In carbon mineralization, carbon dioxide is injected into concrete during production, usually by specifying proprietary methods.  When introduced, the CO2 becomes chemically converted into a solid mineral – meaning that the CO2 is permanently trapped within the concrete. This technology requires a concrete specification without any prescriptive water / cement ratios.

Examples of specialty services and technologies include Green Sense, CarbonCure, Solidia, and Blue Planet.

Note – many of these technologies and services are specific to certain regions and are not available nationwide. 

Lower Embodied Carbon Steel Reinforcement (Rebar)

The embodied carbon of metals depends upon the electricity source used in production.  Rebar is typically produced in electric arc furnaces (EAFs). EAF steel generally has a higher recycled content and lower carbon footprint than blast oxygen furnace (BOF) steel.

Many rebar producers have published EPDs, so it is becoming increasingly viable to directly specify a maximum GWP for rebar. The EC3 tool can help project teams set appropriate thresholds depending on their region.

Recycled Content 

Recycled water can be permitted by including a reference to ASTM C1602 (instead of specifying potable water). Also for some members like footings and foundations, recycled aggregates could be permitted. However, avoid specifying a minimum recycled aggregate content for concrete as it can have an adverse impact on performance.

Note – the embodied carbon benefits of these strategies are hard to quantify.

Performance Specifications

The NRMCA has launched the P2P Initiative, an acronym for ‘Prescriptive to Performance’ specifications.  The initiative’s goal is to shift the concrete industry to performance-based specifications, to more easily allow for concrete mix design optimization to meet sustainability goals.  See the ‘WBS Strategy 3 – Performance Specifications’ section for further information.

Concrete Baselines:

• NRMCA Member National and Regional LCA Benchmark (Industry Average) Report – Version  (Source: National Ready Mixed Concrete Association)
• Carbon Leadership Forum 2021 Material Baselines: (Source: Carbon Leadership Forum)

Concrete Resources:

• The Top Ten Ways to Reduce Concrete’s Carbon Footprint (Source: Build with Strength, The National Ready Mixed Concrete Association)
• Embodied Carbon Impacts of California Concrete Mix Designs (Source: Structural Engineers Association of California (SEAOC))
• Specifying Sustainable Concrete: Using 56 day concrete strengths (Source: The Concrete Centre)
• Model LCA Specifications: (Source: Carbon Leadership Forum)
• Low-Carbon Concrete: (Source: Marin County, CA)
  • Sample Residential Specification (Source: Marin County, CA)
  • Sample Non-Residential Specification (Source: Marin County, CA)
• Resource Library – CarbonCure Specification Language (Source: CarbonCure)
• The Case for Performance-Based Concrete Specs (Webinar) (Source: CarbonCure)
• Specification Guide – Capturing the Value of Low Carbon Mixes (Source: Central Concrete Supply Co., Inc.)
• Guide to Improving Specifications for Ready Mixed Concrete: (Source: National Ready Mixed Concrete Association)
• Guide Specification – Concrete for LEED v4 Projects (Source: National Ready Mixed Concrete Association)
• Carbon Impact of Concrete: (Source: Carbon Smart Materials Palette)
• P2P Initiative: (Source: National Ready Mixed Concrete Association)
• Guide for a Low Carbon Concrete Implementation Strategy: (Source: Carbon Leadership Forum, Magnusson Klemencic Associates)
• Lowering the Embodied Environmental Impacts of Cement and Concrete (Source: MIT Concrete Sustainability Hub)

Specify Sustainable Forest Management Practices

The net carbon benefit of wood products varies greatly with different forest management practices. Poor forest management practices (e.g., logging on illegal or protected land and clearing of forest for non-forest use) can lead to significant carbon emissions in addition to other negative environmental impacts on ecosystem and forest health. Sustainable forest management practices can vary regionally as different jurisdictions attempt to balance ecosystem health, forest resilience, biodiversity, and community needs. A well managed forest  can lead to win-win scenarios of maximizing ecosystem health and minimizing emissions into the atmosphere. It is crucial that sustainable forest management is a core criteria for the procurement of sustainable wood products. 

According to Think Wood, assurance for timber harvested sustainably can be provided by forest certification, responsible fiber sourcing standards, and/or Best Management Practices (BMPs, developed by every U.S. state). Currently, it is common practice among green building designers to specify certified wood (common programs include FSC, SFI, ATFS, and PEFC) as a means of assuring a baseline of sustainable forest management practices. State-developed Best Management Practices may not be as stringent as the requirements of forest certification, but a few strong examples of state codified BMPs include Oregon’s Forest Production Laws: An illustrated manual, Forest Practices Illustrated: A simplified guide to forest practices rules in Washington State and California’s Forest Practices Act.

While forest certification is often seen as having a high level of quality assurance for sustainable forest management, direct climate benefits are not necessarily assured by wood certification since these programs are only recently beginning to specifically address climate mitigation. Additionally, certified products may come with a cost premium. Smaller forest owners and public forests can also demonstrate exceptional, sustainable forest management practices, but may not have the financial means for acquiring certification. The Climate Smart Wood Group provides guidance on additional criteria for climate-smart forestry operations outside of certification which includes non-industrial timber producers such as “US federal, indigenous or tribal, non-profit organizations and land trusts, and family forest owners.” Other recommendations are sourcing from ecological forestry operations where “restoration is occurring to enhance ecological resilience and integrity (which) by definition offer many climate-smart benefits,” and from forests with above-business-as-usual regulations and practices. Examples of the latter include forests governed by Habitat Conservation Plans and those that are third-party-certified carbon projects.

Given the complexity of wood sourcing impacts and challenges with transparency and traceability, practitioners are encouraged to engage with local foresters who are trained, deeply knowledgeable about local forest needs, and can help identify wood sources from forest management operations. Along with source forest disclosure as described in the previous section, it is recommended to request the disclosure of forest management practices.

While there is an ISO Standard (ISO 38200:2018) setting requirements for the supply chain of wood products, there is currently no consensus or widely-accepted standard for sustainable forest management practices. However, there are some clear wins for sustainable wood procurement that practitioners can consider. This includes salvaged and reclaimed wood products (more detail in next section), and feedstocks that are byproducts of targeted harvest, such as small diameter trees not suitable for typical dimension lumber, as part of forestry operations to improve forest health (some examples include Vaagen Brothers Lumber in Washington and Timber Age Systems in Colorado). 

Recommendations:

• Request information about forest management practices from manufacturers for wood products. 
• Avoid illegally sourced wood and wood sourced from deforested lands, old growth forests, and protected lands.
• Proactively engage with forest managers and sawmills, with support from the project team (client, designers, and contractors), to uncover opportunities for available feedstock as byproducts of local forest and ecosystem needs.
• Evaluate sustainable forest management practices against criteria provided by forestry experts, like the Climate Smart Wood Group procurement guide.
• Seek indicators of sustainable forest management practices such as forest certification, responsible fiber sourcing standards, Best Management Practices, ecologically-motivated forestry operations, and climate-smart forestry operations from non-industrial timber producers such as the US Forest Service, indigenous or tribal communities, non-profit organizations, and family forest owners.

Request Forest Sourcing Disclosure

The supply chain for wood products tends to be more opaque and harder to trace than for other structural products. Compared to commodity mineral supply chains, the wood supply chain is often more dispersed and with a larger variation in the involvement, attitudes, and intentions of stakeholders (such as local communities, land owners, loggers, mills, manufacturers, developers, the commoditized lumber market, etc.) while also varying from region to region.  Engineers can leverage their position as material specifiers by requesting a high degree of traceability and transparency within the forest industry. The World Resources Institute defines these terms as follows: 

• Traceability refers to the ability to link a product with information about its history of locations, owners, and transformations between points in the supply chain.
• Transparency refers to the making available of information by any stakeholder. This can include broader information that is relevant in the context of halting and reversing forest loss such as sustainability policies and practices, commitments, land use information, monitoring, or outstanding grievances.

EPDs of structural wood products currently do not include upstream primary data such as source forest disclosure and additional emissions sources.  As summarized in the wood chapter of a recent report by RMI, “Greater life-cycle impacts data and transparency of wood products such as EPDs are needed to make better informed choices. Wood products must be sourced and manufactured in ways that go beyond regulatory minimums and foster regenerative and climate-resilient solutions.” As such, it is necessary to request for traceability and transparency within the wood supply chain until EPDs are updated to reflect this information.

Source: Driving Action on Embodied Carbon in Buildings (RMI)

Some structural engineers are including questionnaires in their bid documents to collect sourcing information. For example, Davies-Crooks Associates provides one example:  

Disclosure Request Template Reference: MASS TIMBER FOREST SOURCING DISCLOSURE QUESTIONNAIRE (8/1/2023)

Introducing a detailed questionnaire during procurement is a relatively new practice for the industry, and therefore the extent of success and difficulty of this approach are not yet known. It is anticipated that the process will be continually refined for streamlined and practical implementation by users over time.The questionnaire may be included in bid documents to give project teams added transparency about the sourcing of mass timber products. The use of this questionnaire can also provide more accurate transportation data to evaluate the climate impacts of shipping timber products over large distances. 

The Climate Smart Wood Procurement Guide, linked in the Wood Resources section, provides more detailed recommendations and guidance on criteria for wood sourcing from a climate-focused perspective.

Recommendations:

• Aim to use the forest sourcing disclosure questionnaire (partially or fully) in your projects.
• At minimum, request information about the source forest from manufacturers for all wood products above a certain threshold (e.g., > 1% of total construction volume or cost). 
• Examine wood product EPDs and acknowledge any missing life-cycle impacts data.

Transportation Emissions

For wood products, transportation impacts can be a notable portion of the total embodied carbon impacts compared to other structural materials, in part due to the inherently lower carbon emissions associated with wood material production. Wood product specifiers should therefore pay particular attention to transportation. 

Engineers should research the availability and sustainable attributes of wood products in their local region, and specify product requirements that are consistent with the project’s carbon reduction goals. Transportation emissions can vary greatly between different wood products and are a function of the total distance traveled and the mode of transportation utilized.  A transportation consideration specific to CLT and some other mass timber products is if it will be sourced from North America or European sources.

Certain studies using WBLCA and EPDs have also indicated that transportation-related emissions can be the majority of the embodied carbon for a given mass timber product. For example, a UK-based study performed by Arup for CLT and Glulam indicated that the transportation of the original raw materials to the manufacturing facility (life cycle stage A2) accounted for 8-10% of the A1-A5 emissions.  Stage A4, transport of manufactured materials to the construction site, accounted for 50-55% of A1-A5 emissions.  It should however be noted that the emissions percentages in this study are representative of the UK market, which typically sources from Europe, and the results should not be directly applied to a North American project (where transport distances and modes will vary widely).  The results of any WBLCA study are dependent upon multiple factors such as the project location, proximity of mass timber suppliers, and modes of transportation, and the calculation should be performed for each project.

Typical carbon contributions to the supply chain for CLT and Glulam products. 

(Note: this study was performed in the UK.)

Source: Embodied Carbon, Timber (Arup)

Recommendations: 

• Weigh the transport emissions (life cycle stage A4) against the manufacturing emissions (A1-A3) to understand the full life cycle emissions associated with use of the wood product. Forell Elsesser Structural Engineers has created this tool to calculate A4 emissions.
• Prioritize locally sourced products whenever possible.
• Investigate whether lower carbon transport modes are available. Consider prioritizing suppliers that use lower carbon transport modes like rail and cargo ship whenever possible.  As the availability of electric vehicles continues to increase, investigate opportunities for electric semi truck transport.

Acknowledge the Impact of Adhesives

It is important to note that specification of adhesive types is beyond the reasonable scope of the structural engineer, as these are either prescribed by standards, performance requirements, or chosen by the supplier.  This section is instead provided to educate and provide insight into this lesser-known topic.

Many types of wood and mass timber products require the use of adhesive technologies.  While explicit information on the GWP of wood adhesives is limited, some research has shown that adhesives contribute significantly to the GWP of wood-based products.  

Adhesives are typically made from oil, which is energy-intensive to extract and refine. CLT and Glulam products typically contain at least 1 to 2.5% by volume of adhesive, yet the specific percentage of a product’s GWP that is due to adhesives is not consistently reported in EPDs.  Researchers at Arup have put together the graphic below from a variety of resources to provide a comparison of GWP/kg between common adhesives and common wood products that require adhesives.

Source: Embodied Carbon, Timber (Arup)

MUF = Melamine-Urea-Formaldehyde*

MF = Melamine Formaldehyde 

PF = Phenol Formaldehyde 

EPI = Emulsion Polymer Isocyanate

PEP = Polyurethane Emulsion Polymer  

PUR = Polyurethane* 

PVAc = Polyvinyl Acetate

* CLT most often uses Polyurethane (PUR), and Glulam most often uses Melamine-Urea-Formaldehyde (MUF).

Acknowledge the Impact of Kiln Drying

It is important to note that specification of kiln drying methods is often beyond the reasonable scope of the structural engineer, as these are either prescribed by standards or chosen by the supplier.  This section is instead provided to educate and provide insight into this lesser-known topic.

Wood is a hygroscopic material that absorbs and releases moisture into the surrounding atmosphere. Wood has improved strength and stiffness when dry compared to wet, and wood shrinks as it undergoes the drying process – causing checks. To ensure dimensional stability, avoid visual cracks, and benefit from the improved mechanical properties, wood is typically dried to approximately 12% moisture content – the typical equilibrium moisture content of a building. Wood for laminations (lamstock) in CLT and glulam is typically dried to a moisture content of less than 16% to ensure proper adhesion of the laminations.

Wood can be dried by air flow (wood stickered and stacked outside), in a solar kiln (wood stickered and stacked within a greenhouse), or dried in a fueled convective kiln. (Stickers are wood spacers placed between the boards to allow the air to circulate.) Air drying uses no additional energy but may take months to reach the desired moisture content. Solar kilns use a small amount of electricity to power fans to circulate air, but may take 4-6 weeks to reach the desired moisture content. Convective kilns require energy input to heat the air, but can dry wood in days. Convective kiln drying is the typical method for commercial wood and laminated wood products.

The kiln drying process consumes around 90% of the manufacturing energy of sawn wood products. The fuel used for the kiln depends on the facility location and operator, but typically will be a mix of incinerated biomass from the facilities process and natural gas. Some studies have indicated that between 35% and 40% of the Global Warming Potential of glulam can be attributed to lamstock production, which is associated with this drying process.

While it is often impractical to specify the drying method, this should be visible in the product EPD. Facilities that use a larger proportion of non-fossil fuels for their kiln or have a local renewable energy grid will have a lower carbon footprint. For unlaminated wood products in some environmental conditions, the emissions associated with drying can also be avoided by specifying green (undried) lumber with a moisture content of 20% or above and allowing the material to dry naturally.

Additional Considerations for Structural Engineers

SHEATHING | The 2020 AWC/CWC industry-average Environmental Product Declarations (EPD) indicates that OSB has over 10% more embodied carbon than softwood plywood. The latest OSB and plywood EPDs can be found through a search of the American Wood Council resources.

OTHER WOOD USES | The construction industry often requires the use of wood-based products for ultimately non-structural applications, most notably including concrete formwork and temporary structures such as shoring.  Structural engineers should consider these other uses of wood when revising procurement documents for sustainability, and apply the strategies listed in this resource where applicable (formwork is often specified in the Division 03 concrete specifications, for example).

WOOD IN LCAs | Always report biogenic carbon separately when performing LCAs (e.g., provide a total emissions value that does not include biogenic carbon storage).  

This is recommended because there is currently no consensus on the valuation of carbon storage for wood products.  There are two approaches: backwards accounting (taking credit for carbon absorbed in the last 40 years, which doesn’t affect our atmosphere today), and forward accounting (considering the carbon storage of newly planted trees as a direct result from harvest).  

It should also be noted that ISO 21930:2017 uses the +1/-1 approach for biogenic carbon in LCA accounting. When a bio-based material leaves nature, its stored biogenic carbon is reported as a negative emission.  When the bio-based material is converted back to emissions, via combustion or biodegradation at end of life, it is then counted as a positive biogenic emission.

Best practice is to report biogenic carbon separately in LCAs to enable effective comparisons between design options, and discourage inefficient use of timber.

OTHER BIOGENIC MATERIALS | There are many complexities associated with wood products from both a climate, environmental, and market perspective. However, there are many biobased materials with a more defensible low-carbon and carbon storage story since they can have shorter growth cycles and less nuanced land use impacts. Consider incorporating other biobased materials into your practice, such as bamboo, grass panels, or load-bearing straw and hemp assemblies.

Additional Wood Resources:

• CLF Wood Carbon Seminar: Trees, Forestry, and Carbon 101: https://www.youtube.com/watch?v=ciY6G5IbAYE (Source: Carbon Leadership Forum)
• Carbon Impact of Wood Products https://materialspalette.org/wood/ (Source: Carbon Smart Materials Palette)
• Forest Certification Update 2021: The Pace of Change https://www.dovetailinc.org/portfoliodetail.php?id=60085a177dc07 (Source: Dovetail Partners)
• Climate Smart Wood Procurement Guide https://www.climatesmartwood.net/ (Source: Climate Smart Wood Group)
• LETI Low Embodied Carbon Specification and Procurement Guide https://www.leti.uk/specification (Source: Low Energy Transformation Initiative)
• Embodied Carbon, Timber https://www.istructe.org/IStructE/media/Public/Resources/ARUP-Embodied-carbon-timber_1.pdf (Source: Arup)
• 10 Questions with Sustainable Forestry Expert, Dr. Edie Sonne Hall
• Carbon Leadership Forum, “Top 10 Questions and Answers from the Wood Carbon Seminars,” Question 10 (https://carbonleadershipforum.org/download/11317/).
• Differences between the Forest Stewardship Council (FSC) and Sustainable Forestry Initiative (SFI) Certification Standards for Forest Management: https://dovetailinc.org/upload/tmp/1581654356.pdf (Source: Dovetail Partners Inc.)
• AF&PA White Paper: Sustainable Forestry and Certification Programs in the United States: https://twosidesna.org/wp-content/uploads/sites/16/2018/05/sustainable-forestry-and-certification-programs-in-the-united-states.pdf (Source: American Forest & Paper Association)
• Wood: Is It Still Good? Part One: Embodied Carbon. https://www.buildinggreen.com/feature/wood-it-still-good-part-one-embodied-carbon (Source: Building Green) Note – purchase is required for this resource
• 2024 Embodied Carbon Data for Timber Products. https://timberdevelopment.uk/resources/embodied_carbon_data_for_timber_products/ (Source: Timber Development UK) Note – UK data only
• When to Include Biogenic Carbon in an LCA. https://www.woodworks.org/resources/when-to-include-biogenic-carbon-in-an-lca/ (Source: WoodWorks)

Supplementary Cementitious Materials

Use supplementary cementitious materials (SCMs) to limit the quantity of Portland cement in the mortar, grout mix, and/or concrete block to reduce CMU’s carbon footprint.  See ‘CIP Strategy 1’ section for more information.

Carbon Mineralization in Concrete Masonry Units

Carbon mineralization by CO2 injection during manufacture applies to CMU as well as cast-in-place concrete. See ‘CIP Strategy 3’ section for more information.

Lower Embodied Carbon Steel Reinforcement (Rebar)

The embodied carbon of metals depends upon the electricity source used in production.  Rebar is typically produced in electric arc furnaces (EAFs). EAF steel generally has a higher recycled content and lower carbon footprint than blast oxygen furnace (BOF) steel.

Many rebar producers have published EPDs, so it is becoming increasingly viable to directly specify a maximum GWP for rebar. See ‘WBS Strategy 1’ for more information on EPDs.

Resources:

• High-Volume SCM Grouts for Masonry: (Source: Jamie Farny, Structure Magazine May 2018)
• Resource Library – CarbonCure Specification Language: (Source: CarbonCure)
• How to Properly Specify Masonry Wall Assemblies for f’m = 2500 psi (Source: FORSE Consulting, LLC)