Green Building and Wood Products  

With growing pressure to reduce the carbon footprint of the built environment, building designers are increasingly being called upon to balance functionality and cost objectives with reduced environmental impact. Wood can help to achieve that balance.

The choice to use wood as a green building material is intuitive. It's abundant, renewable and recyclable, and has a lighter carbon footprint than other construction materials.¹ Wood is also the only structural building material with third-party certification systems in place to verify that products have come from a sustainably managed resource.

In addition to its environmental benefits, wood's natural beauty and warmth have a positive effect on building occupants. In two studies conducted at FPInnovations and the University of British Columbia, for example, the use of visual wood was shown to lower sympathetic nervous system (SNS) activation, which is responsible for physiological stress responses in humans.² As a result, an increasing number of architects are incorporating wood in their designs as a way to achieve goals such as improved productivity and performance in schools and offices, and better patient outcomes in hospitals.³

With all of these attributes, wood is well positioned as a key component of environmentally superior structures. Yet, early efforts to promote green construction resulted in highly variable treatment of wood in green building rating systems—which, at the time, were largely based on long lists of prescriptive standards, typically focused on single attributes such as recycled content. Such variability can still be seen in many of the green building programs in use today. However, these systems are increasingly moving away from prescriptive standards and toward reliance on systematic, multi-attribute assessment of building products, assemblies, and completed structures through life cycle assessment (LCA). The result is greater uniformity between programs and far greater robustness in evaluation, both of which serve to leverage the environmental advantages of wood.

This continuing education course examines key green building rating programs and how wood building materials and components are rated within each. Increased reliance on LCA and environmental product declarations (EPDs), and the implications for wood construction, are also explored.

Green Rating Systems, Codes, and Wood

Of the more than 42 green building programs currently in use in the U.S. and Canada, 12 of the most prominent are examined in this article; the UK BREEAM program—the world's first comprehensive green rating system and basis for many systems worldwide—is also included.

Approaches to Rating Green Buildings

Early green building rating initiatives in North America were based on lists of prescribed measures for reducing energy consumption and various environmental impacts. Among these were Built Green, Earthcraft, Leadership in Energy and Environmental Design (LEED), and the NAHB Model Green Home Building Guidelines—precursor to the National Green Building Standard. Arranged within categories such as Energy, Water, Indoor Air Quality, Materials and Resources, and Site, prescriptive lists of recommended or required measures outlined the path toward environmentally better buildings. Each measure typically addressed a single concern or attribute such as recycled, recycled content, rapidly renewable, and sourcing. Recommendations for improving environmental performance of buildings and construction practices varied among the initiatives, as did recommendations for the use of wood and wood products.

In more recent initiatives, there has been a noticeable shift away from prescriptive measures and toward systematic, performance-based assessment using LCA. This shift is reflected in the latest version of LEED, Green Globes and several other rating systems, and is discussed later in this article.

Green Building to Code

Given broad interest in reducing the environmental impacts of buildings and their construction, it is not surprising that provisions of voluntary green building rating systems are beginning to find their way into building codes. The State of California became the first state to codify green building provisions with its California Green Building Standards Code (CALGreen), which applies to all occupancies within the state. Model code language has also been developed in the form of ASHRAE 189.1 and the International Green Construction Code (IgCC). Washington, D.C., for example, has adopted the 189.1 standard as part of its city building code, while Florida requires compliance with the IgCC in the construction of state-owned buildings. Other states and municipalities, such as Maryland, Rhode Island, Phoenix, and Scottsdale, have endorsed the use of the IgCC on a voluntary basis.

CALGreen provisions and model code language within the ASHRAE and IgCC standards are similar to those in voluntary green building rating systems. However, a comparison of all three shows greater incentive for wood use under the IgCC than CALGreen or the ASHRAE standard. For example:

▶ The Materials Selection section of the IgCC standard specifies that at least 55 percent of the total materials used in each building project (based on mass, volume, or cost) must be any combination of used, recycled-content, or recyclable materials, or bio-based materials, where the bio-based content is not less than 75 percent and where wood materials are environmentally certified.

▶ ASHRAE 189.1 contains a similar requirement, specifying that at least 45 percent of materials must be low-impact materials, with low impact defined as recycled content, regional, or bio-based materials; bio-based materials are required to comprise a minimum of 5 percent of the total cost of materials.

▶ CALGreen awards voluntary credits for the use of bio-based materials.

All of these initiatives emphasize use of rapidly renewable materials, defined as materials that renew in 10 years or less, rather than 11 years or more (i.e., they favor materials other than wood), although they also reward the use of certified wood. None of these programs require comprehensive environmental certification of rapidly renewable materials or of any construction material other than wood.

New Developments in Green Building Rating Systems & Codes

The following developments within major green rating systems demonstrate the shift toward LCA-based tools and data.

LEED v.4. In the Materials and Resources category of LEED v.4 (2013), prescriptive measures that were part of the previous version of the system—for material reuse, recycled content, and rapidly renewable materials—have been replaced with optional credits related to LCA, LCA-based environmental product declarations (EPDs), material ingredient verification, and raw material extraction (see sidebar on page 3). EPDs need only be collected to gain credit; there is no requirement that they be understood or acted upon, though such requirements will presumably appear in a subsequent version of LEED. Prescriptive elements also remain.

According to Dr. Jim Bowyer, director of the Responsible Materials Program at Dovetail Partners, “The two rating systems that have long incorporated systematic assessment into their programs—BREEAM and Green Globes—have more robust LCA provisions.”

Green Globes v.1.3. The newest version of Green Globes (version 1.3, 2014) offers two paths to satisfying material selection requirements. One option is to conduct LCAs in the conceptual design phase of at least two building designs (core and shell including envelope), with selection of the lowest impact option. Alternatively, EPDs that comply with standards put forth by the International Organization for Standardization (ISO), third-party certifications to multi-attribute consensus-based standards, and/or third-party-certified, ISO-compliant life cycle product analyses focused on appropriate characteristics for the building system or application must be used as a basis for selection of specified products.

BREEAM. Within the Materials section of BREEAM, credits are awarded on the basis of a building's quantified environmental life cycle impact through assessment of the main building elements—i.e., exterior walls, windows, roof, upper floors, internal walls, and floor coverings and finish. Impacts can be quantified either through use of an ISO-compliant LCA tool (wherein building designers must demonstrate that they know how to use the LCA tool and document how the building design has benefitted from its use), or through selection of building components based on either an LCA-based Green Guide developed and maintained by BRE, and/or ISO-compliant EPDs. Life cycle greenhouse gas emissions (in kilograms of carbon dioxide, or CO₂ equivalent) for each element must also be reported based on a 60-year building life.

The shift toward performance-based assessment is also reflected in ASHRAE 189.1, the IgCC, and CALGreen.

The ASHRAE guidelines provide alternative prescriptive and performance pathways. The performance option requires that LCAs be conducted for a minimum of two building design alternatives. Assessment must demonstrate at least a 5 percent improvement in at least two categories, including land use (or habitat alteration), resource use, climate change, ozone depletion potential, human health effects, ecotoxicity, eutrophication, acidification, or smog. Completion of an LCA eliminates the need to adhere to prescriptive low-impact material requirements outlined earlier.

Similarly, the IgCC guidelines also offer the option to pursue either a prescriptive or performance path. Here, choice of the performance pathway requires a whole building LCA and demonstration that a given project achieves not less than a 20 percent improvement in environmental performance as compared to a reference design of similar usable floor area, function, and configuration that meets the minimum energy requirements of IgCC and structural requirements of the International Building Code. Environmental performance improvement is required in global warming potential and at least two of the following impact measures: primary energy use, acidification potential, eutrophication potential, ozone depletion potential, and smog potential. As in the ASHRAE program, fulfillment of this requirement eliminates the need to document adherence to a number of prescriptive elements related to material selection. CALGreen contains a similar provision.

Life Cycle Assessment: Getting to a Material’s Real Green Quotient

Life cycle assessment is sometimes described as mysterious and complicated. Yet, what is involved is simply a thorough accounting of resource consumption, including energy, and emissions and wastes associated with production and use of a product. For a “product” as complex as a building, this means tracking and adding up inputs and outputs for all assemblies and subassemblies—every framing member, panel, fastener, finish material, coating, and so on. Further, to ensure that results and data developed by different LCA practitioners and in different countries are comparable (i.e., that results allow apple-to-apple comparisons), LCA practitioners must strictly adhere to a set of international guidelines set forth by the International Organization for Standardization.

Tracking products and co-products through a supply chain and properly allocating resource use, emissions, and wastes to various outputs can indeed be complicated and expensive. However, a growing number of LCA tools have made LCA a viable option for any designer. User-friendly, low-cost (in most cases free) tools, such as the Athena Impact Estimator for Buildings (IE), provide life cycle impact information for an extensive range of generic building assemblies, or designers can choose to undertake full building analyses. LCA-based data is also available in the form of standardized, easy-to-understand EPDs for a wide range of products.

The wood industry has been an early adopter of EPDs, undertaking research and developing life cycle information that verifies the environmental impact of wood building products. EPDs on wood products are available from the American Wood Council (www.awc.org) along with transparency briefs summarizing the most critical data presented in each. (For more information on EPDs, see the continuing education course Wood and Environmental Product Declarations.⁴)

Increased use of LCA in the evaluation of building design alternatives and material selection greatly favors wood in all types of construction, since environmental impacts across a large spectrum of indicators tend to be significantly lower for wood products than alternative materials.⁵ Scientific comparisons of functionally equivalent buildings, components, and subassemblies have been remarkably consistent in this regard, with wood almost invariably found to be the low-impact option.

As an example of the environmental performance of wood structures in comparison to those constructed of other materials, a highly regarded and commonly used LCA tool, the Athena EcoCalculator, was used to evaluate three alternative configurations of a simple building. Designed for the Atlanta geographical area, the building footprint was 20,000 ft² (100 ft x 200 ft). Two stories in height, the structure was 20 feet tall with 40,000 ft² of total floor area. To simplify analysis and comparison of materials in particular, the theoretical building was analyzed without windows, doors, or internal partitions. Of the three configurations, one was wood, one steel, and one concrete. All were assumed built on a concrete foundation and slab.

This analysis involved systematic assessment using life cycle methodology of all building assemblies beginning with raw material extraction through primary and secondary manufacturing, transport at all stages of the production chain and to the job site, and building construction. Differences among the wood, steel, and concrete structures are shown in the table above.

Impacts associated with the steel design as compared to the wood design are mostly 1.3 to 1.6 times greater, with a range of 1.02 to 3.0 times. Lower impacts are indicated for the wood design in every impact category. Comparison of the concrete vs. wood design shows even greater differences. In this case, environmental impacts associated with the concrete design range from 1.9 to 5.8 times greater than for the wood design. Again, impacts across all indicators are lower for the wood design. The impact categories in the tables closely match those specified in rating tools and code language where use of LCA is encouraged or rewarded.

One reality consistently revealed by LCA is the low embodied energy in wood structures compared to those built of steel or concrete.⁶ The term 'embodied energy' refers to the total consumption of energy linked to production of a building, including resource extraction, manufacturing, transport, and installation of building materials. According to Dr. Bowyer, “The embodied energy of wood assemblies has consistently been found to be 20 to 70 percent lower than functionally equivalent steel or concrete assemblies.”

Comparison of the environmental impact measures linked to material selection obtained through LCA (tables on page 6) and material selection factors typically considered in prescriptive-based rating systems (page 2) reveals a startling reality. None of the prescriptive-based rating systems reward superior performance, or even consider in building material selection, such things as fossil fuel consumption, global warming potential, total resource use, non-renewable resource consumption, acidification or eutrophication potential, ozone depletion, smog potential, or water use. The reason is that any rating system that does not incorporate LCA or LCA-based tools and information does not have the capacity to consider these things. The same is true of prescriptive pathways within systems that do incorporate LCA and LCA-based tools and information, but do not require their use.

Wood and Carbon

Although LCA recognizes products associated with low CO₂ emissions, long-term carbon storage is not one of the metrics measured. Wood performs well on both counts—but its benefits are most evident when the forest/wood cycle is viewed as a whole.

In the process of photosynthesis, trees absorb carbon dioxide from the atmosphere, release the oxygen and incorporate the carbon into their trunks, branches, leaves and root systems. Trees that decompose and die in the forest release this carbon back into the atmosphere slowly, and it is released more quickly in forests that succumb to insects, disease or wildfire. However, if the trees are harvested and manufactured into lumber and other forest products, these products continue to store carbon while the forest regenerates and once again begins absorbing CO₂. In the case of buildings, this carbon is stored for the lifetime of the structure—or longer, since wood also lends itself to adaptation, salvage and re-use. Wood can also be used as a low-carbon substitute for fossil energy.

The second aspect to wood's relatively light carbon footprint is that it grows naturally and requires comparatively little additional energy to manufacture into products. This gives wood an environmental advantage over construction materials such as steel, cement and glass, the production of which requires temperatures of up to 3,500° F and large quantities of energy, resulting in substantial greenhouse gas emissions.⁷

Taking advantage of wood's carbon and other environmental benefits in building construction comes with a bonus—namely that, in many cases, wood construction is less expensive than other building solutions. For example, one high school in Arkansas documented in a case study saved $2.7 million by changing the design of its new school from steel and masonry to wood—while achieving a carbon benefit of 11,440 metric tons of CO₂.⁸

New Materials Create New Possibilities

The emergence of mass timber products such as cross laminated timber (CLT) is allowing designers to create a broader range of lower-impact structures. Internationally, for example, CLT's relatively light carbon footprint is helping to drive a trend toward taller buildings, such as the eight-story Bridport House in the United Kingdom and the 10-story Forté in Australia.

Although relatively new in North America, CLT has been used in a variety of building designs, from the LEED Gold-certified Earth Sciences Building at the University of British Columbia to the new Fort McMurray Airport, where designers have taken a “first principles” approach to sustainability, blending best practice with the monitoring approaches of various green building rating systems. In the U.S., examples include the The Crossroads, a 52,000-square-foot staff and visitor facility at the LEED Gold-certified Promega Feynman Center in Wisconsin, and a two-story school in West Virginia.

What the Future Holds

With growing concerns over climate change and the environmental impact of buildings, it stands to reason that green building concepts will be increasingly incorporated into structures of all kinds. What began as an interest in reducing energy consumption to save money in the 1970s has led to today's net zero energy objectives, and net zero carbon is another frontier. With attention turning away from the prescriptive approach to sustainable design and toward LCA-based tools that identify the lowest impact alternatives, more designers will become familiar with the environmental advantages of wood, and wood products will be a building material of choice for a growing range of applications.

For more information on the themes described in this article, download the Green Building with Wood Toolkit at rethinkwood.com.

The reThink Wood initiative is a coalition of interests representing North America's wood products industry and related stakeholders. The coalition shares a passion for wood products and the forests they come from. Innovative new technologies and building systems have enabled longer wood spans, taller walls and higher buildings, and continue to expand the possibilities for wood use in construction. www.rethinkwood.com