This course is part of the Mass Timber Academy

Building Sustainably Using Wood

When designing to meet green buildings standards, wood adds value on a number of levels. Wood is the only major building material that is renewable. As trees grow, they release oxygen and absorb CO₂. And when those trees are harvested, the carbon is sequestered and held in the wood for as long as the wood remains in use.

When wood is used for the structure of a building, it reduces the carbon footprint in two ways – through carbon storage and through avoided greenhouse gas emissions, since wood products use less energy from fossil fuels in their manufacture than other materials. In fact, wood products comprise 47 percent of the industrial raw material manufactured in the U.S., yet consume only 4 percent of the energy needed for production. Every ton of wood used in place of an alternative building material potentially avoids 2.1 tons of carbon, or 7.7 tons of carbon dioxide equivalent (CO₂e).

Wood building materials also provide benefits for the ongoing operating efficiency of a building. Wood has a relatively high R-value which provides good insulating value. Wood also has lower thermal conductivity than other materials such as steel or concrete, making wood-framed buildings more efficient to insulate. Light-frame wood, mass timber, and prefabricated wood construction can all be used to create an efficient building envelope.

One of the most effective ways to improve indoor air quality is to reduce the use of materials that emit pollutants. Wood’s natural beauty can also be left exposed to a building’s interior, eliminating the need for additional interior finishes which could add VOCs. The Oregon Conservation Center in Portland, Oregon, left its wood structure exposed to the interior, creating a natural finish for tenants.

Demand for wood products provides an incentive for forest owners to keep their lands actively managed and healthy. A young, growing forest produces 1 ton of oxygen and absorbs 1.4 tons of CO₂ for every 1 ton of wood. When a forest is left unmanaged, older trees eventually stop absorbing carbon from the atmosphere and then release carbon when they decompose or die from wildfire, insect damage, or disease.

Wood Building Systems

Wood construction can support green building of many occupancy types, including multifamily, mixed-use, office, industrial, education, healthcare, and more. Three common framing types all provide green building benefits.

Light-Frame

Light-frame is the most common form of wood construction, accounting for roughly 80 percent of wood-framed construction in the U.S. Wood can be used in structural applications such as roof, floor, and wall framing, but also for other components such as doors and windows, exterior and interior finishes, trim, siding, and decking.

The two predominant types of light-frame wood construction are:

• Type III, used primarily for multifamily residential building
• Type V, which permits wood to be used for all structural elements of the building

The Orchards at Orenco project in Hillsboro, Oregon, is a good example of a light-frame wood structure that added sustainable value through its energy efficiency. Designers used 2x10 dimensional lumber to frame the walls, which gave them width to add extra insulation, reducing overall energy use of the complex by 60 to 70 percent.

Mass Timber

Mass timber framing uses large, solid wood members for walls, roofs, and floor structures. Common mass timber building components include glue-laminated (glulam) beams, cross-laminated timber (CLT), nail-laminated timber (NLT), dowel-laminated timber (DLT), and others. Mass timber fits green building goals in several ways. It replaces other more carbon-intensive structural materials with a renewable and carbon-sequestering option. Mass timber components are fabricated off-site to precise specifications, which reduces waste, speeds construction, and lowers labor costs; it also results in improved energy efficiency because the controlled fabrication process means they can build a tighter building envelope with fewer air gaps. Type IV, also known as heavy timber construction, first included CLT in 2015. The 2021 International Building Code (IBC) allows mass timber structures of 8, 12, and 18 floors. Tall wood structures make efficient use of building sites, another consideration for green building certification.

Prefabricated or Modular Wood

Use of prefabricated assemblies and full modular construction is growing because both approaches offer many benefits to green building in terms of efficient, cost-effective approaches. Both are suitable for light-frame wood and mass timber construction of multifamily, office, retail, and other types of building occupancies. A 2019 study by McKinsey & Company found that modular construction can trim costs by 20 percent and speed schedules by as much as 50 percent. Precise fabrication also improves the quality of the building envelope by improving airtightness. Plus, the controlled material procurement process ensures that certified materials are used.

How Much Carbon Are We Talking About?

The WoodWorks Carbon Estimator and Carbon Calculator use project wood volume to estimate the total wood mass in a building and the associated carbon impacts, measuring both the carbon stored in the wood building products and the greenhouse gas emissions avoided by choosing wood instead of more energy-intensive building materials.

Stella, a 244-unit multifamily housing development in Marina del Rey, California, avoided nearly 10,000 metric tons of CO₂ emissions and stored almost 4,500 metric tons of CO₂ by framing with wood. This equaled the carbon equivalent of taking more than 2,600 cars off the road for one year.

Green Building Codes

When building codes initially began focusing on green building, they were concentrated primarily on improving energy efficiency. Now, building codes reflect a broader global movement to mitigate the impacts of the built environment by promoting material resource efficiency, waste reduction, indoor air quality, site selection and utilization, and more. Green building codes are designed to take a project beyond the minimum requirements in terms of energy efficiency and sustainable features. While many jurisdictions have their own requirements for sustainable building, the IgCC and CALGreen are among the most influential.

The Orchards At Orenco Case Study

Location: Hillsboro, Oregon; Architect: Ankrom Moisan Architects, Inc.

This three-story, 57-unit, wood-framed, multifamily, affordable living project is certified Passive House, meeting stringent energy efficiency requirements. In fact, at the time it was completed, The Orchards was the largest certified multifamily Passive House project in North America. Wood framing helped the project achieve its aggressive energy efficiency goals; 2x10 wall framing allowed designers to superinsulate the walls, adding blown-in fiberglass insulation to the stud cavities as well as 1½ inches of rigid mineral wool insulation to the exterior. By taping the plywood sheathing to reduce air gaps, the project team created a tight air barrier. At the roof, exterior walls were framed to the bottom of the roof sheathing so that the air barrier and insulation could continue up and over the light-frame wood trusses, which were ledgered to the inside face of the exterior wall. Designers also relied on wood’s inherent insulating properties, incorporating details to encapsulate fasteners that would have otherwise created thermal bridges. Overall, through design choices, the expected energy use from heating was reduced by 90 percent, and overall energy use was reduced by 60 to 70 percent, compared to that of a similar building.