Seattle’s Bullitt Center has been called the “greenest commercial building in the world,” and for good reason. The facility includes several cutting-edge sustainability features including a high-performance building envelope, radiant floor heating with passive cooling and natural ventilation, a Net-Zero water approach, and even a closed-loop geothermal system.
One of its most important sustainability features, however, is a bit less high-tech but equally effective: The ubiquitous use of wood in its construction.
Designed by the Miller Hull Partnership in 2013, the six-storey, 52,000 sq. ft. building features four storeys of Type IV heavy timber framing built on two storeys of reinforced concrete. It also includes Douglas fir glulam beams, a timber floor deck made of 2×6 lumber, and a timber roof deck comprised of 2×4 lumber. It all adds up to nearly 400,000 board feet of wood within the structure, equal to around 545 metric tons of carbon stored on-site within the wood itself (something we’ll discuss further below), and 1,703 metric tons of potential CO2 emissions saved – enough to power a home for 145 years.
The building momentum of mass timber
The Bullitt Center is just one example of wood’s growing momentum in the building sector, largely driven by sustainability concerns as well as evolving government policy favouring the material in public and other building projects. Buildings are huge consumers of energy, accounting for 76 percent of all electricity produced and 40 percent of all carbon emissions in the U.S. These numbers account not just for operating energy, but also for the energy required to produce the materials of which they’re made – most notably concrete and steel, with concrete responsible for eight percent of the world’s carbon emissions in 2018.
The silver lining, according to the American Wood Council, is that 75 percent of U.S. building stock “will be either new or renovated” between 2010 and 2035 – providing a unique opportunity for builders, architects and specifiers to use more carbon-friendly materials such as wood in new building construction or renovations.
Part of wood’s growing momentum is the popularity of mass timber, a type of wood product manufactured into large pieces that easily fit together and provide massive performance boosts compared to traditional wood beams. Mass timber comes in several variants including glue-laminated (glulam), cross-laminated timber (CLT), laminated veneer lumber (LVL), nail-laminated timber (NLT), and dowel-laminated timber (DLT). Mass timber can meet or even exceed the performance of concrete and steel, and there’s even an 80-storey mass timber tower proposed in Chicago.
Legislation in various countries has also encouraged the use of wood such as heavy timber or mass timber in various building projects, including:
- France, where new public buildings must have at least 0.2 cubic metres of wood for every square metre of floor
- New Zealand, where wood products must be considered as the main structural material in new government-funded buildings up to four storeys tall
- Japan, where wood products must be considered as the main building material for government-funded projects up to three storeys
- Canada, where the provincial governments of Ontario, B.C. and Quebec encourage wood’s use in public buildings
But recommending wood as a building material doesn’t just make for good public policy. It also provides significant environmental and other benefits for building owners and managers.
The benefits of wood as a building material
The benefits associated with wood as a building material are varied. Lifecycle assessments (LCAs), the internationally recognized and standardized method of evaluating a product’s environmental impact (from extraction, to production, to use, to final disposal) has proven that wood materials are tied to significantly lower emissions than similar non-wood products. Wood also serves as a supremely efficient carbon storage vehicle, thanks to photosynthesis – one research paper by the USDA’s Forest Products Laboratory concluded that a 2,062 sq. ft. wood home could store 10,500 pounds of carbon or sequester 38,500 pounds of CO2. And Forest research center FPInnovations estimates that one 100,000 sq. ft. wooden building is equal to taking 1,400 cars off the road each year.
“The reduced carbon emission impacts associated with woody biofuel use and storage of carbon in long-lived wood products result in lower net carbon emissions of wood products compared with non-wood product alternatives,” the report read. “For the cases we evaluated, the combined GHG emissions reductions due to biofuel usage, carbon storage, and avoided fossil emissions are always greater than the wood product manufacturing carbon emissions.”
Indeed, the benefits of wood as a building material are varied and widespread, and are even recognized by the UN’s Intergovernmental Panel on Climate Change (IPCC), including:
1) Lower carbon footprint: Wood requires less energy to extract, process, and transport, while buildings made of wood typically need less energy to build and maintain.
2) High performance: Wood, especially mass timber and other types of wood-related materials, are lightweight yet incredibly strong and versatile, easy to work with, and have excellent load-bearing capacity.
3) Better thermal efficiency: The cellular structure of wood (which includes many very small air pockets) is tailor-made for thermal efficiency, and is 400x better than steel and 10x better than concrete at resisting heat. It also requires less insulation than other materials.
4) Carbon storage: Trees intake CO2 and release oxygen during photosynthesis, storing that carbon in the wood indefinitely (until the wood burns or decomposes). Wood used in construction essentially becomes a carbon storage vehicle.
5) Encouragement of carbon sinks: Selective harvesting of wood products from old-growth and other forests help remove decomposing and rotting wood, which emit carbon. A poorly managed forest can even be a net carbon emitter – and is much more at-risk for large carbon release events such as forest fires – while a well-maintained forest is usually considered a carbon sink, which is a forest that removes more carbon from the atmosphere than it emits.
6) Recycling and reuse: Forest products can more easily be recycled, reused, or recovered for energy than most non-wood building materials.
Indeed, many experts see the increasing use of wood as a way to significantly enhance the capacity for carbon storage within the built environment. And, as mentioned, we’re already seeing tremendous momentum in this direction, with the world’s tallest timber tower – 85.4 metres tall – constructed in Norway in 2019.