The sustainable building sector is looking to the past for ideas on how to decarbonize materials the industry relies on to create buildings and homes. An old player that is becoming increasingly key? Wood.

While wood sequesters carbon and is touted as a sustainable building product, there are problems associated with some of the wood products being used for taller buildings now going up in cities worldwide.

In Vancouver, there are two timber highrises, and countries like Sweden, Norway and Austria have similarly built towering structures with wood.

However, many wood products are coated with toxic chemicals that make them difficult to recycle at the end of their life, which is a problem, said Naomi Keena, an assistant professor at McGill University’s Peter Guo-hua Fu School of Architecture. There are promising natural alternatives popping up that she says need more attention.

Steel, cement and plastic are just a few of the carbon-intensive materials used to construct everything from small homes to towering skyscrapers. On the other hand, wood sequesters carbon and architects are using it to build higher than ever before with the material.

The wood used to create buildings is often cross-laminated timber and glue-laminated timber. Unlike raw wood slabs, they are made by layering small sheets of wood with glue or by shredding up wood and mixing it with a binder to create a solid piece.

The glue used is synthetic, while wood is a natural fibre, and Keena said that’s a combination that should be avoided. In a circular economy (where materials are reused, repaired and recycled), natural fibres fit into what’s known as the biological cycle and synthetics fit into the technical cycle, and the two don’t overlap.

“A simple example … with food, we have composting because we know it can biodegrade because it's a biological product. With synthetic resins, they're part of the technical cycle,” she explained, adding there are proven recycling processes for materials like steel, but not when it's mixed with other materials.

Around 11 per cent of global emissions come from the built sector of the building industry, rather than emissions from operations, which includes the production of building materials such as cement, along with the construction process. As timber becomes an increasingly attractive choice for the building sector to cut back on its emissions, it’s a pivotal time to consider how to make the product recyclable at the end of its life, said Keena. If not, the use of new composite wood products will lead to significant waste in the future.

While wood sequesters carbon and is touted as a sustainable building product, there are problems associated with some of the wood products being used for taller buildings now going up in cities worldwide. Toxic resin coats many of these products.

“People don't think about 50 years time, but if we keep building the way we're building now … not thinking about the end of life, then … we're going to have the same issues in 50 years time,” she said.

“...As we develop these new products that are being lauded as green … we have to think about how they can have a long-term, sustainable life cycle.”

Sustainable alternatives

There are ways to create strong wood products that don’t rely on synthetic resins, explains Keena. One way is by using dowel-laminated timber, where wood is joined together with dowels. Another option is to use natural resins, which fit into the biological cycle with wood.

Agricultural waste, algae, coconut husks, mycelium (which consists of fungal threads) and more all fit the bill, explains Mae-ling Lokko, assistant professor at the Yale Center for Ecosystems and Architecture. All of those materials contain lignin, a naturally occurring binder in plants.

Lokko has studied a range of alternatives at Yale and is also the founder of Ghana-based Willow Technologies, a startup that turns agricultural waste into bio-based building materials. A company called Ecovative, a US-based company with facilities in Europe, has developed a mycelium strain that is used by the building industry.

Lokko said mycelium is an especially exciting alternative because there are “over 10 million strains and each of them has adapted to eat very specific things where it grows.” Because mycelium works as a binder by eating sugar components to form bonds, using local varieties with local wood will make the product more efficient.

As of now, bio-based binders are expensive and lack diversity, said Lokko. There is only one mycelium binder on the market and it isn’t ideal in all climatic conditions, she said.

Mae-ling Lokko is an assistant professor at the Yale Center for Ecosystems and Architecture. Photo by Shannon Straney

“The mycelium composite in Ghana, where it's hot and humid, gets moldy much quicker because it just wasn't trained to do that,” she explained.

Bio-binders shouldn’t be grown in one country and exported on a massive scale because that would put too much pressure on one crop in one area on top of the issues that come with it being used in an area it’s not endemic to. Instead, Lokko said, countries should be finding ways to localize the production of sustainable building materials combined with sustainable forestry practices.

The manufacturing of the formaldehyde-based chemicals used in mainstream composite wood materials can also cause health problems. Once disposed of, they can contaminate soil and waterways.

Canada is in an especially good position to lead the “bio-based revolution,” said Keena, who along with Lokko, was an author of a September report from the United Nations Environment Program and the Yale Center for Ecosystems and Architecture that said governments across the world need to decarbonize building materials for the sector to reach net-zero emissions by 2050.

“There are already a lot of industries creating mass timber in Canada … there's also this huge opportunity in agricultural waste … because there are lots of crops that are used in agriculture in Canada that are suitable for building products.”

Updates and corrections

| Corrections policy
December 19, 2023, 12:00 pm

This article has been updated to clarify that Ecovative is a US-based company with facilities in Europe.

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Two things.

First, building tall with wood is great, but I have yet to see any in the commentariat include, in their touting, the observation that the world's forests are already in crisis, and present a way that adding another stressor -- substituting reinforced concrete and steel -- with wood -- will be beneficial for forest habitat and biodiversity.

Second, if architects were truly interested in sustainable design using wood, they would find agreement on ways to standardize building elements, such as beams, that can be reused -- i.e. Lego -- when a building is determined redundant and is "consciously disassembled".

Q: How much recycling of old wood us done, presently, when an excavator rips down a SFH in an hour to redevelop the land?

It's one thing to build a detached home with wood products, but I have an issue with large multi-family dwelling built exclusively with wood. Despite fire separations and other measures to slow the spread of fire, it just takes one careless home owner to burn out everyone else in the structure. I have seen these enough times to where I would never consider buying a unit in a multi-family structure.

The toxic nature of the material used preserves the material from rotting and also acts as a fire retardant.

I am sure there are ways to recycle most materials, despite the toxic nature of agents used to bond the material, especially if used to build other structures that contain the same materials.

Another way to prolong the sequestering of carbon in wooden structures is to avoid the bulldoze and rebuild cycle common in developed countries, especially North America. As in many European cities, recycle the [wood] building in-place via retrofit. This preserves the streetscape, the building envelope and the floor partitions - areas where there is most intensive use of structural wood laminates. Additionally, new wood buildings will be designed with energy conservation in mind and tall buildings are designed with service distribution columns that also favour re-work with minimum waste.
BTW laminate beams do come in standard widths and lengths and, at least in Vancouver, you can buy recycled beams.

I would have loved to have read comments in this article from a structural engineer with expertise in materials and toxins. Vancouver architect Michael Green has loads of experience with mass timber buildings, and has a 23-storey student housing tower at UBC using mass timber in his portfolio, among other projects. He is likely able to quote all sorts of data on carbon sequestration, carbon savings and the toxic content of some glues and binders.

Timber and resin-wood composites are incongruently mixed together in one paragraph above. They are not the same and their differences really need an explanation. Glue-laminated beams and cross-laminated timber columns and slabs are nearly completely made of wood. The glue is a very thin layer sandwiched between members, and highly compressed. The wood-glue ratio is something like 100:1. Glue-lam posts and beams are ultimately recyclable if they are still in good condition after a century of protection in an enclosed, heated and dry building. That is not unlike large dimension Douglas fir timbers from 100-year old sawmills and warehouses becoming a salvage treasure and recycled for another 100 years into new structures as focal architectural features unto themselves. The same recycling principle applies to glue-laminated posts and beams and cross-laminated columns and slabs, items only the very ignorant would not save / sell for future use and instead bins them for the dump.

Joining mass timber beams, posts and slabs is done with steel fasteners and plates. Yes, one can use wooden dowels in low rise buildings (love those Japanese and Korean hand-built houses, especially when they form a three-sided courtyard!) where the seismic stresses are not nearly as strong as for taller buildings. But steel is far, far superior in strength. Steel is permanently recyclable. The problem is with the energy used to melt scrap metal. Canada is just now moving to electric arc furnaces (electrolysis is another possible method) from coal-fired blast furnaces. We need green steel plants as part of a domestic climate strategy.

Composites contain a wood-resin ratio that hovers around 5:1, or even as low as 1:1. Oriented strand board (OSB) is essentially waste wood chips pressed together with glue into standard panels as a substitute for the more expensive but structurally superior plywood. OSB looses structure if the wood component rots out when saturated on a wet construction site or if exposed to dampness inside walls that do not adhere to modern rainscreen ventilation specifications. Plywood is far better in all considerations in my view, including recycling into new structures.

The Parallam brand product is mainly liquid resin poured into a pressurized beam mold with wood strands added. However, they too can be salvaged and repurposed as internal load-bearing lintels over large openings in walls. I can't imagine anyone trying to compost or throw out a perfectly good major structural component made from Parallam. However, I think they are ugly. I much prefer exposed glu-lam beams made from Douglas fir or pine and lightly rubbed with linseed oil or Danish oil. Beautiful!

There are glues and resins with lower levels of volatile organic compounds (VOCs), just as there is low VOC water-based paints. All of these chemistries need to be explored more fully for toxicity. Also think about it in the context of mass timber members usually not ending up in compost piles or the dump when they are orders-of-magnitude more valuable when reused. These are not the rotted out 2x4s in poorly maintained older houses, usually in the lower wall areas where a century of rain splash back became a serious structural problem. Having said that, Douglas fir studs from old houses that were dismantled at a far higher cost than demolition / dumping are often recycled into furniture.

Regarding fire, large mass timbers do not burn through. The surface gets charred, but the internal lignin 15mm below the surface is still intact. Timber buildings do require vigilance with respect to moisture penetration and long-term damp rot, though they can take the occasional flood from a plumbing failure as long as they dry out quickly. Most mass timber towers hav thick cross-laminated floor slabs with membranes and a concrete levelling course poured over it. Sound and fire-deadening are thus incorporated.

The mass timber architecture and building industry uses primarily second class wood grown in dedicated plantations, and keeps demand for freshly cut old growth timber out of the domestic market. The same cannot be said about the rip 'n ship method of exporting huge old growth logs overseas, a practice that continues despite decades of criticism and protest.

A problem not mentioned in this article is that wood comes from trees and trees capture carbon and store it throughout their lifetimes. Once trees are felled they cannot capture any more carbon plus about 60% of carbon that was stored in a forest (in the trees, undergrowth, soil etc) is released during logging and does not get sequestered at all in the eventual wood products made. In comparing the "green-ness" of different building materials, the loss of future carbon capture and storage and the loss of carbon during the logging process need to be taken into account.

Exactly. A living tree continues to capture and store carbon -- and importantly for us, release oxygen.

We have a drastic shortfall of trees in Canada. I'm unaware of any country whose forests are at this point taking up so much CO2 that they are compromising their own air ... quite unlike what humans are doing.

It seems to me that NO old-growth "mother" trees should be cut at all, no forests of any sort should be clearcut and burned off, and wood chips should be made only with the wood from slash heaps.

It seems to me that we need to be growing forests that are sustainable in terms of sustaining the local ecologies, and their own longevity, as opposed to thinking of "sustainable" logging. Right now, in Canada, there is no such animal as "sustainable" harvest of trees. There aren't enough growing right now. Leave'em be, instead of making a brand new market for forest destruction.

As for the synthetic glues and resins used in building materials, they are virtually all toxic, and they offgas for a long time.

"...the loss of future carbon capture and storage and the loss of carbon during the logging process need to be taken into account."

Amen to that. Industrial logging in this country needs to change. There are methods to maintain a working forest that don't resort to clearcutting, which is the main culprit that forces carbon and moisture to be released from the soil, and that damages soil structure. The soil is just as important as the trees that grow in it. The mycorrhizae fungi in the soil is densely connected to tree roots and transports nutrients and water to and among the plants, and in fact make a forest one gigantic conjoined root system. Soil is not well understood -- or is ignored -- in most government and corporate forestry management strategies that are concerned primarily on cash trees. Money does grow on trees, until the trees are gone.

Selection logging is an important answer, as is understanding the diverse symbiotic relations between tree species and with herbaceous plants enough to plant mixed species with beneficial properties, not necessary all cash crops. Site reclamation projects are also very important and can utilize the nutrient-fixing and erosion control capabilities of several pioneering species to "repair" damaged sites (e.g. red alder in BC). Selective thinning can be practiced for centuries on forest lots while maintaining a continuous tree canopy cover.

And, of course, all logging in old growth forests, unique ecosystems, conservation areas and in stream bank riparian areas must stop. If that means a net loss of jobs, so be it. You cannot base a forest economy on rip n' ship and unsustainable forest practices without running headlong into a job fall down when massive clearcut sites have been damaged beyond repair. It's far better to stabilize a local forest economy with fewer, but permanent jobs with widespread sustainable forest practices.

Further, climate change requires a new attitude about forestry, one that focuses on learning more about ecology and bringing that knowledge into the forest. A warming planet will affect the forest, and it's possible to anticipate which species will survive better. West Coast cedars are already giving way to firs and pines. A major effort needs to be taken as part of a national climate action plan to research and field test forest plots with various compositions, and have plots without continuous canopy cover to compare them to. The ancient Indigenous practice of controlled burns is also a valuable part of historical wisdom to learn from.

Maintaining a continuous cover was mentioned above, and one important attribute of it is the permanently shaded forest floor and the maintenance of relatively undisturbed soils. This was an important part of Dr. Suzanne Simard's book, 'The Mother Tree' which resulted from decades of deep diving into traditional forestry practices and what the ecology tells us. She didn't negate forestry; she instead promoted a powerful science-based argument to change it by using nature as a guide.