Last Updated: Jul 16, 2021
Wood is widely used for outdoor purposes in North American home construction — for decking, siding, and fencing, to mention just a few uses.
What is sustainable (or not) about using wood outdoors? Lumber comes from trees—a natural, renewable product considered carbon-neutral because trees absorb carbon dioxide from the atmosphere before eventually decomposing and releasing that CO2 back into the air. The problem is that wood can expand, contract, and decay—making it a less durable and potentially unhealthy building material. So, the home building industry's solution to make it last longer has been to treat lumber chemically. But that solution has led to health concerns, and so new methods of treating wood are coming to light. Before telling you what you need to know about thermally treated wood, known as thermally modified wood, let's review the conventional option problems.
Thermally modified wood is a natural, non-toxic alternative to chemically treated wood products for the home. Unlike pressure-treated lumber, thermally modified wood undergoes a thermal modification process. The wood is essentially "cooked" in oxygen-free ovens at temperatures over 400 degrees Fahrenheit.
The thermal modification process changes the cellular and molecular makeup of wood by removing all organic compounds from the wood cells. So, wood products will no longer be able to absorb water. In turn, this reduces the wood's ability to expand or contract, which is one of the leading causes of water absorption and rot in outdoor uses. This lack of moisture content in the wood means that the wood's cellulose will no longer act as a "food source" for fungi or insects. In essence, the cooking of the wood with high heat creates a naturally durable wood product that will, in theory, be permanently resistant to water, fungus, insects, and decay.
From an aesthetic standpoint, thermally modified wood offers some unique benefits and characteristics. The process of treating the wood at a high temperature gives it a natural deep brown hue. This beautiful color negates the need for chemical varnishes or stains, most of which off-gas various volatile organic compounds (VOCs) and require ongoing maintenance. Many homeowners find that the natural dark brown color resembles tropical hardwoods. The use of a sealant to protect the wood from UV rays is often all that is needed to maintain this beautiful natural color.
Thermally modified wood is different from other types of preservation techniques that require burning the wood. The Japanese method of Shou Sugi Ban, for example, chars the wood and makes for beautiful charred siding.
The bending strength of the wood is reduced during the heating process. So, thermally modified wood is not suitable for structural applications. It is commonly used for non-load-bearing applications such as decking, siding, flooring, wall paneling, ceilings, accent walls, and even doors (interior and exterior)!
Much of the lumber used in the building industry today is treated with a wide array of chemicals. These wood preservatives used include:
These chemicals help extend the lifespan and durability of wood by protecting it from bacteria, fungi, and other pests that can cause wood to rot and decompose. However, dozens of health and environmental problems are associated with the use of these chemicals and pesticides.
The Center for Disease Control (CDC) finds that chromated copper arsenate (CCA) can contaminate local soils through leaching. Young children are at a higher risk for consuming CCA, which can accumulate in soils around decks and other outdoor wood structures that have been treated with this product. Arsenic found in CCA has been found to increase the risk of cancer. This risk is incredibly high in children who may be exposed to chemically treated wood in outdoor settings.
In addition, for exterior applications where pressure-treated lumber comes into contact with metal, it can corrode the metal. When working with pressure-treated lumber for exterior projects, be sure to use fasteners that are "hot-dipped" and meet the ASTM A653 standard to ensure that they will last as long as the structure.
One of the main benefits of thermally modified wood products is that it offers a natural and non-toxic way to preserve wood products without using unhealthy pesticides and fungicides.
As the cooking process removes moisture from the wood, the timber is much more lightweight than regular lumber, making it easier to work with from an installation standpoint. Because it no longer has any water or moisture, the wood is much less susceptible to warping or chipping, making it much more structurally sound. From a durability perspective, thermally modified wood has maximum weather resistance properties. Thermally modified decking can be guaranteed for up to 25 years.
Another added benefit is that you can use a wider variety of tree species. This versatility allows homeowners to make conscientious decisions about sourcing their lumber from local sources. In many cases, fast-growing local forest stands can be thermally modified and subsequently used for lumber for almost every home construction project.
While there are dozens of established companies in Europe producing high-quality thermally modified wood, the carbon emissions associated with shipping lumber across the Atlantic increase the embodied energy of your home. Fortunately, many manufacturers are now producing this type of wood across North America. Below, we review a few of the top manufacturers of thermally modified wood.
This New Brunswick-based company is relatively close to the Rise headquarters and has produced Thermalwood for the Canadian market for several years. They offer a wide array of thermally modified wood products, including wood destined for exterior use, such as decking, siding, and outdoor furniture. In addition, they provide interior wood products for flooring, indoor furniture, and even specialty woods for musical instruments.
Americana by Bingaman is a family-run company from the Appalachian mountains of Pennsylvania. They have been making thermally modified wood products for over a decade for siding, decking, and porch flooring. Americana is passionate about helping homeowners move away from human-made plastics and move toward natural, renewable resources for outdoor living.
This company ships its products from Europe for sale in the US. While this does entail a higher carbon footprint, you will be getting thermally modified lumber products produced from companies with years of experience. Thermory specializes in decking, cladding, and porch flooring with unmatched longevity and lasting natural beauty. It is clear from the company's website that it takes sustainable forestry seriously, as they carefully select and source the raw materials from renewable northern forests.
Arbor Wood Co. sources its raw material from sustainably harvested US forests before undergoing the thermal modification process. The result is high-quality, performance-driven timber, which sustains the natural beauty and design element of wood. Arbor Wood Co. offers thermally modified wood products for decking, siding, and flooring. This Minnesota-based company has even installed a boardwalk through a natural wetland made from its thermally modified decking boards to show its products' durability.
Thermally modified wood has been a popular option in Europe for decades. As it makes inroads in the North American market, it's worth checking out for your next home building project.
Tobias runs an agroecology farm and a natural building collective in the mountains of El Salvador. He specializes in earthen construction methods and uses permaculture design methods to integrate structures into the sustainability of the landscape.