As architects and designers across North America seek materials that balance performance, aesthetics, and sustainability, thermally modified wood is emerging as a convincing option.
Wood remains a top choice for building in North America thanks to its natural look, versatility, and renewability. However, traditional wood can warp, rot, or require frequent upkeep, especially in harsh climates. Thermally modified wood (TMW) addresses these issues by using heat and steam to boost durability and stability, all without chemicals.
To explore why TMW is gaining ground in North America, we turn to the expertise of Simo Kurki and Tomi Vainikka from Jartek, whose work with wood producers across the U.S. and Canada offers a valuable insight into this growing trend.
So, whether you’re designing a residential deck in Oregon, a commercial façade in Toronto, or a public space in Austin, here are five reasons why TMW deserves a place in your next building project.
1. Dimensional stability
TMW expands and contracts up to 60% less than regular, untreated wood. That means it’s much more stable when the weather changes, and less likely to warp, crack, or shift. Untreated wood can swell or shrink noticeably with moisture, which can lead to problems over time.
“TMW’s stability comes from a special process that uses just heat and steam (no chemicals) to change the wood’s structure. This makes it absorb far less water and keeps it in shape, even in places with extreme humidity or dryness, like the rainy Pacific Northwest or the arid Southwest,” says Tomi Vainikka, Quality and Development Manager, at Jartek. “It’s a smart choice for things like cladding, decking, and window frames, where long-term durability really matters,” he adds.
Image Courtesy of Arbor Wood Co.
2. Resistance to decay and pests
TMW’s durability is about biological resilience. “The high-heat, oxygen-deprived process alters the wood’s cellular structure, converting natural sugars and into acids into compounds that are no longer a food source for fungi, mold, or insects. The outcome is a wood that is naturally resistant to rot, decay, and infestation,” explains Simo Kurki, Sales Manager, at Jartek.
Unlike pressure-treated wood, which relies on chemical preservatives, TMW achieves this resistance without any additives. It’s a clean, natural solution that aligns with growing demand in North America for non-toxic, sustainable building materials, especially in applications like saunas, bathrooms, and outdoor furniture where moisture is a concern.
3. Sustainability
TMW absorbs can store up to five times more carbon dioxide (CO₂) than it emits, making it a smart, climate-friendly choice. Unlike tropical hardwoods like ipe, which are often harvested from sensitive ecosystems and come with a high environmental cost, TMW offers a low-impact alternative like thermo ash (North American ash). It upgrades widely available, lower-grade wood into a durable, attractive material that performs like premium hardwoods.
TMW is also lightweight, recyclable, and made using a chemical-free process. Vainikka notes that for North American producers, it’s a way to add value to local wood while meeting the growing demand for sustainable building materials: “In regions like Canada and the northeastern U.S., where responsible forestry is a priority, TMW helps extend the life of domestic wood and reduce reliance on imported tropical species.”
4. Certification and quality assurance
“Twenty years ago, the thermally modified wood market resembled a wild west,” recalls Kurki. “Producers lacked standardized methods and durability benchmarks, flooding the market with inconsistent products. A few poor-quality offerings damaged TMW’s reputation, causing architects and designers to hesitate before adopting it.”
Today, that landscape has changed dramatically. The International ThermoWood® Association has introduced clear standards and rigorous certification processes. Producers who want to use ThermoWood® trademarked products must meet strict criteria and undergo annual third-party audits, ensuring that both the production process and the final product meet high performance and safety the applicable standards.
“For North American specifiers, this certification offers peace of mind,” says Vainikka. “It guarantees that the material is not only high-quality but also traceable and accountable – a crucial factor when specifying materials for commercial, residential, or public projects.”
Image Courtesy of Arbor Wood Co.
5. Process innovation and customization
“Not all wood species respond the same way to thermal modification. Through years of collaboration with producers, Jartek has developed tailored “recipes” for different species, adjusting parameters like; temperature, air flow, moisture, and timing to optimize results,” says Vainikka.
For example, processors must apply a slow, conservative process to Red Oak, widely available in North America, to preserve its integrity, while some other species can be thermally treated modified even 2-3 times faster. This flexibility lets producers add value to local timber and experiment with new species using test chambers.
“The result is a material that’s not only high-performing but also adaptable to regional resources and design needs. It is a significant advantage for North American sawmills and wood processors looking to differentiate their offerings,” underlines Vainikka.
Bonus: Aesthetic versatility
TMW starts with a rich, dark tone that extends throughout the entire thickness of each board, giving it the appearance of exotic hardwoods. “Over time, when exposed to UV radiation, it naturally grays, creating a weathered look that many designers find stylish. This change does not affect durability, and the original color can be preserved with UV-blocking oils coatings if desired,” states Kurki.
TMW is also easy to work with. It can be glued, painted, or finished like standard wood, and it’s compatible with most milling and fabrication processes.
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