Heat treated wood (also called thermally modified wood) is lumber that has been heated to temperatures between 180°C and 220°C in an oxygen-free environment, permanently changing its chemical structure. The process makes the wood more resistant to moisture, rot, and dimensional changes, all without adding any chemicals. It’s increasingly popular for decking, siding, and outdoor furniture as an alternative to pressure-treated lumber.
How the Process Works
The process starts with kiln-dried lumber placed into a sealed treatment chamber. In North America, most thermal modification happens in a vacuum, which prevents the wood from catching fire at high temperatures. The treatment follows three stages: the wood is gradually heated while remaining moisture is driven out, then held at peak temperatures (180°C to 220°C) for the actual modification phase, and finally cooled and reconditioned with a small amount of moisture to stabilize it for use.
The peak temperature matters. Wood treated above 200°C develops significantly greater decay resistance than wood treated at lower temperatures. But higher heat also means more strength loss, so manufacturers balance durability against structural integrity depending on the intended application.
What Changes Inside the Wood
Wood contains a component called hemicellulose, an amorphous sugar-based material that loves water and serves as a food source for fungi. Hemicellulose begins breaking down around 160°C. As it degrades, it releases organic acids that further break apart the bonds holding the wood’s internal structure together. This chain reaction transforms wood from a material that readily absorbs moisture into one that resists it.
The practical result is twofold. First, the wood absorbs far less water from the surrounding air. Studies on thermally modified hardwoods found that equilibrium moisture content dropped by 22% in hickory and up to 59% in red maple compared to untreated wood. Second, with less of that amorphous sugar material available, decay fungi essentially lose their food source. The wood becomes inhospitable to rot without any added preservatives.
This reduced moisture uptake also means the wood swells and shrinks less with seasonal humidity changes. Ash showed particularly dramatic improvement, with anti-swelling efficiency above 70%, meaning it moved less than a third as much as untreated ash in response to moisture.
Strength Trade-Offs
The same chemical changes that improve durability come at a cost to mechanical strength. Bending strength (the load a board can handle before snapping) typically drops by around 14%, though this varies widely. Treatment duration plays a major role: pine treated at 180°C for 2 hours lost only 4% of its bending strength, while the same species treated for 12 hours lost 38%. Some studies have documented reductions as high as 50% under aggressive treatment conditions.
This is why thermally modified wood works well for decking, cladding, and furniture but is generally not used for structural framing, load-bearing beams, or posts set in the ground. The wood is also more brittle than untreated lumber, so it’s less forgiving of impact and more prone to splitting if handled roughly during installation.
How It Compares to Pressure-Treated Wood
Pressure-treated wood is infused with chemical preservatives under high pressure, forcing fungicides and insecticides deep into the wood fibers. It’s effective, especially for ground-contact applications like fence posts and structural supports, but those chemicals can leach into soil and water over time. Heat treated wood uses no chemicals at all, making it a genuinely chemical-free option.
For applications like decking and exterior cladding where the wood stays above ground, heat treated wood performs as well or better than pressure-treated alternatives. It’s more dimensionally stable, won’t leach chemicals onto your patio, and has a rich, warm color that many homeowners prefer. However, it’s not a direct substitute in every situation. Pressure-treated wood remains the better choice for ground contact, structural applications, and anywhere termite resistance is critical, since thermal modification doesn’t reliably deter insects.
Which Species Are Used
Almost any wood species can be thermally modified, but some respond better than others. Independent testing at the CATAS laboratory in Italy found that four North American hardwoods, ash, tulipwood, soft maple, and quarter-sawn red oak, achieved the highest possible durability rating (Class 1, “very durable”) after thermal modification. That rating puts them on par with tropical hardwoods like Ipe, which is notable because these domestic species cost significantly less and are more sustainably sourced.
Softwoods like pine and spruce are also commonly heat treated, particularly in European markets where the technology has been established longer. The species choice depends on the application: ash and oak are popular for decking, while lighter softwoods often end up as cladding or interior feature walls.
Where Standards Currently Stand
Unlike pressure-treated wood, which has decades of established standards through the American Wood Protection Association (AWPA), thermally modified wood is still working its way through the standardization process. The AWPA created Guidance Document N, which outlines data requirements for listing thermally modified wood with enhanced durability, but formal use-category standards are still being developed. The industry is working to define specific use classes for thermally modified products and establish quality control testing methods.
In practice, this means you should look for products from established manufacturers who can provide third-party durability test results. The lack of a universal standard doesn’t mean the product is unreliable, but it does mean quality can vary between producers.
Installation and Maintenance Tips
Because thermally modified wood is more brittle than untreated lumber, pre-drilling for screws is often necessary to prevent splitting, especially near board ends. Stainless steel fasteners are the recommended choice for any outdoor application. Standard galvanized or carbon steel fasteners can corrode and stain the wood surface, particularly when exposed to rain.
The wood’s natural color after treatment is a deep, caramel-to-chocolate brown, depending on species and treatment intensity. Left unfinished outdoors, it will eventually gray from UV exposure, just like any natural wood. If you want to preserve the original color, a UV-protective oil finish applied periodically will slow that process. The wood itself won’t rot or degrade from being unfinished; the color change is purely cosmetic.
One maintenance advantage over pressure-treated wood: thermally modified boards are less likely to warp, cup, or twist over time because of their reduced moisture movement. Decks and cladding installations tend to stay flatter and tighter at the joints across seasons.