What Is Thermally Modified Wood? The Heat Treatment That Creates Premium Siding

Thermally modified wood is natural wood that has been heated to high temperatures in a controlled, oxygen-free environment to enhance its durability, stability, and resistance to moisture and decay.

At Edge, we've spent years identifying the perfect process to create siding that's not just beautiful, but built to last. Here's how we do it.

Above: ForgeWood thermally modified wood in Coal and Tungsten

The Science Behind the Heat

The secret lies in what happens inside the wood during modification.  The wood is heated to extreme temperatures in an oxygen-free environment, removing moisture and changing the molecular structure so it becomes considerably more durable and dimensionally stable. Ultimately, it's resistant to rot and decay.

What makes the difference? The process degrades hemicellulose, which is the component in wood that allows it to absorb and lose moisture in changing weather conditions. Once that degradation occurs, you get wood with improved stability as temperature and natural elements fluctuate.

The process also transforms the color throughout the entire thickness of the wood, creating rich, darker colors reminiscent of tropical hardwoods.

Our ForgeWood collection of thermally modified wood

How It Happens: The Step-by-Step Process

Step 1: Moisture Removal

We start by gradually heating the wood, evaporating substantially all of the moisture from it. This is a critical foundation for what comes next. 

Step 2: The Heat Treatment

Here's where the magic happens. Depending on the species, color profile, and durability required for a project, the wood is heated to temperatures up to 415 degrees Fahrenheit for a set period of time. Essentially, we use different “recipes” to achieve the desired outcome. 

We introduce steam or nitrogen as the wood reaches higher temperatures to force out oxygen and prevent combustion. This controlled environment is where the hemicellulose breaks down and the wood's cellular structure fundamentally changes.

Step 3: Cooling and Rebalancing

Finally, we cool the wood in a controlled environment until it arrives at its new equilibrium moisture content (EMC) of less than 7%. Compared to regular wood that can swing 20%+ EMC from season to season, it’s a major difference.

ForgeWood thermally modified wood in Raw

The Performance Difference You Can Count On

Dimensional Stability That Actually Works

Once reacclimated to its "modified EMC," thermally modified wood is highly stable—its moisture content won't vary by more than 2-3% in virtually any conditions. It's a night and day difference versus unmodified wood that can vary by 20%+ EMC from season to season.

This means less potential for warped siding or gaps that appear after the first season. 

Rot Resistance Without Chemicals

The temperatures in the modification process kill existing fungal or insect infiltration that may exist in the unmodified wood. The wood becomes largely hydrophobic after modification, too, reducing water from absorbing back into the wood. 

The thermal modification process removes moisture and organic compounds (like hemicellulose) that fungi need to grow, making the wood far less susceptible to decay..

This is why we are proud to be able to offer a 20-year limited warranty against rot and decay. 

What Makes Edge Different in Thermal Modification

A few highlights that set us apart:

• Majority of our raw materials are sourced within North America
• Various "recipes" to meet consumers' needs while maintaining quality and consistency

Environmental Benefits of Thermally Modified Wood:

• Chemical-free process. Heat and water or nitrogen are the only things we introduce from start to finish
• Carbon sequestration benefits because the wood is so incredibly durable
• North American sourcing reduces transportation environmental impact
• No ongoing chemical treatments required

ForgeWood thermally modified wood in Tungsten

The Bottom Line on Thermal Modification

We've taken wood (one of nature's most versatile building materials) and made it better through controlled heat treatment. No chemicals, no compromises. Just enhanced performance that delivers on jobsites and looks beautiful.

The process transforms readily available North American species into premium building materials, giving you the beauty of exotic woods without the supply chain headaches or sustainability concerns.

Installation advantages:

• End-matched T&G profiles for faster installation and less waste
• Prefinished options eliminate on-site finishing delays
• Consistent moisture content means predictable performance
• Matching trim available for complete project solutions

Performance you can specify with confidence:

• 20-year limited warranty against rot and fungal decay
• Dimensional stability that minimizes callbacks
• Lighter in weight with improved thermal insulation properties

Ready to see what thermally modified wood can do for your next project? 

Explore ForgeWood, our premium line of thermally modified siding that combines cutting-edge modification technology with the craftsmanship building professionals demand. Find where to buy today!