Thermally Modified Wood Siding Explained: Pros, Cons & Cost
Many homeowners love the look of real wood siding but worry about maintenance, moisture, rot, and how long it will last. Thermally modified wood is designed to address those concerns while keeping the look of natural wood.
In the video above, Jon Heyesen of Arbor Wood Co. walks through his company’s thermally modified siding line: species, profiles, finishes, and maintenance. This page covers what the material is, its pros and cons, what it costs, and how grain choice affects performance.
What Is Thermally Modified Wood?
Thermal modification uses heat and steam, with no chemical preservatives, to permanently change the wood’s cell structure. The process breaks down the sugars that fungi feed on and reduces how much water the wood can absorb.
The result is still real wood, unlike composite or engineered products, but it is far more stable and decay-resistant than untreated lumber. It is used for siding, cladding, decking, and architectural details.
Pros of Thermally Modified Wood Siding
Moisture resistance: it absorbs much less water, so it is less prone to rot, mold, fungal decay, warping, cupping, and swelling.
Stability: less movement with changing humidity keeps boards flat and joints tight.
Durability: many manufacturers estimate a service life of 25 to 30 years or longer when properly installed and maintained, and the treatment runs through the whole board, not just the surface.
Real wood appearance: heat creates rich, warm color through the board rather than a surface stain.
No chemical preservatives: an attraction for homeowners who want a more environmentally conscious material, especially when sourced from responsibly managed forests.
Cons of Thermally Modified Wood Siding
Higher cost than standard wood siding: it costs more than pine and other untreated siding, and often close to or above cedar.
More brittle: the heat treatment reduces strength and makes boards easier to split, so installers pre-drill fastener holes and handle boards carefully.
Fastener requirements: stainless steel or coated fasteners are needed to prevent corrosion staining.
Color change: left unfinished, it weathers to silver-gray. Keeping the original color means reapplying a UV-protective finish.
Not structural: it is for siding and cladding, not load-bearing framing.
What Thermally Modified Wood Siding Costs
Pricing varies widely by species, board size, profile, and finish. Material typically runs about $5 to $12 per square foot. Installed, most projects land somewhere around $11 to $24 per square foot, depending on the profile, whether a rainscreen or clip system is used, and local labor.
Compared with cedar, thermally modified wood usually costs more for the material, but installed costs are closer, since labor is similar. Many homeowners justify the difference with lower maintenance over the life of the siding. For conventional wood and other materials, see our siding replacement cost guide.
Tight Grain vs. Flat Sawn: Why Grain Matters
How a board is cut from the log affects how it looks and performs:
Tight grain (vertical or edge grain): cut so the growth rings run roughly perpendicular to the face. The grain shows as fine straight lines. These boards move less, are less prone to cupping, and hold finishes well.
Flat sawn: cut so the growth rings run across the face, showing the arched "cathedral" grain pattern. Flat-sawn boards cost less and have more visible character, but they move and cup more with moisture.
For siding, tight-grain boards are the premium choice for stability and finish life, while flat-sawn boards are the budget-friendly option with more grain character. Ask your supplier which cut each profile uses before comparing prices.
Species, Finishes, and Profiles
Several species can be thermally modified. White ash is valued for its strength and hardness. Southern yellow pine is known for its availability, affordability, and toughness compared with many other pines. Each species brings its own grain pattern and color.
Finishes include natural, charred, blackened, brown-toned, whitewashed, and gray-toned options, including finishes inspired by traditional Japanese charred-wood techniques. Common profiles include tongue-and-groove, hidden-fastener systems, horizontal and vertical siding, and modern cladding profiles.
Maintenance
Like all exterior wood, thermally modified siding benefits from routine care, but it needs less than untreated wood. You have two choices: let it weather naturally to a silver-gray patina, or apply UV-protective oils and finishes to keep the original color.
Depending on the product and climate, refinishing intervals may run every two to four years for vertical siding when you want to keep the original tone.
How It Compares
Thermally modified wood sits between natural cedar and engineered products. It is real wood with better moisture resistance than untreated lumber. Engineered wood costs less and resists impact well, but it is not solid wood; see our engineered wood siding guide. For how all the main materials compare, see the best siding for your house.
For cedar, redwood, and other natural wood options, see our wood siding guide.
Final Thoughts
Thermally modified wood siding offers the beauty of real wood with far better moisture resistance and stability than untreated lumber. It costs more than standard wood siding and needs careful installation, but for homeowners who want genuine wood with less upkeep, it is worth a close look. For guides on every siding material, visit our siding project hub.
Considering wood siding for your home? Home Upgrade Pros can connect you with licensed siding contractors for free, no-obligation estimates, or call (725) 745-8553.
Frequently Asked Questions
What is thermally modified wood siding?
Real wood treated with heat and steam, without chemical preservatives, to improve moisture resistance, stability, and durability.
How much does thermally modified wood siding cost?
Material typically runs about $5 to $12 per square foot, and installed projects often land around $11 to $24 per square foot, depending on species, profile, finish, and labor.
Is tight grain or flat sawn better for siding?
Tight-grain (vertical-grain) boards move less, resist cupping, and hold finishes longer, so they are the premium choice. Flat-sawn boards cost less and show more grain character but move more with moisture.
How long does thermally modified wood siding last?
Many manufacturers estimate 25 to 30 years or longer when it is properly installed and maintained.
Does thermally modified wood rot?
The treatment greatly improves resistance to rot, fungal decay, and moisture damage compared with untreated wood.
Can thermally modified wood be left unfinished?
Yes. It weathers to a silver-gray patina. To keep the original color, apply a UV-protective finish periodically.
