Thermally Modified Ash vs. Treated SPF vs. Ipe: Real Prices and What the 25-Year Claim Means

I've replaced the same pressure-treated deck boards twice on a friends' place outside Bastrop. Same board, same rot line, twice. Treated lumber is cheap and it's everywhere, but "cheap" has a payment plan, and the sun down here collects.

So when "thermally modified wood" started showing up on big-box shelves and online decking retailers, I paid attention. The pitch is simple: cook wood hot enough, without oxygen, and you change what it wants to do with water. No chemicals pumped in. No rainforest cut down. Sounds good. Now let's check the claims against the price tag.

What "Thermal Modification" Actually Means

The process is real, and the science is recent enough to be credible. A 2024 Virginia Tech study published in BioResources took six Appalachian hardwoods, including white ash, and ran them through an industrial thermo-vacuum system. The lumber went in kiln-dried to 6–8% moisture content, then got heated to roughly 200–240 °C in a vacuum atmosphere. That heat irreversibly degrades hemicellulose, which is the part of wood that loves water. Fewer hydrophilic hydroxyl groups means the wood stops drinking.

The results for ash were the standout in the study. Anti-swelling efficiency hit 71.4%, the best of the six species tested. Anti-shrinkage efficiency in ash was 65.6%. In plain terms: this board moves a fraction as much when the humidity swings from March to August. If you've ever watched a flat deck board cup into a taco by July, you know what that's worth.

So claim one, the stability claim, checks out. Peer-reviewed, independent lab, actual numbers. Good.

The 25-Year Claim, Unpacked

Here's where I get picky, because "25 years" gets thrown around like it means one thing. It doesn't. It means at least three different things, and the vendors aren't eager to separate them.

Thermory, the brand behind a big chunk of the market, says its thermo-pine lasts "upwards of 15 years" and thermo-spruce 25+ years against decay; its distributor, J Gibson McIlvain, credits Thermory's thermo-ash with an EN 350 Class 1 rating for 25+ years of decay resistance. That's the top tier on the European standard, the same class where Thermory puts teak and ipe. Larch and cedar sit down around Class 4. WoodPlank, an online retailer selling thermo-ash decking, offers a "25-Year Durability Guarantee" on its product page.

Those are not the same number. A decay-resistance class tells you how slow the fungus eats it. A structural lifespan depends on framing, fasteners, drainage, and whether you swept leaves into the corners for a decade. A warranty tells you how hard the company will fight your claim in year twenty. Keep them separate in your head, because the marketing sure doesn't.

And here's the part I respect Thermory for saying out loud: "not all thermowood is created equal," and a top durability rating "is not automatically guaranteed unless the material has been properly tested," citing lab testing through CATAS. Translation: the 25-year Class 1 rating belongs to boards from kilns that tested for it, not to the process in general. If you buy no-name thermowood, you're betting on someone else's QC. The vendor admitted it. That's either honest or a legal hedge. Probably both.

What It Costs, per Square Foot

WoodPlank sells its thermally modified ash decking, 5/4" thick with a 5-1/4" net face, in 15 sq ft boxes at $299.99 on sale, $399.00 list. That's about $20.00 per square foot at the sale price and $26.60 at list. Boards come sealed on all four sides, square-edge or pre-grooved for hidden fasteners, random lengths from 6 to 12 feet. The retailer says you can leave them unsealed to silver out.

Home Depot stocks a big-box version: WellDone's "5/4 in. x 6 in. x 8 ft. Thermally-Treated Premium Ash 4-Sides Oiled Decking Board," which the catalog describes as a five-board bundle covering 19.2 sq ft, with a 6-foot bundle at 14.4 sq ft. I have to be straight with you: Home Depot blocked the product pages when this was researched, so I couldn't confirm even the packaging details firsthand, let alone a shelf price, and I'm not going to guess. Pull the live price for item 300606619 before you budget anything. One flag while you're there: the listing's descriptive copy calls it "Premium Oak" while the title says Ash. That's either a sloppy listing or a wrong product, and neither builds confidence. Read the spec sheet, not the marketing copy.

For comparison's sake, Thermory's own positioning says its Benchmark knotty pine decking is "priced similarly to STK cedar in most markets" and, in the Northeast, lands in the same general range as many mid-tier composite products, while its ash costs the same as or less than tropical hardwoods. Mataverde, another vendor, claims thermally modified decking "costs much lower than cedar." Those two statements can both be true depending on species and grade, but notice they're both vendors talking their own book. The one hard number I'll trust is the one printed on a price tag, and the only one I could verify is WoodPlank's $20–26.60 per square foot.

Treated SPF: Still the Cheap Benchmark

Pressure-treated SPF is the floor, and it hasn't moved much. Buy Ipe Direct's 2026 comparison table puts pressure-treated pine at $2–5 per square foot in materials, with a 10–15 year lifespan and two to three replacements expected over 30 years. Mataverde's March 2026 price alert reports treated SYP decking showed little to no year-over-year price increase, and calls treated lumber the lowest-cost decking option by far.

The math on that cheapness is less flattering when you stretch the timeline. Two to three deck rebuilds over 30 years means paying the labor bill two or three times, and around here the labor bill is the one that stings. Add the maintenance cycle, and the $2 per square foot starts compounding.

On the chemicals: Thermory's pitch is that treated wood "introduces chemical preservatives" that "may leach into the environment," while thermowood uses "only high heat and steam." That leaching characterization is the vendor's, and I didn't find an EPA or extension-service source behind it in this research, so treat it as a claim, not a citation. My personal position, which is opinion and nothing more: I like the idea of deck boards that got their durability from an oven instead of a pressure cylinder, especially on a deck where the kids eat off the surface. But I'm not going to tell you treated lumber is dangerous, because the sourced data here doesn't support either direction.

Ipe, and Where the Ceiling Sits

Ipe is the top of this ladder and the site has already covered the CITES paperwork mess, so I'll keep it to price. Buy Ipe Direct lists ipe at $10–21 per square foot, with 1x6 wholesale at $5.00–7.50 per linear foot and 5/4x6 at $6.50–9.50, and claims 75+ years of lifespan. A 400 sq ft deck in 1x6 ipe runs $4,200–6,600 in materials, or $5,600–8,400 in 5/4x6.

Thermory markets thermo-ash explicitly as the "no rainforest wood" alternative at comparable or lower cost than tropical hardwoods. But next to ipe's sourced range, WoodPlank's $20–26.60 per square foot sits at the upper end of the field, overlapping the top of ipe's range rather than sitting below it. That's the actual positioning as the verified prices tell it: it's not competing with treated pine, it's priced alongside premium tropical hardwood, and vendors pitch it on conscience and availability.

The Trade Nobody Puts on the Sticker

Stability costs you stiffness. J Gibson McIlvain, a Thermory distributor, publishes the before-and-after for thermo-ash versus kiln-dried: bending strength down just 1%, but stiffness (MOE) down a full 20%. Weight drops 20%, density 12%, and radial shrinkage goes to 0.1% with tangential at 0.4%. Janka hardness holds at 1,325 lbf, so it still dents about like regular ash, and it mills, glues, screws, and finishes the same way, just browner, silvering to gray if you skip the finish.

A 20% stiffness hit on a joist-spanning deck board means you might notice more give underfoot on wider spans. You're trading flex for stability, and for most decks that's the right trade. Know you're making it.

The Verdict

If your budget forces treated SPF, buy it, build it square, and plan on replacing boards around year twelve. If you want real wood that survives without a chemical pump and you've got $20-ish a square foot, thermally modified ash is the most defensible premium option I've seen priced: the stability numbers are independently verified, the decay rating is real for properly tested product, and it prices into the same territory as 5/4 ipe. Just verify three things with your own eyes before you buy: the live big-box shelf price, the species on the spec sheet, and whose lab actually certified the durability class. The process is honest. The listings sometimes aren't.