Thermory Alder Sauna Wall Boards, profile STS4 (natural alder)

Natural alder wall boards from Thermory, an Estonian maker that publishes declarations of performance, a verified EPD and forest certificates. Nothing is added, but no one tests what the hot boards give off, and prices vary by store.

Best for Builders who want plain hardwood on the walls and ceiling of a sauna and prefer untreated wood to heat-treated wood.

Our rating: Good (3.9 of 5)
Price range: Mid-range
Size: 0.59 in (15 mm) thick tongue-and-groove wall boards with narrow angled grooves; 4.4 in or 3.2 in coverage; bundles of six boards, 4.9 to 8.8 ft at Finnish Sauna Builders (to 9.8 ft at Sauna Marketplace)
Made in: Estonia (made by Thermory in Reola)

approx. $169 for a bundle of six 4.9 ft boards, 4.4 in coverage (about $5.76 per linear foot), Finnish Sauna Builders, 2026-10-23

See it on Thermory’s site → (opens in a new tab)

We are not affiliated with Thermory. This link goes to the maker’s own page, with no tracking code. How we're funded.

Also sold at Finnish Sauna Builders ($5.76 per linear foot at 4.4 in coverage ($169.34 for six boards of 4.9 ft); $3.62 per foot at 3.2 in coverage), Sauna Marketplace ($5.41 per linear foot for the 1x5 board (4.3 in coverage); $3.62 per foot for the 1x4 (3.2 in)), Haven of Heat ($201 for six 5 ft boards of the 1x4 (3.2 in coverage), about $6.70 per linear foot) .

Reader ratings & reviews

– out of 5

Loading ratings…

Have this product? Please rate it.

Reviews are personal experiences, not research, and they never change our quality ratings. Every review is read by our team before it appears.

How we evaluated it

How it scores Our rating, metric by metric Good, 3.9 of 5

Materials 45% of the rating 4 of 5

Natural common alder with no surface coating, per Thermory’s own declaration, which also gives formaldehyde class E1; its EPD reports no REACH SVHC above 0.1 percent. Grade and origin are stated only across all species, and no emissions test of the hot boards exists.
Durability 20% of the rating 4 of 5

Thermory calls alder tough, water-repellent and non-overheating, and says its sauna woods resist mold and do not splinter; no warranty or test of these boards is stated.
Safety 10% of the rating 3 of 5

Thermory’s own declaration (EN 14915) gives alder cladding reaction to fire D-s2,d0 and formaldehyde class E1; the stores give no clearance or fire guidance, which sits with the heater maker.
Trust 10% of the rating 4 of 5

Thermory publishes declarations of performance, a third-party-verified EPD and FSC, PEFC and ISO certificates, and no CPSC recall or Prop 65 notice turned up, but it sells only through resellers whose prices differ by up to 85 percent.
Value 15% of the rating 4 of 5

About $14 to $16 per square foot at Finnish Sauna Builders and Sauna Marketplace (about $25 at Haven of Heat), the lowest wall-wood price we list; wood freight can add a lot.

The rating, 3.9 of 5, is the weighted average of these scores. See how we rank building a sauna and our standard.

Where to buy Stores and the prices we saw 3 stores

Finnish Sauna Builders (opens in a new tab) approx. $5.76 per linear foot at 4.4 in coverage ($169.34 for six boards of 4.9 ft); $3.62 per foot at 3.2 in coverage. Illinois sauna retailer that lists Thermory as a brand. The page does not say natural or unoiled, so ask. We could not confirm it is on Thermory USA’s distributor list.
Sauna Marketplace (opens in a new tab) approx. $5.41 per linear foot for the 1x5 board (4.3 in coverage); $3.62 per foot for the 1x4 (3.2 in). Bundles of six, 4.9 to 9.8 ft. Says natural alder with no surface coating. Wood ships for a $795 flat rate per order; further wood in the same order is marked “Part of existing shipment”.
Haven of Heat (opens in a new tab) approx. $201 for six 5 ft boards of the 1x4 (3.2 in coverage), about $6.70 per linear foot. Says unoiled European alder. About 85 percent more per foot than the other two stores for the same 3.2 in board. Packs run 5 to 10 ft.

Prices as we saw them on October 23, 2026. They change often. These links carry no tracking code and earn us nothing.

What it’s made of What touches your food, water, skin or air 3 materials

Natural solid alder (common alder, Alnus glutinosa), declared by Thermory as cladding “without surface coating” for interior use; thermally modified and black-wax versions are separate products
No glue, finish or coating on the natural board per Thermory’s declaration, Sauna Marketplace (“no surface coating”) and Haven of Heat (“unoiled”); Thermory also sells waxed, oiled and coated versions, so order the natural one
Species named. Thermory’s EPD says its natural boards are 100 percent bio-based and mostly Grade A, from raw material grown in Estonia, Latvia, Lithuania, the UK, Sweden and Ukraine; that list covers all species, not alder alone

Who tested it What each test found, and who paid for it 3 tests

Maker’s own test Thermory (declaration of performance LM2021-Dv1, EN 14915, system 4) March 5, 2024

Thermory’s own declaration for natural solid alder cladding without surface coating, for interior use: common alder (Alnus glutinosa), reaction to fire D-s2,d0, formaldehyde class E1, thermal conductivity 0.12 W/(m·K), biological durability class 5 (EN 350, the lowest of five classes). Pentachlorophenol and release of other dangerous substances are “NPD” (no performance determined). System 4 means the maker declares it with no outside body, and it says nothing about what the boards give off when heated. Open ↗ (opens in a new tab)
Paid certification EPD Hub (third-party-verified EPD HUB-4842, natural solid wood boards) January 13, 2026

Life-cycle declaration for Thermory’s natural solid wood boards without surface coating (several products averaged), made in Reola, Estonia, verified by an EPD Hub verifier on 2026-01-13. It states no REACH SVHC above 0.1 percent and says air, soil and water impacts in use were not studied. Open ↗ (opens in a new tab)
Paid certification Preferred by Nature (FSC® certificate PBN-COC-009078; PEFC certificate PBN-PEFC-COC-000132) June 8, 2026

Chain-of-custody certificates for Thermory AS. The FSC certificate (valid to 2030-03-31) lists solid wood boards and sauna products under FSC Mix Credit and says a product counts as FSC only when the claim is on its sales documents; none of the three stores’ pages makes an FSC claim. The PEFC certificate lists sawnwood, flooring and terrace products, not sauna boards. Neither tests what the boards give off. Open ↗ (opens in a new tab)

Who paid, and why it matters

Independent The tester took no money from the maker for it: a consumer group, a journalist, a university or a regulator.
Paid certification An outside lab or certifier tested or listed it, and the maker paid for that.
Maker’s own test A lab report or certificate of analysis the maker commissioned and published.

A certification is a floor, not a verdict: it checks what its program checks and nothing else. We weigh tests by someone with no stake in the result most.

What people ask Worries raised online and elsewhere, and what we found 5 questions

Is the wood treated or finished?: Thermory’s declaration is for natural alder cladding “without surface coating”, and Thermory USA (opens in a new tab) says its sauna paneling is non-modified or thermally modified at a medium level. Thermory’s sauna page (opens in a new tab) also lists waxed, oiled and coated finishes. Finnish Sauna Builders’ listing does not say which you get, so ask for natural, unoiled boards in writing.
Where does the alder grow, and is it certified?: Thermory’s EPD (opens in a new tab) lists raw material from Estonia, Latvia, Lithuania, the UK, Sweden and Ukraine for its natural boards, not alder alone. Its FSC certificate (opens in a new tab) covers solid wood boards but says a product is FSC only if the sales documents say so, and none of the three stores does. Ask the seller for the chain-of-custody claim if it matters to you.
Does it give off fumes when hot?: We found no emissions test of these boards. Thermory declares formaldehyde class E1 for them itself, and natural alder is untreated wood. A 2022 lab study found that thermally treating wood changes what it emits (see our thermo-aspen page), which is one reason we score plain alder above heat-treated boards.
Can I buy it direct from Thermory?: Not through its main site: Thermory says it supplies exclusively to official distributors and does not sell to private customers. Thermory USA’s store (opens in a new tab) sells sauna accessories, not boards, and its sauna page says to contact it about sauna materials; we did not contact anyone. One buyer in a 2024 r/Sauna thread (opens in a new tab) says they bought a pallet straight from Thermory at a price below clear cedar, a forum claim we could not check. Prices vary a lot by reseller (see the store list).
Do the sellers have ties to Thermory?: Sauna Marketplace says in its own Reddit post (opens in a new tab) that Thermory hosted its first Partner Alliance Summit in Estonia in September 2026 and that it attended, and the store hosts the r/saunamarketplace forum. We found no such statement from the other two stores. Read a retailer’s praise of the brand as a retailer’s.

Why we chose it What put it on our list 4 points

Plain, named wood: natural common alder (Alnus glutinosa) without surface coating, not a composite, with no glue or finish.
Thermory says alder is hardy, water-repellent and does not overheat, and that it sells only sauna-suitable woods that resist mold and do not splinter.
Thermory publishes its declaration of performance, a third-party-verified EPD and its forest and ISO certificates.
It is the least expensive wall wood we list, at about $5.76 per linear foot ($3.62 for the narrower board) at the cheaper stores.

Trade-offs What to weigh before you buy 4 points

The declarations and certificates cover fire class, formaldehyde class, forest chain of custody and management systems, not what the boards give off when heated.
Grade and origin are not stated for alder itself, the FSC claim depends on the sales documents, and the thickness is 0.59 in, thinner than bench stock.
Thermory’s maintenance page says to treat sauna surfaces at least once a year with a product meant for the purpose. For the plain-wood standard we leave that optional, and the stores sell the boards unoiled.
Thermory does not sell to the public, so you pay a reseller’s price, and lumber freight can add a lot ($795 flat rate at one store).

What to know Details worth knowing first

Alder is a hardwood with little resin, and Thermory, an Estonian maker of sauna and cladding wood, says it is tough, water-repellent and does not overheat. This is the natural board: untreated alder in a narrow-groove wall profile, 0.59 in thick. Thermory’s own declaration lists it as common alder (Alnus glutinosa) without surface coating. It is the least expensive wall wood we list, at about $5.76 per linear foot from one US reseller and as little as $3.62 for the narrower board.

What is missing is any test of what the boards give off when hot. The declaration, the third-party-verified EPD and the forest certificates cover fire class, formaldehyde class E1, chemicals of concern and chain of custody, not heated emissions. Thermory’s FSC certificate counts a product as FSC only when the sales documents say so, and none of the three stores we checked makes that claim. Thermory also sells waxed and black-wax boards, so order the natural one and ask for it in writing.

Thermory does not sell these boards to the public, so the price depends on the reseller: the 3.2 in board was $3.62 per foot at two stores and $6.70 at a third, and one store charges a $795 flat rate per order for wood freight. Our heat therapy page covers the Finnish studies, which were of saunas at 80 to 100 °C. Heat speeds off-gassing, and a closed hot room is where anything the wood or its finish gives off ends up.

What we checked Each claim, and how sure we are of it 7 checks

Tested by: Thermory declarations and certificates (fire class, EPD, forest chain of custody); no heated-emissions test

How Thermory Alder Sauna Wall Boards, profile STS4 (natural alder) measures up against our building a sauna criteria
What we checked	What we found
Untreated, low-resin wood Maker’s claim	Natural common alder (Alnus glutinosa) without surface coating, per Thermory’s own declaration; the stores say unoiled or no coating. Thermory calls alder “hardy, water repellent and doesn’t overheat”.
Species, grade and origin named Maker’s claim	Species named in the declaration. Grade (“mostly Grade A”) and origin (six countries) are given only for all of Thermory’s natural boards, and the stores state neither.
Emissions when hot Not verified	No test of what the boards give off when heated. Thermory declares formaldehyde class E1 for alder cladding itself (system 4, no outside body); its EPD lists no REACH SVHC above 0.1 percent.
Environmental declaration (EPD) Verified	EPD HUB-4842 for natural solid wood boards is third-party verified (2026-01-13). It covers production and end of life and says impacts in use were not studied.
Forest certificates Maker’s claim	The FSC certificate lists solid wood boards and sauna products under FSC Mix Credit; a product is FSC only if the sales documents say so, and the stores do not. We did not check the FSC database.
Fire and heat guidance Maker’s claim	Reaction to fire D-s2,d0 in Thermory’s own declaration. The stores give no clearance guidance, which sits with the heater maker.
Recalls and notices Verified	No recall naming Thermory in the US Consumer Product Safety Commission data and no Prop 65 notice in the California Attorney General’s search, both on 2026-10-23. We could not search class actions (the site needs JavaScript).

What the labels mean

Verified We opened an independent record that confirms it: a certifier’s listing, a regulator’s list or a lab report. It is linked under Sources.
Maker’s claim The maker says so on its own page or label. We read it but found no independent record.
Not verified We looked and could not confirm it, or the maker does not say. Treat it as unknown.

Sources Where our facts come from 16 sources

Thermory: sauna wall panels and bench boards. Maker’s page. Species, finishes, the sauna-suitable-wood statement and the distributor-only note. Open ↗ (opens in a new tab)
Thermory: wood species. Maker’s page. Alder “hardy, water repellent and doesn’t overheat”. Open ↗ (opens in a new tab)
Thermory: certifications and testing. Maker’s page. Lists the certificates, EPDs and declarations of performance. Open ↗ (opens in a new tab)
Thermory declaration of performance: natural solid alder cladding (LM2021-Dv1). Maker’s page. Species, fire class D-s2,d0, formaldehyde class E1, system 4. Open ↗ (opens in a new tab)
EPD Hub HUB-4842: Thermory natural solid wood boards without surface coating. Test report. Third-party-verified EPD: raw material origin, no REACH SVHC above 0.1 percent. Open ↗ (opens in a new tab)
Thermory FSC® chain-of-custody certificate PBN-COC-009078. Maker’s page. Scope (solid wood boards, sauna products) under FSC Mix Credit; the claim must be on sales documents. Open ↗ (opens in a new tab)
Thermory: sauna maintenance. Maker’s page. Treat the wooden surfaces at least once a year with a product meant for the purpose. Open ↗ (opens in a new tab)
Thermory USA: sauna materials. Maker’s page. Sauna paneling is non-modified or thermally modified at a medium level; contact Thermory USA about sauna materials. Open ↗ (opens in a new tab)
Thermory USA: distributors. Maker’s page. US lumber distributors; we could not find the three stores below on it. Open ↗ (opens in a new tab)
r/Sauna: Thermory wood? (2024 thread). Independent source. A buyer says they bought a pallet directly from Thermory; forum posts, not tests. Open ↗ (opens in a new tab)
r/saunamarketplace: We spent three days with Thermory and sauna people from 23 countries (2026 post by the store). Independent source. Sauna Marketplace says Thermory hosted its first Partner Alliance Summit in Estonia. Open ↗ (opens in a new tab)
Finnish Sauna Builders: Alder sauna wall boards, profile STS4 by Thermory. Independent source. Store price by the foot and by the bundle, sizes and 0.59 in thickness, seen 2026-10-23. Open ↗ (opens in a new tab)
Sauna Marketplace: 1X5 Alder STS4 wall cladding by Thermory. Independent source. Store price per linear foot, 4.3 in coverage, freight terms, seen 2026-10-23. Open ↗ (opens in a new tab)
Haven of Heat: Thermory alder nickel gap sauna wall cladding, 1x4, 6-board pack. Independent source. Store price by pack and length, “unoiled”, “European alder”, seen 2026-10-23. Open ↗ (opens in a new tab)
US Consumer Product Safety Commission: recall search. Certifier or regulator listing. Returns no recall for Thermory (2026-10-23). Open ↗ (opens in a new tab)
California Attorney General: Proposition 65 60-day notice search for Thermory. Certifier or regulator listing. No matching records (2026-10-23). Open ↗ (opens in a new tab)

Alternatives Other options to compare 2 options

Thermory Thermo-Aspen Sauna Bench Boards (SHP) Heat-treated aspen for benches. About $5.70 to $6.84 per linear foot.
ProSaunas Western Red Cedar vertical grain bench boards Untreated western red cedar for benches or walls, about $7.48 to $13.20 per linear foot.

Reader ratings & reviews

How we evaluated it

Weekly research digest