Thermowood vs cedar for a sauna
Cedar resists decay because of what grows in it; thermowood gets there with heat and steam. Sun Home builds its outdoor Solaris in thermo-alder, while Almost Heaven builds most of its barrels in cedar and its Harvia Collection in thermally modified pine.
The short version: cedar and thermowood are two routes to the same goal — wood that stays stable and resists decay outdoors. Cedar has natural decay resistance. Thermowood is ordinary wood, such as pine, spruce or alder, modified with heat and steam. Premium makers use both. The practical questions are which class of thermowood you are buying, and whether it is named ThermoWood or a generic "thermally treated" claim.
What the ThermoWood process is
ThermoWood® is the International ThermoWood Association's registered name for its process. The association describes wood modified at elevated temperature in the presence of steam, under atmospheric pressure and without added chemicals. It says the process permanently changes the wood's structure and how it takes up moisture — lowering its equilibrium moisture content, improving its dimensional stability and increasing its resistance to biological decay. It also describes the result as resin-free, with consistent color through the wood and reduced thermal conductivity, and as suitable for indoor and outdoor use in all climates.
Thermo-S and Thermo-D
The association describes two classes. Thermo-S emphasizes dimensional stability and appearance, and the association lists sauna interiors and benches among its typical uses. Thermo-D offers higher biological durability and weather resistance, for demanding outdoor conditions — so it is the class to ask about for an exterior.
What cedar brings
Cedar's case outdoors is its natural resistance to decay and insects, which our cedar vs hemlock vs spruce page covers with its sources — and it gets there without processing. Most of Almost Heaven's barrels are rustic red cedar, and three of the four picks here are Canadian cedar.
Who uses which
| Maker and model | Wood | Published warranty on the structure |
|---|---|---|
| Sun Home Solaris (outdoor traditional) | Thermo-alder | 3 years, residential, on cabinetry — interior and exterior wood |
| Sun Home Luminar (outdoor infrared) | Canadian red cedar inside an aluminum exterior | Limited lifetime on cabinetry and heaters, up to 7 years; 3 years on the exterior cabin |
| Almost Heaven Harvia Collection (Magnus, Takoa, Kuuma, Evander) | Thermally modified Nordic pine | Limited lifetime on sauna room components |
| Almost Heaven canopy and classic barrels (Vienna, Audra, Charleston and others) | Rustic red cedar | Limited lifetime on sauna room components |
From Sun Home's and Almost Heaven's product and warranty pages, read on 2026-09-11.
The premium makers do not agree on a winner. Almost Heaven offers the same warranty on its thermally modified pine as on its cedar. Sun Home gives the thermo-alder Solaris three years on its cabinetry, interior and exterior wood, and the cedar-lined Luminar a limited lifetime on its cabinetry but three years on the exterior cabin — so outdoors, both exteriors carry three years.
Questions to ask about thermowood
- Is it named ThermoWood, or a generic heat treatment? "Thermally treated" on a listing does not say which process or class was used; the HOSAUNA pick is an example.
- Which class? For an exterior, ask which class the maker uses — the association positions Thermo-D for demanding outdoor conditions.
- What finish does the maker specify? Sun Home's care advice for the Solaris includes refreshing the exterior finish on the schedule your climate demands.
- What is warranted? Compare the structure warranty, not just the wood's reputation — see sauna and cold plunge warranties compared.
Not the same thing: carbonized composite
Some outdoor infrared cabins on Amazon use a carbonized wood-plastic composite, which one listing describes as carbonized wood combined with food-grade plastic. That is a composite, not thermally modified wood — a different material with a different set of questions.
Which to choose
- Cedar, if you want natural decay resistance without processing, and the cedar aroma.
- Thermowood, if stability and a resin-free, even-colored surface appeal — and the maker names the process and the class.
Alternatives you can buy today
The first pick is the one barrel we found on Amazon built in thermally treated spruce. The other three are Canadian cedar barrels with named heaters: a wood-fired barrel with a glass rear dome, a six-to-eight-person panoramic barrel with a Harvia 8 kW, and a four-person barrel with a Harvia 6 kW. None is a Sun Home or Almost Heaven product.
Thermally treated and cedar barrels on Amazon
HOSAUNA 4–6 Person Outdoor Barrel Sauna, Thermally Treated Finnish Spruce, 9 kW
HOSAUNA
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The one barrel we found on Amazon built in thermally treated spruce, with a 9 kW heater and a curved glass door and window.
| Outside | 80 × 80 × 80 in |
|---|---|
| Interior | 76 × 70 × 66 in |
| Heater | 9 kW, UL certified, with volcanic stones |
| Wood | Thermally treated Finnish spruce |
| Glass | Curved panoramic tempered door and window |
| Extras | Bluetooth speakers, LED lighting, LED control panel |
| Seating | 4–6 people |
| Voltage and circuit | Not stated in the listing |
What works
- Thermally treated wood outdoors
- 9 kW for a large barrel
- Interior dimensions published
What doesn't
- Voltage and circuit are not stated — confirm before ordering
- Does not say whether the wood is licensed ThermoWood, or which class
- The listing's health claims are the seller's
Skip it ifyou need the electrical requirement confirmed before you order.
Duthss 4–7 Person Outdoor Barrel Sauna, Canadian Cedar, Wood-Fired, Panoramic Dome
Duthss
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Thick Canadian cedar, a wood-burning stove and a glass rear dome — the cedar barrel for a site with no power.
| Outside | About 71 × 71 × 71 in in the description; 72 × 72 × 84 in in the spec table |
|---|---|
| Weight | 600 lb |
| Heater | Wood-burning stove with sauna stones |
| Wood | Canadian cedar, about 2 in thick |
| Glass | Panoramic rear dome, about 1/3 in insulated glass |
| Roof | Black rain-resistant shingles |
| Support, per the listing | 2 years stove system; 5 years wood structure |
What works
- Thick cedar staves
- No electrical run
- Glass rear dome
What doesn't
- A wood stove needs a chimney and fire planning
- Dimensions are given two ways
- A 4–7 capacity depends on who is sitting
Skip it ifa wood stove is impractical where the sauna would stand.
ZONEMEL 6–8 Person Panoramic Barrel Sauna, Canadian Red Cedar, Harvia 8 kW
ZONEMEL
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An 88-inch interior length — enough for two people to lie down, per the listing — in red cedar, with a named Harvia heater and a published circuit.
| Outside | 75 W × 94.5 D × 75 H in |
|---|---|
| Interior | 69 W × 88 D × 69 H in |
| Heater | Harvia SW80, 8 kW, ETL certified |
| Electrical | 220/240 V, 40 A |
| Temperature | 195 °F, per the title |
| Wood | Canadian red cedar |
| Roof | Wooden shingles |
| Back wall | Acrylic panoramic panel |
| Bands | 304 stainless steel |
What works
- 88-inch interior length
- Named Harvia heater and a stated circuit
- Red cedar outdoors
What doesn't
- An acrylic rather than glass back wall
- Needs a 40 A, 240 V circuit
- 69-inch interior height
Skip it ifyou cannot run a 40 A circuit to the garden.
4-Person Outdoor Barrel Sauna, Canadian Cedar, 6 kW Harvia
Generic Amazon listing
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Cedar shell, named-brand heater, four-person capacity — the three things that actually decide whether a barrel sauna is still good in year five.
| Capacity | 4 person |
|---|---|
| Height | 78" |
| Wood | Canadian cedar |
| Heater | 6 kW Harvia |
| Type | Traditional, vertical barrel |
| Circuit | 240 V — required for a 6 kW heater |
| Roof covering | Not published in the listing |
What works
- A Harvia heater means a published spec sheet and a real parts channel
- Cedar is the correct wood for a structure that lives outside year round
- 6 kW is properly sized for a four-person barrel volume
What doesn't
- A 240 V run to the yard is a real, permitted expense
- Roof covering is not specified, and barrels leak at the top first
- Freight delivery with a curbside drop — you move it yourself
Skip it ifyour yard has no realistic path for a 240 V run. Trenching a circuit to the back of a garden regularly costs more than the sauna, and it is not a corner worth cutting.
Our honest position
Both woods are legitimate, and premium makers use both. The difference that matters to a buyer is disclosure: a cedar barrel is cedar, but a "thermally treated" barrel could mean several things. Ask which process and class, and what the structure warranty says. Our Sun Home Solaris review looks at the best-documented thermowood sauna we know of.
Questions people actually ask
Is thermowood good for a sauna?
Is thermowood better than cedar?
What is thermally modified wood?
Does thermowood need a finish outdoors?
Sources
- International ThermoWood Association — What is ThermoWood — checked Sep 11, 2026
- Sun Home Saunas — Solaris 4-Person product page — checked Sep 11, 2026
- Sun Home Saunas — warranty information — checked Sep 11, 2026
- Almost Heaven Saunas — Takoa Half Moon Classic Barrel Sauna, Harvia Collection — checked Sep 11, 2026
- Almost Heaven Saunas — Magnus 6 Person Canopy Barrel Sauna, Harvia Collection — checked Sep 11, 2026
- Almost Heaven Saunas — warranty terms — checked Sep 11, 2026
- Amazon.com — thermally modified wood sauna listings — checked Sep 11, 2026
- Amazon.com — thermowood outdoor sauna listings — checked Sep 11, 2026
Read next
Every pick on this page comes from published specifications, not from hardware we own. Read how we pick — including what spec-led ranking cannot tell you.