What is Thermo-Aspen?
Aspen baked rather than treated. Heated to 180 to 215 degrees in an oxygen-free kiln using heat and steam alone, it becomes more stable, more durable and a deep caramel brown right through. It is the wood we line our saunas with.
The material
Baked, not treated
Thermo-Aspen is natural aspen that has been thermally modified: heated to roughly 180 to 215 degrees Celsius in a carefully controlled, oxygen-free environment, using nothing but heat and steam. No chemicals. No preservatives.
That process permanently changes the cellular structure of the timber. What comes out is stronger, far more stable, and much better suited to the one environment that punishes wood hardest: a room that is heated to full temperature and then allowed to cool again, thousands of times over its life.
The same process applied to other species is what the industry calls Thermowood. We use thermally modified aspen for cabin linings and benches, and thermally modified spruce for the staves of our barrel saunas.
The process in three numbers
215°C
Peak kiln temperature. The chamber is oxygen-free, so the wood changes rather than burns.
Zero
Chemicals or preservatives used. Thermo-Aspen is modified with heat and steam alone.
100%
Of the board's thickness carries the caramel colour, so it cannot wear away.
Why it matters
Thermal modification is what turns a soft, pale, everyday timber into a material that holds its shape and its looks for years in a sauna.
Three reasons we use it
What Thermo-Aspen does for your sauna
Stability
It holds its shape
The greatest gain from thermal modification is stability. Thermo-Aspen absorbs significantly less moisture than untreated wood, so it resists the expansion and contraction caused by repeated heating and cooling.
In practice that means far less warping, twisting, cupping and cracking. The tight fit and clean lines you see on delivery day are still there years later, which is the whole point of building a cabin properly.
Colour
A rich caramel brown, right through
Untreated aspen is a pale cream. Thermal modification develops a deep caramel to chocolate-brown tone through the entire thickness of the board, so it is the colour of the wood rather than a stain sitting on top.
It gives the interior a warmth and visual depth closer to fine furniture than to a timber cabin, and it is the reason a lot of people choose it before they have heard a single technical argument for it.
Comfort
Kind to sit against at full heat
Aspen has naturally low thermal conductivity, and thermal modification keeps it that way. The wood stays comfortable against bare skin even when the cabin is at full temperature, which is exactly what you want from a bench, a backrest or a wall you lean on.
It is why we use it for benches, backrests, armrests and interior panelling, in traditional and infrared cabins alike.
The usual comparison
Thermo-Aspen or cedar?
Thermo-Aspen
Neutral
Quiet, stable, modern
Almost no aroma, so nothing competes with the heat or your own oils. Heat-treated for stability, resin-free, knot-free and a deep caramel brown throughout. The choice if you want the room to feel calm and contemporary.
Cedar
Aromatic
Fragrant and traditional
Cedar's strong, distinctive scent is loved by traditionalists and is the main reason people pick it. That same aroma can be overpowering in a small cabin, and is a problem for anyone sensitive to strong wood scents.
Heat and steam. Nothing else.
Thermal modification improves timber using only heat and steam, with no chemical preservatives. It extends the life of the wood, keeps it a natural product, and means the material in your sauna is exactly what it looks like: aspen, changed by heat.
Thermo-Aspen FAQs
Thermowood is the general name for timber that has been thermally modified: heated to around 180 to 215 degrees Celsius in an oxygen-free environment using heat and steam, with no chemicals. The process makes the wood more stable and more resistant to moisture and decay, and darkens it throughout. Thermo-Aspen is aspen treated this way; thermo-spruce is spruce treated this way, which is what our barrel sauna staves are made from.
Neither is objectively better, they are different. Cedar’s appeal is its strong aroma. Thermo-Aspen is chosen for stability, a resin-free and knot-free surface, a rich caramel colour through the full depth of the board, and a neutral scent that leaves the room feeling calm. If you dislike strong wood smells, or you want to use aromatherapy oils, aspen is the easier wood to live with.
No. The stability and decay resistance come from the heat treatment itself rather than from a coating, so there is no oiling or sealing routine to keep up. Ordinary sauna hygiene is enough: sit on a towel, let the cabin dry out after use, and wipe the benches down.
The caramel tone is created through the full thickness of the timber rather than applied to the surface, so it does not wear off the way a stain would. Like any natural wood it can lighten slightly with prolonged direct sunlight, which is worth bearing in mind for a cabin with a large glass front.
Thermally modified aspen lines our cabins and forms the benches across the range. Our barrel saunas use thermally modified spruce for the curved outer staves, with aspen inside, and our garden cabin saunas pair a weatherproof black WPC shell with an aspen interior.
A sauna heats to temperature and cools again thousands of times over its life. Every one of those cycles moves the timber a little. Wood that absorbs moisture and moves will eventually warp, split or open up gaps at the joints, and a resinous wood will weep and smell. Choosing a stable, resin-free timber is the difference between a cabin that still looks and feels right in ten years and one that does not.
Feel it for yourself
The surest way to understand a material is to sit on it. Run a hand along a Thermo-Aspen bench at full heat and the argument makes itself.
Saunas lined in Thermo-Aspen
