Why Thermo-Treated Wood Works So Well for Outdoor Saunas
An outdoor sauna places unusual demands on timber. Its exterior must withstand rain, sunlight and seasonal changes, while the interior repeatedly moves between high heat, humidity and cooler resting temperatures. Thermo-treated wood is well suited to this combination.
The timber is modified using carefully controlled heat and steam. This changes how it reacts to moisture, making it more dimensionally stable than untreated wood. It is less prone to the swelling and shrinking caused by changes in humidity, which is a valuable quality in both an outdoor building and a frequently heated sauna.
Thermal modification also improves the wood’s natural durability, reduces its resin content and creates a rich brown colour throughout the timber. No chemical preservatives are added during the modification process.
It is still real wood, with natural variations in colour, grain and texture. That is part of its character—and one of the reasons every thermo wood sauna looks slightly different.
More Stable in Changing Conditions
Wood naturally absorbs and releases moisture as its surroundings change. Over time, this can cause movement, including swelling, shrinking and surface checking.
Thermal treatment reduces the timber’s ability to absorb moisture. As a result, thermo wood responds less dramatically to changing humidity and is more likely to retain its shape.
That does not mean it will never move or develop small cracks. All timber remains a living-looking natural material, particularly when used outside. The advantage of thermo-treated wood is that this movement is reduced, giving the sauna a more stable starting point for long-term outdoor use.
Enhanced Natural Durability
The modification process changes the wood in a way that makes it less favourable to the organisms responsible for decay. This improves its natural durability without relying on chemical preservatives during treatment.
For a garden sauna exposed to British weather, that added durability is a meaningful benefit. It does not replace good design, roofing, drainage or surface protection, but it helps the timber perform more reliably in demanding conditions.
Correct assembly and maintenance remain essential. The sauna should stand on a properly prepared base, rainwater must be able to drain away, and the timber should not remain in direct contact with soil or standing water.
A Rich Colour Created by Heat
Thermo-treated wood has a noticeably deeper, warmer colour than untreated Nordic spruce. The brown tone is produced by heat rather than being applied as a surface stain, so it runs through the timber and brings out the natural grain.
This gives a thermo wood sauna a distinctive appearance from the day it is assembled. The material works equally well with the curved profile of a traditional barrel sauna and the sharp, architectural lines of a modern cubic model.
Sunlight will gradually change the colour of any external timber. If thermo wood is left without UV protection, the surface will fade and eventually develop a natural silver-grey patina. Some owners like this weathered appearance. If you want to retain more of the original brown colour, use a suitable pigmented exterior treatment with UV protection.
Lower Resin Content
The high temperatures used during modification remove much of the resin naturally found in Nordic spruce. This is particularly useful in a sauna, where untreated resinous timber may release sap when exposed to heat.
No natural timber can be guaranteed to be completely resin-free, but the thermal treatment substantially reduces its resin content. It also gives the wood a characteristic, gently toasted scent that becomes more subtle over time.
Barrel, Cubic or Oval?
The Palmako thermo wood sauna collection includes three distinct shapes, allowing you to choose a model that suits both your garden and the way you plan to use it.
Thermo Wood Barrel Saunas
The rounded Anita barrel sauna is compact, recognisable and efficient in its use of material. Its curved interior creates a comfortable, enclosed atmosphere, while the smaller models fit easily into gardens where space is limited.
Different lengths provide a choice of internal space, and selected models are available with a glazed front for more natural light and a wider view from the sauna.
Thermo Wood Cubic Saunas
The ASTRA range combines thermo-treated wood with a clean, contemporary shape. It is a strong choice for modern gardens, terraces and properties where straight architectural lines suit the surrounding space.
Several sizes are available, including versions with a large glass wall. The glazing gives the compact interior a more open feel and allows the garden to remain part of the sauna experience.
Thermo Wood Oval Saunas
The ANETTE oval sauna offers more room while retaining the softened lines associated with barrel designs. Its elongated shape creates a spacious interior and a distinctive exterior that sits naturally within a landscaped garden.
Before choosing, compare the complete external dimensions as well as the internal floor area. Allow enough room around the sauna for assembly, safe access and future maintenance.
Caring for a Thermo Wood Sauna
Thermal modification improves the timber, but it does not make an outdoor sauna maintenance-free. Correct protection from the beginning will help preserve both its appearance and performance.
We strongly recommend applying a protective treatment to the exterior and interior timber surfaces immediately after assembly.
For the exterior, choose a breathable treatment intended for outdoor timber. A pigmented product with UV protection will help slow fading and preserve more of the wood’s original brown colour. Pay particular attention to end grain, joints and areas that are regularly exposed to rain.
For the interior, use only a treatment specifically designed for sauna interiors and suitable for exposure to high temperatures and humidity. Ordinary exterior preservatives, paints and varnishes must not be used inside unless the product manufacturer expressly confirms that they are safe for sauna use.
Follow the treatment manufacturer’s instructions regarding preparation, coverage, drying time and future reapplication.
Everyday Cleaning and Ventilation
A sauna should be allowed to dry properly after every use. Keep the ventilation openings clear, wipe away excess water and leave the door open once the sauna has cooled.
Use a soft brush or cloth and a mild cleaning product intended for sauna timber. Avoid aggressive household chemicals, pressure washing and abrasive methods that could damage the wood’s surface.
The exterior should be inspected at least once a year. Check the surface finish, roof covering, seals, metal fixings and areas around the base. Reapply the protective treatment when the coating begins to wear rather than waiting for the timber to become heavily weathered.
Choosing the Right Heater
Heater options vary between sauna models. Depending on the product, you may be able to choose an electric heater or a wood-burning stove.
An electric heater offers straightforward operation and precise temperature control. It must be connected by a qualified electrician using a supply suitable for the heater’s specified output.
A wood-burning stove provides a more traditional atmosphere but requires the correct flue, heat shields, safety distances and ventilation. Installation must follow the stove manufacturer’s instructions and all applicable building and fire-safety requirements.
Always match the heater to the internal volume of the sauna. A heater that is too small will struggle to reach and maintain the desired temperature, while an unnecessarily powerful unit may heat the space too aggressively.
Check the individual product page before ordering, as heater compatibility and the contents of each package differ.
Preparing the Base
A thermo wood sauna needs a firm, level and stable foundation that supports the structure evenly. Suitable options may include a concrete slab or a professionally prepared paving base, depending on the model and ground conditions.
Drainage is just as important as strength. Water should flow away from the sauna rather than collect underneath it. A poorly prepared or uneven base can affect assembly, door operation and the long-term stability of the structure.
Check the model’s exact dimensions and installation instructions before preparing the site. Remember to allow for roof overhangs, access to the entrance, heater installation and enough space to maintain the exterior surfaces.
A Natural Material, Carefully Improved
Thermo-treated wood does not try to disguise the qualities of timber. It improves the way wood performs while retaining the grain, warmth and individual character that make it appealing in the first place.
Its stability, enhanced durability, reduced resin content and rich natural colour make it a particularly strong choice for an outdoor sauna. Combined with a suitable base, correct treatment and regular care, it creates a sauna designed to remain an attractive and much-used part of the garden for years to come.
Explore the Palmako collection to compare thermo wood barrel, cubic and oval saunas, including compact models and designs with panoramic glazed fronts.
