If you have been comparing saunas, you may have come across spruce, thermo-spruce, aspen and Thermowood, sometimes without much explanation of why a manufacturer has chosen them.
The wood can affect how comfortable the seating feels, the appearance and scent of the cabin, how the timber responds to moisture and the care it needs. Those considerations apply to traditional and infrared saunas alike, although their importance changes depending on how the cabin is built and whether it lives indoors or outside.
Understanding the descriptions helps you judge what you are paying for.
Making sense of the names
Spruce, aspen, alder and cedar are names for types of wood. Terms such as thermo-spruce and thermo-aspen also tell you that the timber has undergone thermal modification: a controlled heat treatment that changes some of its properties.
Manufacturers use these descriptions in different combinations. Hekla specifies thermo-spruce for its Cube frames and thermo-aspen inside its infrared cabins. Yakia’s 400 S specification lists Thermowood walls and floor, with thermo-aspen benches and backrests.
ThermoWood® is a registered trademark associated with a particular thermal modification process and quality system. The broader term is thermally modified timber. Neither identifies the original wood species, so a description that simply says Thermowood leaves that question unanswered.
You will also find descriptions of surface texture and colour. Tylö offers brushed thermo-aspen and a darker thermo-aspen option alongside its other timbers. These details help explain the finished appearance, although a dark colour alone cannot establish whether wood has been thermally modified.
A sauna may use several different woods
The benches, internal lining and exterior do different jobs, which gives manufacturers good reasons to combine materials.
Harvia’s Family cabin has solid spruce construction with linden interior fittings, also described as lime wood on its product page. Yakia’s Ultima L uses alder-lined walls and thermo-aspen benches and backrests.
The seating needs to provide smooth, comfortable surfaces for bare skin and withstand perspiration and cleaning. Wall and ceiling linings experience the cabin’s changing temperature and humidity. In a garden sauna, exposed external timber also has to cope with rain, sunlight and seasonal changes.
Construction affects the comparison too. Solid timber walls form the enclosure itself; in an insulated cabin, the visible wood is one layer of a larger wall assembly. Board thickness and timber choice are therefore only part of the explanation of how well a sauna retains heat.
When a description gives one timber name for the whole sauna, it is worth establishing which components it refers to.
Comfort starts with the seating
Wood is useful for sauna seating because it transfers heat to your skin much more slowly than metal at the same temperature. Its thermal properties vary with density, moisture content and grain direction. Harvia identifies low-density woods as common choices for benches because they are comfortable to sit or lean on.
A wooden bench still warms up during use. The temperature of its surface and how readily heat passes into your skin both affect how it feels.
Timber selection and workmanship also become apparent here. Knots can feel hotter against the skin, which is why Harvia recommends minimising them in seating. They may give a wall an attractive character while being less welcome beneath your legs or against your back.
Smooth boards, comfortable edges and a well-finished backrest are relevant in any cabin, including infrared models operating at lower air temperatures. When viewing a sauna, take a few minutes to sit down, lean back and feel the bench edge. You will learn more about its comfort than you can from photographs.
What thermal modification changes
Wood absorbs and releases moisture as conditions around it change, causing it to swell and shrink. Thermal modification can reduce this movement and improve resistance to fungal decay, with the results depending on the wood and treatment used.
The ThermoWood system has different treatment classes for applications ranging from sauna interiors and benches to exposed external cladding. The treatment should suit the component’s intended use.
Thermal modification can also reduce thermal conductivity, affecting how readily heat passes through the wood. That can be relevant to sauna use, although it does not mean a treated surface will remain cool.
There are compromises. The process can reduce bending strength and impact resistance, making the timber more brittle. Manufacturers need to account for those changes in their design and handling of the material.
This goes beyond ordinary kiln drying, which primarily brings timber to a suitable moisture content. Thermal modification uses controlled heat treatment to alter the wood’s properties and also darkens it. A surface stain can produce a similar colour, so the written specification is the place to establish which process has been used.
Traditional, infrared and hybrid cabins
A traditional sauna generally operates at a higher air temperature than a dedicated infrared cabin. Pouring water over its hot stones also increases humidity. An infrared-only cabin does not have that source of moisture, although its seating still encounters perspiration and needs cleaning.
The wood needs to suit those conditions. Thermally modified timber is used in both types: Hekla specifies thermo-aspen inside its infrared cabins, while other manufacturers offer different choices. Sunlighten’s UK range, for example, includes eucalyptus and basswood.
For an infrared cabin, the timber specification tells you about the enclosure and its surfaces. Assessing the heating system means looking at the emitters and their arrangement. A more expensive wood does not, by itself, establish better infrared performance.
Hybrid cabins combine infrared emitters with a traditional sauna heater. Harvia offers systems that can be controlled separately, allowing either experience within the same room. The cabin materials need to suit the operating conditions permitted by the manufacturer.
Outdoor suitability is a separate consideration for all of them. The complete structure must be designed for that setting; choosing a particular timber cannot make an indoor cabin suitable for the garden.
Appearance and scent
Aspen produces a pale interior, alder brings warmer reddish-brown tones, and thermally modified woods can create a darker space. Grain, knots and the width and arrangement of the boards contribute to the overall appearance.
These differences may be a substantial part of the appeal. The cabin will occupy a place in your home or garden, and you will spend time looking at its interior whenever you use it.
Scent is more personal. A noticeable wood fragrance may be something you enjoy or something you would rather avoid. Treatment can contribute its own smell: Yakia advises that its thermally treated wood has a characteristic scent that diminishes over time but does not disappear completely.
If fragrance is likely to influence your choice, try to experience a warm cabin before buying. Written descriptions are a poor substitute for finding out how you respond to it yourself.
Movement, weathering and care
All timber needs to be prepared and used with its moisture behaviour in mind. The amount it moves depends on factors including the type of wood, grain direction and drying. Good construction accommodates that movement; the manufacturer’s instructions should explain any inspections or adjustments required during ownership.
Outside, exposure can gradually turn timber grey. This surface weathering is different from fungal decay, which can penetrate more deeply and weaken the wood. Greying alone does not tell you that a cabin is failing, but it is still necessary to follow its care instructions.
Yakia requires external timber to be treated with a suitable preservative immediately after installation as a warranty condition, despite specifying Thermowood. That is a useful example of why a durable material should not be assumed to need no maintenance.
Inside any sauna, establish how to clean the seating, ventilate and dry the cabin, and whether particular surfaces need protection. Harvia recommends paraffin oil for its appropriate untreated bench timbers, for example. Other materials or factory finishes may have different requirements.
Use the products approved for the cabin and surface involved. An exterior wood treatment should never be assumed suitable for the interior or seating.
Is a more expensive wood worth paying for?
A higher price may reflect a timber’s appearance, its selection and finish, or a treatment that gives it useful properties. You might also simply prefer its colour or scent.
What deserves questioning is an unexplained claim of “premium wood”. To compare it meaningfully, you need to know what it is, where it is used and what difference it makes.
Before deciding, establish:
- Which woods are used for the cabin, lining, benches and backrests.
- Whether they have been thermally modified or given a surface finish.
- What care is required at installation and during ownership.
- What else differs in the construction and equipment.
At Hot Tubs Oxfordshire, we would also encourage you to spend some time inside the cabins you are considering. Read the specification, but give yourself the chance to judge the seating, finish and atmosphere in person. Those are the qualities you will encounter each time you close the door and settle in.
