The Short Answer

Thermowood is spruce that has been pre-treated with heat, at around 200°C, before it is cut into sauna logs. The process gives the timber greater stability, natural resistance to rot and a deep, even brown colour, so it asks very little of you once it is built. Untreated Nordic spruce is the classic sauna timber: lighter in colour, lower in price, and with a simple protective coat after assembly it will serve you well for years. If your sauna will sit in an open spot, or you want more evenings in the heat and fewer weekends with a brush in hand, Thermowood is usually worth the extra. If you love the traditional pale timber look, or would rather put more of your budget towards a larger cabin, untreated spruce is an excellent choice.

Below: what Thermowood actually is, how it compares with untreated spruce on the points that matter, what each costs across our range, and how to decide.

Jump to:

• What is Thermowood?
• Thermowood vs untreated spruce at a glance
• How much care does each need?
• Does the timber change how the sauna performs?
• Is Thermowood worth the extra cost?
• Which should you choose?
• FAQ

What is Thermowood?

Thermowood is ordinary softwood, usually spruce or pine, that has been thermally modified using only heat and steam, with no chemicals added.

 

The timber is kiln-heated to around 200°C. At that temperature the wood loses its resin and much of its ability to absorb moisture, and the cell structure changes permanently. Because the change runs through the whole thickness of each board rather than sitting on the surface like a stain or paint, a scratch or a cut end does not expose untreated timber underneath.

The process was developed in Finland, and it is no accident that it is widely used for saunas, cladding and decking across the Nordic countries. It takes a fast-growing, widely available timber and gives it the kind of outdoor durability that would otherwise mean importing a naturally durable hardwood or cedar.

Thermowood vs untreated spruce at a glance

The short version: Thermowood, which arrives pre-treated from the factory, wins on durability, stability and upkeep; untreated spruce wins on price and a lighter, classic look.

	Untreated Nordic spruce	Thermowood (heat-treated spruce)
Colour	Pale, honey-blonde; a protective finish keeps it bright	Deep, even brown throughout; a UV-protective finish keeps it rich
Natural rot resistance (EN 350)	Class 4, slightly durable	Exterior grades rated Class 2, durable
Movement with moisture	Normal for softwood; swells and shrinks with the seasons	Noticeably less swelling and shrinking
Resin	Can weep resin near knots in strong heat	Resin removed during treatment
Chemicals	None in the timber itself	None; heat and steam only
Strength	Stronger and more forgiving to work with	Slightly more brittle, so fix gently near board ends
Exterior upkeep	A protective coat after assembly, refreshed every few years	A UV-protective finish to keep the colour
Price	Lower	Around 19% to 26% more across our range

Durability classes from EN 350 as published by timber producers, referenced below.

One small note for build day: heat treatment makes timber slightly more brittle. It makes no difference to the finished sauna, but it is worth taking a little extra care when fixing close to the end of a board.

How much care does each need?

Both are simple to look after. Untreated spruce needs a protective coat after assembly and a refresh every few years; Thermowood needs little more than a UV-protective finish to keep its colour.

Our untreated spruce saunas arrive as natural, bare timber, ready for whichever finish you choose. A coat of wood preservative as soon as the build is complete seals in that fresh timber and protects your investment through the British winter, and it is also what keeps your structural warranty in place. From there, a fresh coat every few years, depending on how exposed your garden is, keeps the cabin looking its best. Doors and frames benefit from the same attention.

Thermowood asks even less of you. It arrives pre-treated and already resistant to moisture and rot, so its care is really about colour. Left as it is, the surface mellows over time from rich brown to a soft silver grey, a weathered look some owners love. To keep the warm brown for longer, our manufacturer recommends a UV-protective finish. Your sauna manual sets out the exact care steps for your model.

Inside, both are cared for in the same way: benches and interior walls are left natural or finished with a sauna-specific product rather than exterior stains or paint.

Does the timber change how the sauna performs?

Only slightly. Wall thickness and build quality matter far more to heat retention than whether the timber is heat-treated.

Thermowood producers report modestly improved insulation, because the treated timber is a little less dense and holds less moisture. In practice, the bigger factor is how much timber sits between you and the cold. Our Thermowood models use the same 42mm solid timber walls as their untreated equivalents, so the heat-up and heat-holding experience is very similar. Where you will notice a difference is over the years: stable timber that swells and shrinks less keeps its joints tight, which helps a sauna keep performing the way it did on day one.

Inside, the bather experience is much the same either way. The darker interior of a Thermowood sauna feels warmer and more enclosed; untreated spruce feels brighter and more traditional. Many people also notice a faint toasted scent from Thermowood when it first heats up, which fades with use.

Is Thermowood worth the extra cost?

For most outdoor saunas, yes: the extra is roughly a fifth of the price, and it buys lower upkeep and better long-term stability.

Here is how the difference works out across the models we offer in both finishes, using starting prices without a heater at the time of writing (October 2026):

Model	Untreated spruce	Thermowood	Difference
Tyyni Solo	£2,438	£2,890	+£452
Usva 1.1m²	£2,390	£2,890	+£500
Usva 1.4m²	£2,590	£3,250	+£660
Usva 2.0m²	£3,250	£3,990	+£740
Löyly Four-Person	£5,337	£6,490	+£1,153

Prices correct at time of writing; see each product page for current pricing.

When weighing it up, think about time as well as price. With untreated spruce, the odd weekend goes on refreshing the finish; with Thermowood, that time goes back into using the sauna. Over the life of the cabin, that narrows the gap considerably.

Which should you choose?

Both make a beautiful garden sauna, so it comes down to what matters most to you.

Thermowood is likely the better fit if:

• Your sauna will stand in an open, exposed or damp part of the garden
• You would rather spend your time in the sauna than looking after it
• You like a darker, contemporary look that sits well against modern planting and grey paving
• You want the timber to stay as stable as possible over many winters

Untreated spruce is likely the better fit if:

• You would like the most sauna for your budget
• You enjoy the lighter, classic Nordic look
• You enjoy the ritual of caring for natural timber
• You would like to finish it in a colour of your choice

You can browse all our Thermowood saunas in one place, or compare finishes side by side on models like the Tyyni Solo in Thermowood and the Löyly Four-Person in Thermowood. If you are still deciding on shape, our barrel saunas are where most of the Thermowood options sit. And if you are planning to have it built before the coldest months, our guide to using a garden sauna through a UK winter covers lead times and preparation.

Frequently asked questions

Is Thermowood pre-treated? Yes. Thermowood is pre-treated with heat and steam before it leaves the factory, so it arrives ready to resist rot without a chemical preservative. Untreated spruce arrives as bare timber, ready for the protective finish of your choice after assembly.

Is Thermowood safe for saunas? Yes. Thermowood is made using only heat and steam, with no chemical preservatives, and it is one of the most widely used timbers in Nordic saunas. Removing the resin also means no sticky sap weeping from knots when the sauna is hot.

Does Thermowood go grey? Over time, yes, if it is left unfinished outdoors. UV light gently mellows the surface to a soft silver grey. It is purely cosmetic and does not affect durability, and a UV-protective finish, as our manufacturer recommends, keeps the rich brown for longer.

Do I need to treat a Thermowood sauna? Not to protect it. Thermowood is already resistant to rot, so it does not need a preservative the way untreated spruce does. To keep its rich colour for longer, our manufacturer recommends a UV-protective finish, and your sauna manual sets out the care for your model.

Is Thermowood the same as pressure-treated wood? No. Pressure-treated timber is impregnated with chemical preservatives. Thermowood is modified by heat alone, so nothing is added to the wood.

Can I paint or stain an untreated spruce sauna? Yes. The exterior takes a coloured wood protection finish well, so you can match it to your garden or home. Keep the interior natural, or use sauna-specific finishes only.

Does Thermowood keep the heat in better? Slightly, as treated timber has lower thermal conductivity, but wall thickness has a much bigger effect. Our Thermowood and untreated models use the same 42mm walls, so heat performance is very similar.

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Sources

• International ThermoWood Association and Stora Enso, product information on ThermoWood: heat and steam process, dimensional stability, durability and resin removal.
• Lunawood, technical data for Thermo-D (exterior) and Thermo-S (interior) grades, including EN 113 durability classes and care guidance.
• International Timber, Thermowood product data: Class 2 durability under BS EN 350, weathering to silver grey, and brittleness when machining.
• Metsä-Kortelainen, S. (VTT Publications 771, 2011), doctoral research on the durability and strength of thermally modified Norway spruce and Scots pine.
• BS EN 350, durability classification of wood species, including Norway spruce.

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Written by Peter Gibbs, founder of North Cabin.