What is Thermowood?

Thermowood (also known as thermally modified timber, heat-treated wood, or thermal wood) is the term used to describe any wood product that has been treated with heat to improve its stability and durability. The process, which involves exposing wood to extreme temperatures (around 200 degrees Celsius), is a totally natural one that removes all moisture from the timber.

Once the wood is heat-treated in this way, its resistance and enhanced structure make it an ideal material to use for various construction and outdoor furniture projects. In this article, we explore what you can expect from this unique material.

How is Thermowood Made?

Thermowood is created when all the moisture in wood has been evaporated. This is done by placing the wood into a chamber and exposing it to extreme heat for several hours. Temperatures range between 185-212 degrees Celcius and after heating, the wood is rapidly cooled again.

During the process, all OH (Hydroxyl) and H20 (Water) is removed from the wood cells. Hydroxyl is what typically draws free water cells into the wood and causes its instability. By removing these groups, you end up with a far more stable, resilient product. It also means that there isn’t any resin, which helps leave a beautiful finish too.

What are the benefits of Thermowood?

After processing, the properties of wood will have drastically changed. The new and improved Thermowood is now more resilient overall, resistant to decay, and has a beautiful finish aesthetically. Let’s look at some of the other benefits more closely.


Thermo-wood is not as vulnerable to moisture as standard wood. The heat treatment process alters the density and structure of wood on a molecular level. This decreases its moisture absorption and release. Put another way, if you place thermally modified wood into a 100% humid environment, it will only absorb a maximum of 12% moisture.

This makes the wood more resistant to extreme weather, humidity, and temperature variations. That’s why so many premium manufacturers are now turning to Thermowood for outdoor saunas, hot tubs, and other furniture alike.

It is also more resistant to fungal damage and less susceptible to mould and rot (properties that are especially crucial for outdoor buildings which are subjected to intense moisture fluctuations).


When working with organic materials – especially given the state of our world today – it’s important to be mindful of our impact on the planet. Working with Thermowood has a sustainability advantage because:

  1. The lifespan of thermally modified wood is much longer.
    It’s more resistant to rot and decay than untreated wood, so by choosing Thermowood products, you’re helping reduce the demand for new lumber. This, in turn, helps to preserve forests and conserve natural resources.
  2. It’s a completely chemical-free option.
    Only steam and heat are used to cure the wood. This means it’s a non-toxic material. Since many people love the look of thermally modified wood (even as it ages) there is no need to stain it or use other harsh chemicals.


Wood that has been treated with heat doesn’t expand and contract as much in response to changes in humidity. The wood is therefore better able to keep its shape and stability for longer because its less likely to warp or shrink over time. Additionally, the heat process removes most of the resin content, which helps to combat resin secretion (a frequent problem for Saunas made with untreated wood where resin or sap begins to seep through the interior walls).

Luxurious finish

Thermowood is a smart choice for those who want a beautiful, natural-wood look without the hassle of traditional wood treatments. That’s because Thermowood has a slightly darker hue than unstained wood. That’s why so many people prefer not to stain it at all.

Left untreated, thermally modified wood will gradually fade to silver/grey over time. If you want to retain the original colour of the wood, you can do so by using a clear treatment that includes a UV filter.

If you choose to stain it a darker shade, you will benefit from the wood being less absorbent than uncured wood, meaning you won’t need to use as much varnish or stain, saving you both time and money.


Even without varnish, Thermowood lasts longer than untreated wood. That’s because chemical impregnation, only affects the outer layers of the wood, whereas thermal modification enhances the timber throughout.

However, to get the most out of any outdoor products such as Sauna cabins, we still recommend varnishing the exterior.

Lighter weight

Because the timber contains no water, it’s lighter weight. This is great when working on larger projects, or even when you consider home sauna kits, since it often means you won’t need to think about additional foundation work.

What Are The Disadvantages of Thermowood?

Although thermally modified wood has many clear advantages, some drawbacks should be considered.

It costs more than uncured wood

Thermowood is more expensive than regular wood. Your location and the type of wood will affect the price difference, so be sure to budget accordingly (it’s worth keeping in mind that this price difference may be offset by the fact that it also lasts longer and requires less maintenance).

It’s more susceptible to colour changes

If you’re looking for a very specific colour or shade, then Thermowood may not be right for you. It’s a lot more susceptible to changing colour over time (especially with a lot of UV exposure) so might not be right for some outdoor projects.


In summary, thermally treated timber does outperform uncured wood in more or less every way, however it does come with a price tag.

If you’re considering which materials to use for outdoor amenities such as Saunas & Hot tubs, it is important to remember the strenuous effects that temperature and moisture fluctuations can have. For these projects we recommend Thermowood over regular untreated wood for its enhanced moisture-repellent and structural properties, which is why Neptune only sells premium Thermo-wood treated Saunas and Hot Tubs.

Our sales team is always happy to help with any questions you may have about our Thermo-wood products. We can provide you with all the information you need to make an informed decision about this exceptional material.

Frequently asked questions

Thermally modified wood is very versatile and can be used in several places. Because of the price difference however, it tends to be used on projects that need wood that’s more robust in nature such as:

  • Exterior paneling
  • Sauna & spa interior/exteriors
  • Decking
Untreated thermally modified wood can last up to 30 years. Its lifespan is much greater than uncured wood.

Whilst it’s hard to produce like-for-like comparisons of wood lifespans (because conditions, wood types etc are so varied), the cellular makeup of thermally modified wood is much stronger and less penetrable, so it will last much longer.

Yes, you can stain thermally treated wood. In fact we advise you do on all our saunas and hot tubs to extend their longevity even further. It’s worth noting that the colour will change over time though (find out more in the article above). Also you won’t need to use as much stain as on untreated timber, since the wood won’t absorb as much moisture.

Thermally modified wood (or Thermowood) has a whole range of benefits, but why is it the perfect choice for building a sauna? One of the main factors is that when it’s exposed to a 100% relative humidity environment, the wood itself will only ever reach a maximum of 12% moisture. This suppressed saturation rate means the wood is:

  • less susceptible to warping and bending
  • safe against bugs
  • doesn’t rot or allow fungal penetration

Both soft and hardwood can be thermally modified. Popular species include pine, cedar, alder, and spruce.