The Story Behind Thermowood

My name is Bryan and I’m a Product Manager at Redwood Outdoors, as well as a passionate sauna user! 

In 2021, I went on a trip to Scandinavia, the birthplace of the sauna, to research different types of saunas and bring the best ones back to North America for Redwood Outdoor’s customers. It was an exciting trip and I want to give everyone a bit of feedback on what I found out.

While experiencing these new sauna types, I found that none of the European saunas I tried were made out of red cedar (the sauna wood of choice in North America). Instead, European saunas were almost exclusively made from Thermowood. Thermowood refers to wood (typically spruce, but pine and fir can also be used) that has been treated at high temperature to increase its longevity and durability. 

The quality of the European Thermowood saunas really impressed me. I also learned from my European hosts that Thermowood is a much more authentic sauna wood than red cedar. In countries where using a sauna is an important tradition and part of daily life such as Finland, Norway, Estonia, and Russia, red cedar does not grow naturally. Instead, sauna makers in Scandinavia and Russia relied on spruce. Spruce was the wood of choice in this region for many years, however, it has an obvious drawback, it can rot if left outdoors untreated. 

The Thermal Modification process was invented to overcome this issue! Unlike pressure-treated wood, which most North Americans would be familiar with, thermal treatment is a chemical-free method for creating wood that could be left outdoors for long periods and not rot. The Thermal Modification process (described below) also gives the wood a pleasant smell and color similar to red cedar. 

My finding couldn’t have come at a better time. As you may have seen in the news or at your local building supply store, the price of red cedar has been rising. This is partially because of the ongoing COVID-19 pandemic and is partially because red cedar is a diminishing resource. Almost all the red cedar used globally is harvested in Canada. After years of logging, the supply of red cedar in Canada is dwindling, with some estimates saying that ~70% of the available red cedar has already been logged. These two factors have resulted in higher prices across the North American sauna market. 

Sustainability is at the root of everything we do at Redwood Outdoors. To honor our products, we must first respect the environment they come from, and that means protecting the forests. Furthermore, our goal at Redwood Outdoors is to give everyone the chance to enjoy the health benefits of sauna at a low price. With the rapid increase in red cedar prices, this has become more and more difficult.

After coming home after my trip to Scandinavia, I decided it was time for Redwood Outdoors to release a line of Thermowood saunas. Not only are these saunas durable and visually striking, they also help us uphold our commitment to nature. Thermowood is made from abundant wood species which are not threatened and considered a sustainable choice by conservationists. Our Thermowood units also allow us to maintain our commitment to delivering exceptional value to our customers. Because the Thermowood is made from more widely available woods (spruce, fir and pine), it is lower cost source than red cedar, meaning we can offer a great price to our customers. Its rare in life to find an example of getting a better price without sacrificing quality, but in this case we can honestly say Thermowood is providing our customers a double win: high quality and great value. 

What is Thermal Modification?

During Thermal Modification, wood is placed in a chamber and gradually heated between to 410-450º F. The chamber is completely void of Oxygen to prevent the wood from burning. The wood is then left in the chamber for up to 3 days to allow the heat to penetrate all the way to the center of the wood.

The chamber is a vacuum, so it has to be incredibly strong. An example of a Thermal Modification chamber is below.

The Thermal Modification process is completely non-toxic and only requires heat to function. 

By contrast, pressure-treated wood is a common choice for outdoor use in the building industry. Pressure-treated wood is soaked in chemical preservatives to prevent infestation and decay. Through thermal modification, we can offer a sauna with the same durability, but without using any harsh chemicals.

Thermowood Technical Advantages:

The addition of heat alters the wood on a cellular level, and gives the wood a number of technical advantages:

Dimensional Stability – The intense heat dries the wood to the core, removing all moisture and resin. As a result, the wood will no longer absorb water. This reduces the wood's ability to expand or contract, which is a leading cause of water absorption and rot. Further, Thermowood does not react to changes in humidity as drastically as untreated wood (i.e., the risk of swelling, cracking, and shrinkage is decreased), allowing Thermowood to retain its shape better over time. Thermowood also does not secrete sap or resin, even in high temperatures.

Environment Sustainability – The wood used to produce Thermowood originates from well-managed, PEFC-certified forests, and no chemicals or foreign substances are involved during its production. Thermowood is an environmentally friendly material produced by using only natural methods.

Rot and Fungi Resistance – The heating process breaks down hemicellulose, so the wood does not contain the appropriate nutrients to allow rot and fungi to grow and develop.

Increased Longevity – When wood is treated with heat, its average life span is prolonged meaning your sauna will last a lifetime making this the most sustainable sauna product on the market.

Improved Insulation – Wood that has been heat-treated is becomes more porous, vastly improving its insulating properties.


I am very excited by our new range of Thermowood sauna and I’m sure the original designs as well as the low price will make you fall in love as well. 

If you have any questions about this new type of wood or what saunas are best for your needs, please contact us at 1-800-567-8036.