Choosing the Right Wood for a Better Sauna

Gather round, everyone, we need to talk about wood.

One of the top questions we get here at Redwood Outdoors is: “Why don’t you use cedar?” Which is a very fair question, not only because our name has “Redwood” in it, but because cedar is probably the most common lumber choice for outdoor builds and saunas in the U.S. It’s what most of us think of when we think “sauna.”

But here’s the thing: in Finland, you won’t find many cedar saunas at all.

For starters, cedar isn’t native to Europe. And more importantly, the Finns are very serious about their sauna experience. What they’ve found over generations is that softer woods like heat-treated spruce, pine, hemlock, and aspen create a more stable, comfortable heat inside the sauna.

In North America, cedar became popular for outdoor use because it was amply available, and its natural oils offer insect resistance and rot protection. The catch? The quality of cedar available today isn’t what our grandparents had access to. Faster growing cycles mean the wood doesn’t have as much time to develop those qualities we’ve historically relied on.

Since we kiln-dry our wood so thoroughly anyway, we choose species that ultimately insulate better and deliver a more comfortable sauna experience. Our kilning process gives the wood the insect resistance and rot protection people associate with cedar, plus increased durability, longevity, higher insulating value, and a more even heat.

And it comes at a lower price point and is sustainably certified, which is very important to us.

You’ll see the term “Thermowood” all over our site, so let’s break it down.

Thermowood is simply wood that’s been heat-treated to change its molecular structure, making it more durable and stable. In plain terms? We dry it out really well in a lumber kiln to strengthen it and help it last a long time.

For wood to work well in a sauna, it needs to check a few important boxes: it has to be heat-resistant, moisture-resistant, dimensionally stable, and it can’t release resin when things heat up.

We achieve all of that through a pretty fascinating process. First, the wood is gently dried from about 30–50% moisture down to 10–12% at a moderate temperature. Then it moves into a high-heat phase, where it’s gradually brought up to around 415°F in an oxygen-deprived chamber (so it doesn’t ignite or burn). Finally, the wood is cooled and conditioned with steam, which naturally restores a small amount of moisture. The result is a board that’s tougher, more stable, and perfectly suited for the intense heat and humidity of a sauna.

The benefits of this process are:

•  Permanently restructured wood

• Better protection against insects

• Increased dimensional stability, i.e., less susceptible to swelling and shrinking with changes in environmental moisture

• Increased durability

• No exterior treatment needed unless you want to maintain color in a high UV environment

• Surfaces that are more comfortable to sit and lean against, and air that won’t be as harsh to breathe in

So, what wood are we using?

Our saunas and cold plunges come in Hemlock. And no, it’s not the poisonous Hemlock plant. The Hemlock tree is a non-poisonous evergreen conifer in the Tsuga genus (Wikipedia). Our Hemlock is sourced from FSC-certified forests in Canada. We love Hemlock for its light buff coloring, subtle aroma, and exceptional heat retention.

Better heat, better longevity, better sauna. That’s the goal. 

We may not be the world’s biggest wood aficionados, but every product decision we make is thoughtful and centered on your comfort and long-term investment. Our sales team loves to “chop it up,” so if you have any questions about our products, wood choices, or kilning process, don’t hesitate to reach out!