Cedar vs Hemlock vs Thermowood: A Sauna Manufacturer's Guide

A sauna is one of the harshest places you can put wood. It heats to 80-90C (175-195F), then cools to room temperature, over and over. Pour water on the rocks and humidity spikes in seconds. An outdoor sauna takes all of that, then sits through winter on top of it.

So when we judge a wood, we're asking a few questions. Does it resist rot, decay, and insects on its own, or does it need chemical treatment to last? Does it stay put as it heats and cools, or warp, cup, and crack? Does it hold pitch that can bleed out when hot? And does it stay cool enough to sit on without scorching skin?

Two things matter less than people expect. Hardness barely registers - most sauna woods are soft, and that's fine. And aroma is a preference, not a quality grade. Keep the list in mind, because each wood scores well on some lines and poorly on others.

Cedar earns its reputation outdoors, and it does it with no chemical help. The reason is in the wood itself: cedar heartwood carries natural oils that resist moisture, decay, fungus, and insects. That's why a cedar sauna can live outside year-round and hold its integrity for decades. When heat hits it, cedar releases the warm, woodsy scent many people call their favourite part of the ritual. The Western Red Cedar Lumber Association keeps good background if you want to go deeper.

There are two cedars worth knowing, and they're easy to mix up.

Western red cedar

This is the premium outdoor wood. It grows along the Pacific coast, runs a warm reddish-brown, and is prized for its natural durability and that signature aroma. It's soft and light, which is normal for sauna wood, not a weakness. Within the two grades of cedar (Clear and Knotty) there is many grades that relate to quality. Leisurecraft only buys the top grade premium quality of both clear and knotty. Our Dundalk Collection is built in western red cedar.

Eastern white cedar

White cedar is the more accessible cousin - lighter in colour, milder in scent, and grown close to where many of these saunas are built in Ontario, which keeps shipping and lead times down. It shares cedar's natural resistance to moisture and decay, though it's softer and lower in density than western red cedar: comparable protection in a lighter, gentler, more affordable package. Want a real cedar sauna without the premium? This is it. Most of our Canadian Timber Collection has eastern white cedar options with thermowood as well.

One note to clear up confusion: neither is eastern red cedar, the closet-lining wood with the sharp, pungent smell. That's a different species, far too strong for a sauna - so when sauna people say cedar, they mean the western red or eastern white here.

Hemlock gets talked about in extremes - dismissed as cheap and flimsy, or marketed as tough and stable. The honest read sits in between, and it comes down to where you put it.

What hemlock does well: it's light-coloured, fine-grained, holds almost no resin, and is virtually odourless. That last point is the real selling point. If cedar's aroma is too strong for you, or you share your sauna with someone scent-sensitive, hemlock's neutrality is genuinely useful. It's also affordable, which is why it shows up in lower-cost cabins and in infrared rooms, where gentler heat puts less stress on the wood.

Where hemlock falls short is the outdoors. Untreated, it isn't naturally rot-resistant, so exposed to year-round weather it needs chemical treatment to last. That's why our outdoor sauna guide points people away from hemlock, spruce, and pine for outdoor use - and we'll say the same here. Indoors or in an infrared cabin, hemlock is perfectly reasonable; as exposed outdoor cladding in a cold climate, cedar is the safer bet. It's the one wood here we don't build with, and that's the honest reason why.

Thermowood isn't a species - it's a process, and a clever one. You heat a softwood in steam, in a low-oxygen chamber, to around 160-230C, with no chemicals added. That changes the wood's internal structure: it absorbs far less moisture, swells and shrinks much less, loses its resin, and takes on a deeper, even brown colour throughout.

The payoff is real. A non-durable softwood comes out dramatically more stable and more rot-resistant - properties cedar has naturally, reached here through heat instead. There's an honest trade, though, and we'll name it because most sellers don't: thermal modification reduces strength, with bending strength dropping by up to about a third. That's no dealbreaker for sauna panels and benches, which aren't load-bearing, but it's a real property of the material, not a footnote.

That's exactly why thermo-modified wood shines in the right setting - a sheltered indoor room, where stability and a clean modern look matter more than raw strength. It's the thinking behind our CT Element indoor sauna, built from thermally modified Thermo Grandis, thermo-treated in Ontario. We're not describing thermowood from the sidelines - it's a wood we build with, where it fits best.

Cedar, hemlock, and thermowood cover most of the decision, but a few other names turn up. Aspen, basswood, and poplar are soft, pale, and nearly odourless - good for interior benches and backrests, with basswood a favourite for scent-sensitive sauna-goers and aspen a traditional Nordic choice - though none carry cedar's outdoor durability, so they stay inside. Pine and spruce are the cheap, easy-to-find option, but pine especially can bleed sticky pitch when hot, so it's a budget path with a real trade-off attached.

Most wood comparisons online assume a vague "humid climate" and stop there. The thing that actually wears outdoor wood out in a cold climate is the freeze-thaw cycle: water works into the wood, freezes, expands, thaws, and repeats, season after season. Add the swing from dry winter air to humid summer, and an outdoor sauna's walls are moving and stressing constantly.

This is where the woods sort themselves out. Cedar resists the moisture that drives rot in the first place - the reason it's stood up outdoors for generations. Thermowood gets there differently: heat treatment slashes how much water it takes on, so there's far less movement to crack and check it. Both work. An untreated, non-durable wood left exposed to all of it doesn't - which is why, for a sauna that lives outside through hard winters, we keep coming back to cedar.

Here's the whole comparison in one place. Read it by your situation, not by hunting for an overall "best" - the right wood for an outdoor barrel by the lake isn't the right wood for an indoor room in a city basement.

Two honest caveats. "Cost" is relative and moves with the market - cedar especially has been climbing as supply tightens. And every rating assumes the wood is used where it belongs: hemlock's "low" rot resistance only bites outdoors, and thermowood's "excellent" stability is the flip side of that strength trade. Match the wood to the job and the table takes care of itself.

After twenty years of this, our approach comes down to one idea: let the environment pick the wood. For an outdoor sauna that faces every season, we trust cedar's natural durability - western red in the Dundalk Collection for the premium wood and full aroma, eastern white in the Canadian Timber Collection for real cedar at a friendlier price. For a sheltered indoor room, where stability and clean lines matter most, the thermo-modified Element line is built for exactly that.

You don't need to memorize wood science for this. Decide where your sauna will live and how you'll use it, and the wood mostly chooses itself. For a second opinion on the fit, that's what our dealers talk through every day.