For an eco-friendly sauna, choose eastern white cedar, hemlock, or thermally modified wood from fast-growing species, pair them with formaldehyde-free insulation, and tile the floor with ceramic or natural stone. Avoid pressure-treated lumber, plywood, and vinyl flooring, all of which off-gas harmful compounds at sauna temperatures.
The average sauna build uses cedar, and for good reason—it resists moisture, smells great, and lasts decades. But cedar harvested from old-growth forests raises questions about sustainability. If you care about what goes into your body and what your choices do to the environment, the materials matter.
Here is what the research and the building community actually say about eco-friendly sauna materials, from the frame to the benches to the insulation.
Wood choices that balance performance and sustainability
The wood you sit on, lean against, and breathe around for 20 to 45 minutes at a time should be safe, durable, and responsibly sourced. Three species stand out for sauna construction.
Thermally modified wood. This is a newer option worth knowing about. The wood—usually alder, ash, or poplar—is heated to 400 to 450 degrees Fahrenheit in a low-oxygen kiln. The heat changes the cellular structure. The wood becomes more resistant to moisture and rot without chemical treatments. Thermally modified poplar, for example, has a Janka hardness rating close to cedar and costs less. The manufacturing process uses no chemicals, and the wood comes from faster-growing species. Multiple European studies from the 2010s confirmed that thermal modification improves dimensional stability by up to 50 percent compared to untreated wood. If you are building a sauna from scratch, this is worth a conversation with your lumber supplier.
Eastern white cedar. Not all cedar is equal. Western red cedar, the gold standard for saunas, grows slowly in old-growth forests on the Pacific coast. Eastern white cedar grows faster, regenerates more readily, and is harvested from managed forests in the Great Lakes region and northeastern Canada. It has similar rot resistance and a lower density, which means it heats up faster in a sauna. The grain is straight and knots are minimal. If you buy a prefab sauna kit, ask the manufacturer specifically where their cedar comes from. If they cannot tell you, that is an answer in itself.
Hemlock. This is the dark horse of sauna woods. Hemlock is dense, resists checking (cracking from heat cycling), and has virtually no scent. Some people prefer that—cedar aroma can be overwhelming in small saunas. Hemlock is also a fast-growing species commonly harvested from sustainably managed forests in the northeastern United States and Canada. The downside: it does not have the same natural rot resistance as cedar, so it needs proper ventilation and a good finish. For an indoor sauna with controlled humidity, hemlock performs admirably.
What to avoid in sauna wood
Pressure-treated lumber of any kind. The chemicals used to preserve outdoor wood—copper, chromium, arsenic in older treatments, or alkaline copper quat in newer ones—do not belong in a 180-degree environment where you are breathing deeply. The heat accelerates off-gassing. This is non-negotiable.
Plywood. Even "exterior grade" plywood uses adhesives that contain formaldehyde. At sauna temperatures, formaldehyde emissions increase significantly. A 2014 study in the journal Indoor Air found that formaldehyde release from engineered wood products roughly doubled for every 18-degree Fahrenheit increase in temperature. Sauna temperatures run 150 to 200 degrees. Do the math.
Insulation that does not off-gas
Most sauna builders use fiberglass insulation between the studs, then cover it with foil vapor barrier and wood paneling. Fiberglass itself is inert once installed, but the binders used in some batts can contain formaldehyde. Look for formaldehyde-free fiberglass insulation. Johns Manville and Owens Corning both make versions labeled "Formaldehyde-free" or "EcoTouch."
Mineral wool (Rockwool) is another option. It handles higher temperatures than fiberglass—up to 2,000 degrees Fahrenheit—and does not burn. It is made from volcanic rock and slag, a byproduct of steel production, and contains no chemical binders. It also does not absorb moisture, which matters in a sauna environment. The downside: mineral wool is slightly more expensive and harder to cut cleanly. It is worth the trade-off.
Sheep's wool insulation is the natural alternative. It is biodegradable, renewable, and naturally resists mold and mildew. It handles moisture by absorbing and releasing it without losing insulating value. The catch: sheep's wool insulation is roughly three times the cost of fiberglass and less common at standard building supply stores. For a custom build where budget is less of a concern, it is the most environmentally sound option available.
The vapor barrier question
Standard practice is to staple reflective foil over the insulation before installing the interior paneling. That foil is usually aluminum laminated to paper or plastic. It works fine. But if you want to avoid synthetic materials entirely, you can use a "smart" vapor retarder like Intello Plus or Pro Clima. These are made from polypropylene—still a plastic—but they are designed to be vapor-permeable when humidity is low and vapor-tight when humidity is high. They last the life of the building and can be recycled at end of life.
For the truly natural approach, a layer of cork board between the studs and the interior paneling provides both insulation and vapor management. Cork is harvested from the bark of cork oak trees without cutting the tree down. The bark regenerates every nine to twelve years. Cork is naturally fire-resistant, rot-resistant, and has excellent thermal properties. It is not a standard vapor barrier, but in combination with proper ventilation, it works.
Flooring and benches
Benches should be the same wood as the walls, ideally with a gap between slats for airflow and drainage. Cedar is traditional. Thermally modified poplar or ash works just as well.
For flooring, ceramic tile or natural stone is the most durable and easiest to clean. Both are made from natural materials—clay, sand, and minerals—and fire at high temperatures that burn off any organic compounds. They do not off-gas. They do not absorb moisture. They last basically forever. The grout should be epoxy-based rather than cement-based, because cement grout absorbs water and can grow mold in a sauna environment.
Avoid vinyl flooring. At sauna temperatures, vinyl can soften and release plasticizers, including phthalates that act as endocrine disruptors. A 2021 study in Environmental Science & Technology found that phthalate emissions from vinyl flooring increased significantly above 140 degrees Fahrenheit.
What to look for when buying a prefab sauna
If you are buying rather than building, the same material principles apply. Ask the manufacturer these three questions:
- Where is the wood sourced? If they say "sustainably harvested," ask for certification. FSC (Forest Stewardship Council) certification means an independent auditor verified the supply chain.
- What adhesive is used in the paneling? Tongue-and-groove cedar planks should be mechanically joined, not glued. If there is glue, it should be a water-based, formaldehyde-free adhesive.
- Is the heater certified for the space? This is less about materials and more about safety, but an undersized heater running at max temperature for extended periods stresses the electrical system and wastes energy. A properly sized heater is the most eco-friendly choice because it uses less electricity to maintain temperature.
The practical takeaway
You do not need to build a sauna out of reclaimed barn wood and sheep's wool to be environmentally responsible. The simplest high-impact choice is this: use eastern white cedar or thermally modified wood instead of old-growth western red cedar. Use formaldehyde-free insulation. Skip the plywood. Tile the floor. Ask your supplier questions.
The sauna is a tool for longevity. The materials you build it from should support that goal, not undermine it.
Frequently asked questions
What is thermally modified wood and is it safe for saunas?
Thermally modified wood is made by heating species like alder, ash, or poplar in a low-oxygen kiln at temperatures between 400 and 450 degrees Fahrenheit. The process changes the cellular structure to resist moisture and rot without any chemical treatments. Multiple European studies from the 2010s confirmed that thermal modification improves dimensional stability by up to 50 percent compared to untreated wood.
Why should you avoid plywood in a sauna build?
Even exterior-grade plywood uses adhesives that contain formaldehyde, and formaldehyde emissions increase significantly at sauna temperatures. A 2014 study in the journal Indoor Air found that formaldehyde release from engineered wood products roughly doubled for every 18-degree Fahrenheit increase in temperature. Sauna temperatures typically run between 150 and 200 degrees, making plywood a poor and potentially harmful choice.
What insulation is best for an eco-friendly sauna?
Formaldehyde-free fiberglass insulation is the most common practical option, and brands like Johns Manville and Owens Corning make versions labeled as such. Mineral wool handles temperatures up to 2,000 degrees Fahrenheit, contains no chemical binders, and does not absorb moisture. Sheep's wool is the most environmentally sound choice since it's biodegradable and renewable, though it runs roughly three times the cost of fiberglass.
Why is vinyl flooring a bad choice for a sauna?
At sauna temperatures, vinyl can soften and release plasticizers including phthalates, which act as endocrine disruptors. A 2021 study in Environmental Science and Technology found that phthalate emissions from vinyl flooring increased significantly above 140 degrees Fahrenheit. Ceramic tile or natural stone is the recommended alternative because it does not off-gas and does not absorb moisture.