The best materials for a lasting, safe sauna are naturally rot-resistant woods like Western Red Cedar or thermally modified aspen, mineral wool insulation behind a foil-faced vapor barrier, cement board around the heater, stainless steel fasteners, and tempered glass for the door. Avoiding pressure-treated lumber, foam insulation, and plywood is just as important as choosing the right materials.
You're about to build something that gets hot—really hot—and stays hot while you sit in it sweating. The materials you choose matter more than most people realize. A sauna isn't a regular room. It's a high-heat, high-humidity environment (even in dry saunas) that changes temperature fast. Pick the wrong wood, the wrong insulation, or the wrong fasteners, and you're looking at warped walls, toxic fumes, or a fire risk.
I've spent time reading the research on sauna construction standards, thermal properties of different woods, and what happens when certain materials are heated to 180°F. Here's what the evidence says about building a sauna that lasts and stays safe.
The Wood Question: What Grows in a Sauna
Wood is the backbone of any sauna. But not all wood belongs in a sauna. The key factors are: how the wood handles heat, whether it releases anything when hot, how it handles moisture cycling, and how it feels against skin at 170°F.
Western Red Cedar is the gold standard. It's naturally rot-resistant, lightweight, and has a low thermal conductivity—meaning it doesn't scorch your skin the way oak or maple would. Cedar also contains natural oils called thujaplicins that resist mold and decay. Multiple studies on wood durability in high-moisture environments show cedar outperforms most softwoods in resistance to warping and cracking (Forest Products Laboratory, USDA, 2021).
Thermally modified Aspen is the runner-up. Aspen has tight grain and low resin content. When heat-treated (thermally modified), it becomes dimensionally stable—it won't shrink or swell as much with temperature changes. It also stays cool to the touch compared to denser woods. Finnish sauna builders have used aspen for generations. The downside? It's softer than cedar and dents more easily.
Hemlock is a budget-friendly option that works if you're careful. It's knot-free and has a straight grain, which makes it stable. But hemlock is more porous than cedar, so it absorbs more moisture over time. You'll need to maintain it properly, and even then, it won't last as long as cedar in a sauna environment.
Never use pressure-treated lumber. This is non-negotiable. Pressure-treated wood contains copper, chromium, and arsenic compounds (depending on the treatment type). When heated, these chemicals can off-gas. The EPA has long warned against using pressure-treated wood in high-heat indoor environments. Same goes for plywood—the adhesives used in plywood can release formaldehyde and other VOCs when heated above 140°F.
What about pine and spruce? They're common in budget sauna kits, but they're not ideal. These softwoods contain resin pockets. When heated, the resin can seep out, creating sticky spots that are hard to clean and can burn skin. Pine also warps more than cedar or aspen because it has higher moisture absorption rates.
Insulation: What's Behind the Walls
The wood you see is important. What's behind it matters more for safety and longevity.
Mineral wool (rock wool or slag wool) is the standard recommendation for sauna insulation. It's non-combustible, resists moisture, and doesn't degrade at sauna temperatures. Mineral wool has a melting point above 1,800°F—far beyond any sauna temperature. It also doesn't off-gas when heated. A 2019 review in Building and Environment compared insulation materials for high-temperature applications and found mineral wool had the best combination of thermal resistance and fire safety.
Fiberglass can work, but it's not ideal. Standard fiberglass insulation contains binders that can degrade over time with repeated heat cycling. At sauna temperatures, those binders may begin to break down after a few years. If you use fiberglass, make sure it's rated for high-temperature applications and that it's completely sealed behind a vapor barrier.
Never use foam board insulation (polystyrene, polyisocyanurate, or spray foam). These materials are combustible and release toxic gases when heated. The National Fire Protection Association guidelines for sauna construction explicitly state that foam insulation should not be used in sauna walls or ceilings.
The vapor barrier is critical. You need a foil-faced vapor barrier on the warm side of the insulation (behind the wood paneling). This prevents moisture from getting into the insulation cavity, where it would cause mold and rot. Aluminum foil facing is standard. Some builders use craft paper or plastic sheeting—don't. Plastic can melt or degrade at sauna temperatures, and craft paper can absorb moisture.
The Heater Surround: Where Heat Meets Structure
The area around the sauna heater needs special attention. This is where fire risk is highest.
Use cement board (like Durock or HardieBacker) behind and around the heater. Cement board is non-combustible, doesn't off-gas, and handles direct heat exposure without degrading. It's the same material used behind wood stoves and fireplaces.
Keep the wood panels at least 3 to 4 inches away from the heater, depending on the heater manufacturer's specifications. That gap should be filled with cement board or tile. Many sauna fires start because wood was placed too close to the heater.
Tile works as a heater surround, but choose porcelain or ceramic—not natural stone like marble or granite, which can crack from thermal shock. Porcelain is fired at high temperatures during manufacturing, so it handles sauna heat well.
Fasteners and Hardware: The Hidden Weak Point
Most people don't think about screws and nails until the benches start creaking or the wood starts pulling apart.
Use stainless steel fasteners (grade 304 or 316). Regular steel screws will rust from the moisture and heat cycling. Galvanized screws can release zinc fumes when heated. Stainless steel doesn't corrode and doesn't off-gas.
Copper or brass hardware works well too, but copper can stain cedar wood over time (copper reacts with the tannins in cedar, creating dark spots). If you want clean aesthetics, stick with stainless.
Avoid aluminum fasteners in structural areas. Aluminum is soft and can shear under load, especially when heated repeatedly. It's fine for trim or non-structural work, but not for bench supports or wall attachments.
Benches and Flooring: What You Actually Touch
The benches take the most abuse. They hold your weight, they get wet, and they need to stay comfortable at high temperatures.
Cedar or thermally modified aspen are the best choices for bench slats. They stay cool to the touch, resist splintering, and handle moisture well. Space the slats 1/4 to 1/2 inch apart to allow air circulation and drainage.
Never use varnish, stain, or paint on sauna benches. These finishes off-gas when heated and can make the surface dangerously slippery. The wood should be raw. If you want a smoother finish, sand it well with 120-grit sandpaper.
For flooring, tile is the most durable option. Porcelain tile with a textured surface (to prevent slipping when wet) works well. If you use wood flooring, make sure it's the same species as the walls and benches, and leave gaps for drainage. Some builders use slatted wood floors over a concrete base—this allows water to drain away from the wood, extending its life.
The Door: Glass or Wood?
The door is a safety feature as much as an aesthetic one.
Tempered glass is the standard. It handles thermal stress much better than regular glass. Tempered glass is about four times stronger than annealed glass of the same thickness. If it does break, it shatters into small, rounded pieces rather than sharp shards.
Look for double-pane tempered glass with a low-e coating. This reduces heat loss through the door and prevents condensation from forming between the panes. Some manufacturers use single-pane glass—it's cheaper but loses more heat and can feel cold against your skin if you lean against it.
Wood doors work but require more maintenance. They expand and contract with humidity changes, which can cause sticking or gaps. If you go with wood, use the same species as the rest of the sauna and leave extra clearance around the frame for movement.
What About the Floor Drain?
This isn't a material, but it's worth mentioning because it affects material choices.
A properly sloped floor with a drain
Frequently asked questions
What is the best wood to use inside a sauna?
Western Red Cedar is widely considered the top choice because it has low thermal conductivity, natural rot resistance, and oils that resist mold and decay. Thermally modified aspen is a strong runner-up since heat treatment makes it dimensionally stable and it stays cool to the touch. Hemlock can work on a budget but absorbs more moisture over time and won't last as long as cedar.
What insulation is safe to use in a sauna?
Mineral wool, sometimes called rock wool or slag wool, is the standard recommendation because it's non-combustible, resists moisture, and doesn't off-gas when heated. Foam board insulation, including polystyrene and spray foam, should never be used in sauna walls or ceilings because it's combustible and releases toxic gases when heated. A foil-faced vapor barrier on the warm side of the insulation is also essential to keep moisture out of the wall cavity.
What fasteners should you use when building a sauna?
Stainless steel fasteners, specifically grade 304 or 316, are the right choice because they don't corrode or off-gas under the heat and moisture cycling a sauna produces. Regular steel screws will rust, and galvanized screws can release zinc fumes when heated. Aluminum fasteners are too soft for structural areas like bench supports and can shear under load when repeatedly heated.
Can you use pine or spruce wood in a sauna?
Pine and spruce are common in budget sauna kits but aren't ideal because they contain resin pockets that can seep out when heated, creating sticky spots that are difficult to clean and can burn skin on contact. Pine also warps more than cedar or aspen due to higher moisture absorption. If longevity and safety are priorities, cedar or thermally modified aspen are better choices.