Rain, shade and long damp seasons are the hard part of owning an outdoor sauna in places like Seattle, Portland or Bend. Heat inside the sauna is easy. Keeping the outside from rotting is where wood choice, foundation and roof matter. This guide uses published sources and labels each one. Where a fact comes from a search summary we could not open ourselves, we say so.
Wood species and decay
The USDA Forest Products Laboratory Wood Handbook says "Wood deteriorates more rapidly in warm, humid areas" and "Untreated sapwood of substantially all species has low resistance to decay."
| Wood | Decay resistance (heartwood) | Notes |
|---|---|---|
| Western red cedar | Resistant to very resistant | The best natural choice for exterior use |
| Hemlock | Slightly resistant or nonresistant | Usually needs treatment outdoors |
| Spruce | Slightly resistant or nonresistant | Usually needs treatment outdoors |
| Sapwood of any species | Low | Exclude where decay is likely |
The species placement above comes from Table 3-10 of the Wood Handbook as reported by a search summary. We have not yet checked each row against the PDF, so treat it as unverified until we do. The handbook does say that heartwood in the two lower classes "will usually require some form of preservative treatment."
The Forest Products Laboratory's Research Note 153 puts numbers on it. Untreated, very resistant heartwood fence posts "may last 20 or more years," versus "5 years or less" for nonresistant wood. It also says: "When long life is desired of untreated wood under conditions favoring decay, all sapwood should be excluded, regardless of species." When you compare cedar saunas, ask whether the boards are clear heartwood.
Thermally modified wood
Thermally modified ("thermowood") lumber is heated to make it more stable and more resistant to decay. Maker datasheets, which we saw only through search summaries, rate Lunawood thermo pine and spruce at EN 350 durability class 2, and Thermory thermo spruce at class 1 and thermo pine at class 2. Both are rated for above-ground exterior use, not ground contact.
Redwood Outdoors says it heat-treats its hemlock above 400°F (204°C) with "no chemicals involved" and describes the result as "better protected against damage from... weather and insects." It also notes an untreated exterior "will naturally weather to a rustic grey."
Our takeaway
This is our reading of the sources, not a cited rule: for the outside of a sauna in a wet climate, use clear western red cedar heartwood or thermally modified wood, and keep all wood off the ground. Plain hemlock and spruce are better suited to interiors.
Foundations
Every maker we checked wants a firm, level base that sheds water:
- Redwood Outdoors (maker page) lists a concrete pad or footings under each cradle, a solid deck, compacted gravel, or level pavers.
- Almost Heaven (manual, search summary only) asks for a surface rated for 1,000+ lb and says not to set the cradles on bare earth. See its site preparation page.
- Dundalk/Leisurecraft barrel manuals (search summary only) call for a solid, level base with a slight rear slope for drainage: concrete, deck, crushed stone or pavers.
In a rainy yard, the drainage slope and keeping the cradles off soil matter as much as the wood. For pad sizes, depths and material quantities, see what a barrel sauna should sit on.
Roofing a barrel sauna
A bare barrel sheds water, but rain still sits in the joints. Redwood Outdoors ships asphalt shingles as standard. The Dundalk/Leisurecraft assembly instructions (search summary only) describe the optional roof as ice-and-water shield first, then shingles, with no nails through the steel bands that hold the staves together.
We have not found a manufacturer source on metal or membrane (EPDM) roofing for barrel saunas, so we are not recommending either yet. Ask the maker before you change the roof.
Comparing two popular barrels on wood, finish and roof? See Almost Heaven Pinnacle (cedar) vs Redwood Outdoors 6 Person (heat-treated hemlock).
Heater sizing in cold, damp air
Harvia's support guidance (search summary only) gives this rule:
- Start at about 1 kW per cubic metre of sauna volume.
- Add 1.2 m³ to the volume for each m² of glass, stone or other uninsulated surface.
- Multiply by 1.5 if the walls are uninsulated logs.
- Choose a heater whose rated range includes the adjusted volume.
Harvia publishes no separate cold-climate multiplier, so a cold location does not change the formula on its own. A glass front or solid log walls do. We have not yet reviewed HUUM's or Finnleo's sizing guidance. Once you have a heater size, our sauna electrical guide shows the amps and breaker each size needs. The figures come from heater manuals, so they apply anywhere. For the full method in cubic feet, with Harvia and Finnleo room ranges and worked examples, see what size sauna heater you need.
Permits in Washington and Oregon
Building and electrical rules for a backyard sauna depend on where you live. See our guides for Washington, Seattle and King County and for Oregon and Portland, or browse saunas.
Frequently asked questions
What wood lasts longest on an outdoor sauna in a wet climate?
The USDA Forest Products Laboratory rates western red cedar heartwood resistant to very resistant to decay. Hemlock and spruce heartwood are rated slightly resistant or nonresistant and usually need some form of preservative treatment. Sapwood of nearly every species has low decay resistance.
Is cedar sapwood as good as heartwood?
No. The Forest Products Laboratory says untreated sapwood of substantially all species has low resistance to decay, and that all sapwood should be excluded when long life is wanted under conditions favoring decay, regardless of species.
Is thermally modified wood a good choice outdoors?
Maker datasheets (which we could not open directly) rate Lunawood thermo pine and spruce at EN 350 durability class 2 and Thermory thermo spruce at class 1, for above-ground exterior use, not ground contact. Redwood Outdoors heat-treats its hemlock above 400°F with no chemicals.
What should an outdoor barrel sauna sit on?
Sauna makers list a level concrete pad or footings, a solid deck, compacted gravel or level pavers. Almost Heaven says the base should be rated for 1,000+ lb and not to set the cradles on bare earth. Dundalk manuals call for a slight slope for drainage.
How do I size a sauna heater for a cold, damp location?
Harvia support guidance starts at about 1 kW per cubic metre, adds 1.2 cubic metres for each square metre of glass, stone or other uninsulated surface, and multiplies by 1.5 for uninsulated log walls. Harvia publishes no separate cold-climate multiplier.
Keep reading
Browse our saunas collection, see all guides, or ask us a question about your project.