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Sauna Floor – How to insulate without creating moisture/mold problems ?

tundracycle | Posted in General Questions on

For an exterior sauna the typical construction method (in Europe) is a wood frame with the floor boards (usually pine or spruce T&G but I’ve seen oak and other woods used) attached directly to the joists and very slightly sloped towards a trough drain. 

Traditionally these were not insulated so the bottom side of the boards could dry easily but over the past 30 years they’ve learned that insulation helps considerably in reducing interior temp and steam stratification. 

The method that seems best so far is to partially fill the joist bays with mineral wool. Enough to insulate a bit but leaving about 1-2″ gap to the underside of the floor boards. And then something to protect from varmints like a very fine screen + hardware cloth.  This has generally seemed to work well. 

I think some people who use a lot of water (dumping buckets over their head) will lay rubber mats down to get most of it to the drain directly to the drain.

Wonder if anyone has any thoughts on a better way to do this?

Thanks,

——
Note: Concrete slab or duroc/mud on a frame + tile is frowned on. Primarily because it becomes a heat sink that sucks heat out and increases stratification unless it is extremely well thermo isolated (both to reduce thermal bridging and insulated). It is also complicated to build correctly. 

An alternative is slab or mud with hydronic in-floor heating. This is the best solution but adds considerable extra expense/complications that many people don’t want to deal with.

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Replies

  1. gambein | | #1

    Hi - I have the same question and am just wondering what option you finally went with, and if completed, how it seems to be working?

    1. tundracycle | | #4

      I've done a couple just as I described above (actually from Trumpkin's Sauna Notes) and so far they're working OK. I'd like to know how they'll be in 10 years though.

      I think I'm going to try Ben's idea but rather than plywood do boards as I do now, a tin 'pan' covering, and then duckboards floating on that.

  2. benwolk | | #2

    You say no durock/mud but why not build the floor like a shower pan but without the tile on top? Slope the tops of the joists so you don't need a mud layer, put a layer of plywood down and then coat the plywood with an epoxy finish that is compatible with sauna temps (though the floor of the sauna should stay pretty cool). Then float the wood floor on sleepers over that. You maintain the wood floor finish but the drainage layer is waterproof and non-pourous. You could even go with a tadelakt floor finish instead of the epoxy: https://tadelakt.com

    Should be low thermal mass enough to not affect temp stratification and just insulate the floor really well with mineral wool insulation.

    1. tundracycle | | #3

      Thanks Ben, I think something like that could work. I'll have to think through the plywood & epoxy and maybe find an alternative. Even though temps are fairly low near the floor that could still be a lot more off-gassing than would be good in a sauna.

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