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I am designing a 4'x4' sauna. Most information online is for indoor saunas. From what I've gleaned, the layers should be (for example):

Outside ––> cedar exterior ––> tyvex ––> strand board --> 2x4 frame with insulation (what grade?) ––> vapor barrier ––> cedar interior

Is this sufficient? Is it overkill?

It's an outdoor sauna and so should be able to retain heat year round. Are there any special considerations to be taken given that 1) it will be used outside and 2) it will be a near infrared sauna heated by red heat lamps (so much less moisture than a traditional steam sauna, dry rather than wet heat).

I need the sauna as part of a health regime to manage a severe mold sensitivity, and so as you can imagine, insuring there is no mold growth is quite important!

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Since it is a sauna for occasional use, and not a full blown living space, I would build it like old school carpentry like you already suggested with one improvement.

Outside-going in, Cedar siding, tyvek or other breathable air barrier, OSB or plywood subsiding, 2X6 studs with R-19 fiberglass insulation, then your cedar interior finish. This arrangement of materials will allow moisture to pass through the walls so no droplets can form anywhere. If you use any foam board, this will block the passage of moisture through the walls, whether it is the occupants sweating or humidity differences from inside to outside. Heavier concentrations of moisture always want to move to the not so humid side, to maintain equilibrium. If they don't condensation can occur and mold can result.

Last thing, if the outside envelope is not built right, that will allow moisture in as well causing problems.

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Is Tyvek breathable? I thought it was a moisture barrier. – wallyk Aug 13 '14 at 5:43
It is permeable, it allows water vapor to pass through, not water droplets. If I remember right it has a perm rating of 35. Plastic has a perm rating of 0, tar paper, depending on the temperature has a rating from 5 when cold to 15 when its warmer. – Jack Aug 14 '14 at 13:10

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