If our sauna walls could talk: insulate with vapor barrier or let them breathe?

There are a couple schools of thought regarding sauna construction and sauna walls:

  1. Insulate, foil vapor barrier and tape the seams well.
  2. Don’t do anything, let the walls breathe.

Those that prescribe to #1 (insulating and sealing our sauna walls with vapor barrier) believe that we need to keep heat and moisture away from our wall cavities (and nowhere near the inside).

Those that prescribe to #2 (letting their sauna walls breathe) believe that heat and moisture want to escape and permeate through the building (and out the other side).

This casual sauna builder is very much a proponent of #1.  Why? 3 reasons:

  1. Log sauna goodness: Most agree that traditional log saunas feel the best.  Sitting on the sauna bench, solid wood logs give the sauna bather a more solid, dense, soft heat kind of feel.  This is the best way I can describe the affinity towards solid wall saunas.  But the point is that a well insulated stick frame sauna plus a well sealed vapor barrier provides a comparable dense sauna wall, replicating (?) a solid wall sauna building.
  2. No moisture leaking:  Warm wet air will always rush to colder dry air. This is why freezers (before frost free technology) used to always get ice build up. “Close the door!” Warm wet air will always rush to colder dry air.  This is an issue when we don’t seal off our hot rooms.  Warm wet air from our hot rooms will rush to wall cavities and settle there as condensation against cooler wood paneling.  Damp, wet, rot.  Not cool.
  3. Thermal containment: A well insulated hot room with well sealed vapor barrier holds in moisture and holds in heat.  “But our sauna walls aren’t insulated and we can get it up to 100 degrees Celsius (212 degrees Fahrenheit)”  That’s great, but could that be like saying that you can eventually cook a pizza with your oven door open?

“Don’t use vapor barrier, let your walls breathe.”  Hogwash, I say.

We build our saunas once.

As you look at the wall cavities of your sauna building, think about how little insulation costs, and how easy insulation is to install.  Think about how easy it is to foil vapor barrier your hot room and how foil bubble wrap is sauna building secret #5.  And think about how this casual sauna builder has repaired saunas that were insulated properly and sealed off properly 30 years before, and how after opening up walls between sauna hot room and outside, this casual sauna builder sees no sign of any moisture, mold, decay whatsoever.  And think about how this same casual sauna builder has opened up walls that were not sealed properly and have found studs and bottom plates that were rotten from moisture hanging on like a wet rag to an armchair.

If our sauna walls could talk, here’s what they’d say:  “Seal off the hot room, don’t let this moisture get in here!”

Sauna framing with Ken in New Zealand.

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