Should you insulate the floor?
You don’t have to insulate your sauna building floor, but why not do it?
With a new sauna build, I would insulate the subfloor with 2” rigid foam ripped on a table saw to widths to fit between your floor joists flush up to your subfloor. The rigid foam can rest between the floor joist cavities via 2×2 or 1×2 strips screwed into your joists 2” down from the top. Rigid foam has an R-5 value per inch. R10 in the floor is adequate.
This is the stage at which one would install sleepers on the insides of floor joists and lay down rigid foam:
I’m not a fan of batting in floors. Rodents. And if there’s a leak, which will happen if someone tips over a 5 gallon bucket of water, batting becomes a soggy wet sponge and loses its integrity. That said, our cabin has R19 between the floor joists and all has been fine for 23 years and counting.
Let us also keep in mind that many saunas, especially old school saunas, are built with cedar decking as the flooring. Sitting on the sauna bench, one can look down between the pencil spacing between 2×6″ or 5/4″ round cedar decking and see the ground beneath. I have taken many a sauna in these saunas. The heat is perfectly adequate. Fresh air works its way up between the decking, providing the necessary oxygen to fuel our wood burning sauna stoves. The sauna stove sits on pavers atop the decking. This is a simple system. Sure our feet can get cold, especially in winter, but our feet are not on the floor while we sauna.
Those that are bent on a philosophy of “let it breathe” may be apt to build their saunas with the deck floor open to the air below option.
Our changing room floor: our feet get cold first
Cooling down in the changing room between sauna rounds, our wet feet can easily become icicles when winter cold air comes waifing into our sauna building. A cold floor against our wet feet chills our feet fast, much like trying to wash our cars with a wet rag in freezing weather. This is when , instead of a pair of Crocs or sandals, we wish we had a pair of SaunaShoes, with a thermal mass of PCM (phase change material) that radiates warmth to keep our feet warm between sauna rounds.
We build our saunas once
An insulated barrier within our sauna building subfloor is not expensive. My $100,000 sauna plan details a radiant heat system within the floor, using tubing created by Uponor, the Finnish radiant heat company. This system is a nod toward the thermal properties of a heat mass radiating heat deep into our feet. (more on thermal heat here). This system is a nod towards “ahhhh that feels nice” when it’s a bitter cold, star filled sub zero night outside, as we return to our changing room and feel warmth on our feet, crack a cold beverage, and enjoy casual conversation with our sauna guests, barely making out their foggy faces across the room engulfed in waves of misty cool steam. Breathe in this moist cool air. Stand and stretch. Our bodies become rejuvenated as our minds open up to fresh ideas, thoughts, as antithesis of the Forgotten Sanctity of Sauna.
The authentic sauna experience is more than just the hot room
A warm changing room floor allows our feet to stay warm longer. This allows us to enjoy a longer winter cool down session. Why is this significant? A longer cool down session brings our body temperature back to normal before we go back into the hot room. With our bodies completely cooled, we are able to enjoy a longer, more fulfilling hot room session. More: Rubber Band Theory. The alternative is that, with cold feet, we rush ourselves back into the hot room to warm up, but we are less aware that our bodies are still too warm from the previous round.
Simply stated, Jackrabbing our sauna rounds limits the enjoyment and fulfillment of the authentic sauna experience. An insulated subfloor is not critical, but sure feels nice on a cold winter’s night.