Below is an email exchange that we thought worthy of putting out to the universe, for others who are in the planning (but not zoning) phase of building their own sauna.
I hope you are doing well and enjoying winter saunas. I’m still working on plans for an outdoor sauna and would really prefer to build with rectangular logs.
Having trouble finding someone who understands the answer to the following: will thicker log walls (eastern white pine) take longer to heat up due to the extra thermal mass having to first heat the logs, especially in colder weather we have in the UP? For example: 4″ or 8″ logs – will there be a noticeable difference in warm up time? Sauna interior dimensions are 8′ x 8 1/2′ x 7′.
I also realize a thicker log has a higher R value which retains heat better, but the warmup time is the issue as the sauna will only be used for an hour or so.
Would you know anyone, or yourself, who builds log saunas that could help with this question, it affects everything on how the sauna will function.
Thank you so much for your help over the past couple of years, John
John: Happy to help.
You and I both know the answer. It will take longer to heat a solid log sauna. No question.
And you spell out why: thermal mass. I am a big fan of well insulated stick frame w foil reflective vapor barrier. This system offers reasonably fast heat up times and excellent heat retention.
But food from a cast iron frying pan tastes better than from a thin Teflon aluminum pan. Density.
Same for sauna. Lead a blindfolded sauna enthusiast into a sauna built with log vs stick frame and I’d like to think we could feel the difference. Right to our bones.
But lead that same person into the same two saunas fired up, say, an hour before and they’ll prefer to stay in the stick frame sauna. As you mention, log sauna is going to take longer to get up to serving temp.
How much longer? I have a great relationship with the mechanical engineering department at the University of Minnesota. Your Michigan Tech has an excellent mechanical engineering program.
How about we work with them to do some modeling ?
I’m up for helping if you want to lead the charge. Until the math comes forth, assuming identically insulated ceilings and sizes, I predict log sauna would take 65% longer time to get to 180f.
What’s your prediction ?
I will give that some thought about MTU. Also, my brother did heat transfer for a living – he may be able to do the calc’s.
Your experience says a lot and may have to rethink the whole log idea. I like traditional, but don’t want to sit around too long waiting to get a quick sauna.
I have your sauna plans and am also rethinking slab vs wood floor. The slab will suck up some amount of heat, which in the winter is a big negative. Don’t want to make a mistake this time around in building a cold sauna.
Thank you for your help and great insights, John
It sounds like heat up time is a barometer for you. And I hear you there.
If I were to build a sauna with this as primary consideration I’d do this:
Floor: 2×6 green floor and rim joists. Double 2” rigid between joists flush up against 3/4” subfloor. Durarock and skim coat.
Walls: 2×4 16” oc. R13. Foil bubble wrap.
Ceiling; 2×4 or 2×6 with R13 or R19 between rafters, respectively. AND second layer of insulation above hot room ceiling rafters cross wise on top giving double action up there, up to R38.
This is most important. A well insulated ceiling holds more heat than well insulated walls.
And I like the thicker 5/8-3/4” tongue and groove vs the 1/2” marginalized paneling out there. Thermal mass,
Stove: wood for sure. BTU output big leagues.
Stove surround: Here is the opportunity for thermal mass enhancement. Double Durarock walls with air gap between. Face Durarock with cultured stone or thick tile. Here is where the cast iron frying pan concept can resonate to life.
Good chatting, John. You’re off to the races!. g.
Editor’s note: We have a great appreciation for traditional log saunas. We are clear to define the preference for stick frame relative to primarily both heat up times and construction practicality. We recognize the potential superiority of log as a structural preference for a deeper, resonating heat (especially savusauna and all it’s virtues).