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Log sauna, stick frame sauna, and a practical frame up for building your own sauna (with thermal mass considerations)

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.

John:

Hi Glenn,

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 ?

Thanks, Glenn 

Hi Glenn,

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

Hi 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).

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12 Comments on This Post

  1. A few thoughts that may or may not be helpful:
    I built my first log Sauna in 1995, and have done twenty or so ( 23 maybe, not sure) since then.

    Folks always ask “how long will it take ?…”
    My answer : “It is a Sauna, not a Camaro-so what’s the yank?”
    It is always best to plan your Sauna so you have enough time to relax!

    And of course there are some more serious versions to the reply:
    Depends on ambient temperature, type and moisture content of fuel, the diligence of the fire tender, the type of stove, and of course the heat load of the building ( this is based on section R-values, window and door details, and the desired running temperature ( the range should be 80′ to 100′ C./ according to the Finnish Sauna Society, but of course adjusted for individual preference. ( this is ~176′ to 212′ F.)

    Per your above comments:
    *A slab will add significantly to the lack of comfort as well as the warm up time, I will typically use my own version of an insulated light frame deck on which to build.
    *There are rules of thumb – available from eqp’t. manufacturers , that help with the stove size calculation; in some situations I will recommend considering a stove ‘the next size up’ for a log-built stoveroom-easy to do with woodburners , and your stove supplier will know how to do this.Stove output is matched to room volume.
    * A recent comment from one of my log Sauna clients when asked for a summation of the warm up time:
    ~” in the winter ( here in Vermont) about 60 to 80 minutes to get us to the 160′ to 180’F range, and we will often push to 200′ to 205’F as needed- quicker in the summertime, we love our Sauna!”
    * I believe that the log Sauna is the best (!), but in a residential setting, it will take a little longer to warm up and properly ‘temper’…
    For a lodge or public Sauna-hands down/no doubt about it, the log Sauna will be better because the longer time of use per day will take advantage of the wall mass; once warm it will stay warm…and naturally, it is more authentic, right!?
    Think Globally, Sweat Locally!
    PS : Be aware the your pine logs are going to smell awesome ( + )but also bleed pitch! ( – )

  2. I’m with Nils on this one. When you are going for quality heat, why stop short just to save some time. You don’t raise cattle on grass, free range, and dry age the steaks, only to microwave them at the end to save time. The time wasted in prepping a great sauna experience is worth every minute.

    Nils, I’d love to see some photos of the log saunas you’ve built. Sounds like you have some good experience.

  3. Right Chris,thanks, and there are a few photos on my site; You have probably seen them…The site is sort of out of date at this point (!?), but they are there…
    And as to the heat load profile and warm up time: I learn new things every day-from clients, and from the empirical paths that all builders and designers have at their disposal.
    Two pieces I did not include in my previous posting:
    *I tend to use narrower rather than fatter softwood logs
    *And a Finnish Sauna stove, designed to heat via convection more than radiation- a good set-up you betcha…

    With thanks to Glenn for providing a channel to understanding some of these facets of the Sauna realm…

  4. Amen. Nils: Hope to meet up with you on a trip to NE sometime in my future. Sauna Talk and discuss these things on the bench. Really applaud your dedication to craftsmanship and the fact that heat is not heat.

  5. Glenn, I ordered your online blueprints yesterday and aside from confirmation by PayPal that my donation was received, I have not heard anything else.
    I am excited to get started!

    Since I had used a company email, I want to make sure that the plans did not get intercepted in a spam filter.
    Thank you,

  6. Thanks Glenn for the kind words…and lucky us to have a stoveroom we can count on anytime we like! Not everyone is so fortunate.
    Best,
    Nils

  7. Jeff: I emailed you the plans, and sent a separate email, if we’re still log jammed behind your work spam filter, please email me again, we keep trying! Thanks, apologies for any hickups

  8. Hi Glenn,

    I’m working on building a sauna pontoon, and am really struggling to build it at a reasonable weight.

    My pontoon is rated for 2200 lbs and I would like to leave 800 lbs of that for human weight.

    Currently the design is 12*7 with at 7*7 sauna and 5*7 warm room. I would really like to keep those dimensions, but understand that shrinking them would help reduce weight.

    My current weight includes the following materials

    Structure:
    2*4 fir studs -450lbs
    2*4 Green treat base -175 lbs
    1/4 inch marine grade sheathing- 450lbs
    Tin Roof- 80 lbs
    Doors/Windows -200lbs

    Sauna Interior:
    1*6 Cedar – 420lbs
    400 lb stove
    2*4 Benches -100lbs

    Do you have any recommendations where I could cut weight? I’m sitting at 2600 right now and need to get down to the 1400 range.

    Thanks for any input!

    Quinn

  9. Hi Quinn!

    Great design problem…and I have lots of ideas. I’ve been designing and building Sauna for 30 years-but haven’t done a floating version, so this will be advice from a neophyte… ( but just the same you may want to consider the following:)
    *Buy a Finnish sauna stove ( ~ 190# with stones)…
    *Balance the ‘float’, and skip the changing room ( you are already ‘changed’, and will not likely be using this in the cold seasons)
    * 2 x 4 benches are unnecessary ( strength – wise ), and a waste of lumber-I used to tell students” If you go into the woods to cut trees to make lumber, you are duty-bound to use it efficiently-and with little or no waste”
    I have lots of other ideas-but this will suffice for the high spots!
    Good luck!
    Think Globally, Sweat locally-
    Cheers.

  10. Glenn,

    I am currently in the process of building my outdoor sauna. I have 2×4 walls with 2×4 trusses. I am curious if I need to put slats over the foil backed barrier and then nail my tongue and groove to the slats leaving an air gap between the tongue and groove and the vapor barrier or if I can just nail the tongue and groove to the studs and tight to the vapor barrier. Also do I put the barrier on the ceiling in the sauna and in the changing room.

    Thanks for the information,

    Ben

  11. Ben: As detailed in “Sauna Build, from Start to Finnish” we apply our T&G right against the foil vapor barrier. There has been some chatter about micro piercing of foil: how with every finish nail going into every board and then through foil into studs, well, this method breaks the foil vapor barrier.

    I get this!

    On paper and in the brain, it makes sense. And the way around it is to, as you detail, put in slats or firing strips over the foil and nail t&g to the slats.

    But I don’t do this.

    It decreases the footprint of the hot room (albeit ever so slightly). It could be argued that moisture may work its way behind the t&g and settle within that cavity. It could be noted that the paneling is kind of “tin-ny” or thin when leaning against, with air space between studs.

    So, as we revisit the situation, finish nails piercing the foil is a “yuck” but let’s really think about it.

    Moisture will travel only if encouraged. Like when we open a freezer, warm moist air is encouraged to move toward cold dry air. Whereas in our sauna hot rooms, moisture is never encouraged to travel from our hot room, through the wood of our walls, finding a microscopic gap in a finish nail, and into a wall cavity where there is no reason to be there in the first place (no humidity differential, no air pull, etc.).

    So there you have it, some ramblings about the pros and cons of slats. I welcome anyone else’s opinion on this nuance.

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