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

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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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25 Comments

25 thoughts on “If our sauna walls could talk: insulate with vapor barrier or let them breathe?”

  1. Couple questions that aren’t addressed in the booklet.

    1.) I’ve read a few insulation nerds talking on line about leaving an air gap between the reflective insulation and the interior cedar. Do you sandwich that reflective insulation roll directly in between the studs and the T&G Cedar? What’s your opinion on the air gap question?

    2.) I used rolls of reflective bubble insulation in my original sauna build in TX, and it works great…I’m wondering about a product like this with the corners taped and sealed. Would it not perform the same function with similar results??

    Also, As I move towards my second build in MT, I was curious about using stacked stone for parts of the interior walls (below upper bench height, so as to not be at risk of touching it!). It’s pretty hard to find info about that (+ or -) b/c when you search “sauna” and “stone”, you get overloaded with ‘what are the right stones to put in the heater?’ Type of answers…

    Do you have any guidance on using stone internally?? Better thermal mass than wood? Harder (longer) to get to temp??

  2. 1. Air gap between foil and cladding:
    If using a species like spruce, aspen, basswood – yes for sure. These type of woods don’t do well with prolonged moisture. (See bake and breathe method post on SaunaTimes). The air gap is a hold over from Finland where spruce is the primary cladding species.

    Cedar holds up very well in moist conditions. The air gap with this species is not necessary. I know this is true. Decades old saunas and holding up fine.

    This business of needing air gap to reflect the heat back in the room has been refuted by a thermal engineer I know, but I’m not privy to his formulas and chatter at this time. I just remember the conclusion.

    2. Foil non bubble wrap is a better vapor barrier.
    Foil vapor barrier is less expensive than foil bubble wrap. It is easier to work with, and more free range organic than the bubble wrap. I was foil bubble wrap happy back in the day when this is all we could get, and readily available via pushing our cart down isles at the big box like Depot Menards Lowe’s.

  3. Glenn, I tend to disagree here. First, lets be clear, as you note, real sauna foil is better than foil bubble wrap- which is a polyethylene sandwich, and polyethylene melts at 150° F. It is also easy to purchase by clicking on any sauna supplier’s web page—way easier than pushing a cart through a big box store.
    But as for the air gap (maybe I am am that “insulation nerd”), 1: it is essential to the radiant function of the foil since foil in contact with hot cedar wall will conduct the heat away from the wood and towards the wall cavity and 2: cedar is not as holy as you claim . Not only can it rot (I, too, have deconstructed many saunas) but letting the wood breathe will help keep it stable and happy (cedar can and does shrink, expand, cup and crack).
    I have done experiments in the shop to verify #1 and built too many saunas and other structures to not notice #2.
    Sure, anyone can build a sauna any way they want, and many old Finnish saunas were just that— built by farmers and miners with only basic carpentry skills, but these days we do have building science to back up good craftsmanship; something any good builder should adhere to. I certainly do.

  4. Hi Rob:

    I have a good friend who is a thermal engineer. We wrestled to the ground whether an air gap vs. non air gap effects the foil radiating heat back into the room. He was going on and on about the temperature at the hot room side of the wood paneling, vs. the temperature at the back side of the wood paneling, and how the relatively cooler air in that air gap cavity is a good thing, but doesn’t affect the radiant capabilities of the foil.

    So, that aside, air gap to help keep the paneling dry and long lasting is a great thing. Especially Aspen, poplar, basswood and the common white spruce from the Motherland (Finland).

    I do know that several saunas lined with cedar in my summer home terrain of NE Minnesota without air gap have held up for several decades with zero degradation. Good ventilation and bake and breathe method are likely the reason.

    I respect the insulation nerd in you, and share your devotion to “optimizing” our materials and design for best performance. I get weird about this also. Eg. I can feel a hot room difference between 1/2″ paneling vs. 3/4″ paneling. It may be interesting to try to quantify this some way. Some thermal couples inside the wall cavity, etc. My thermal engineer buddy did this. He got a frown from his wife, but he made some mock wall panels and put them in front of his open oven, and did some thermal modeling/testing, and found that the air gap vs. non air gap was minimal difference in heat loss through the wall, which, as I remember, was his way of evaluating whether foil was adding heat back to the room, or just doing its job as a vapor barrier.

    I do like where you are going with all this, and we are kindred spirits of this kind of stuff.

  5. I am looking into building my first sauna, which I intend to build outdoors, off-grid and in the most traditional / authentic way possible. Would you recommend building a solid wood log sauna with no insulation over a stick-frame build with vapor barrier and insulation? I have access to as much lumber as I need in any size, and have really been liking some of the builds I’ve been seeing on youtube (https://www.youtube.com/watch?v=aN47VMnRc-U) or (https://www.youtube.com/watch?v=Gf3921HtCUk). I have constructed buildings of this type before, and so I was wondering if there is an advantage for using this simple method instead of stick frame construction with insulation and vapor barrier. I am hoping to use the sauna year round.

  6. Hi Jessica:

    Log sauna is considered the top of the line, vs. stick frame. The traditional smoke saunas (savusaunas) are log. There’s a lot of theory and practice as to why this is better. A lot of it has to do with lämpömassa (and all its goodness). If you’ve got the logs, and the “oomph” to make this happen, I’d say for sure do it. The only downside is that it takes longer to build up the heat mass of the logs, but once you’ve heated this log sauna, you’ll feel a really dense, fulfilling heat. And you’ll need a kick ass sauna stove and I recommend the Kuuma (it’s the bomb).

  7. Hi there! I’ve been loving your information, I had a question regarding climates. I live in on South East Australia. Our summers can get to a humid 35 degrees Celsius/95 Fahrenheit semi regularly, and our winters at night drop into the single digits on occasion.

    For me, the insulation is definitely angled at keeping the heat in, but I’ve heard some things that in humid climates, one might rethink the vapour barrier. I personally don’t see even at a 35 degree humid day moisture building up in a wall to the point of causing rot, but I could be wrong.

    I’m just wondering if you’d have any differing advice in the installation for someone in a climate like that, compared to North America.

    I know your stance on vapour barrier against or at least close to the inside panelling of the sauna. My consideration was initially inside panelling > 1/2 inch gap> foil backed phenolic foam board > 40mm rock wool batting > to outside panelling (in this case untreated pine)
    (I know phenolic isn’t common, let me know if this would not be recommended, and also if adding the foil vapour barrier over the top of the foil backed foam board would be better.

    This is meant to be for an easy set up, and pack up design. I appreciate any input you can give me.

  8. I hear you. Most all my sauna action and building is in cold climates. So, I’m trying to assimilate to your climate situation. and here’s what I think: You could get by with no insulation and no vapor barrier and just go to town. That’s as easy of a set up as their is, and it just may get you where you need to go in your warm, humid climate.

  9. Thanks so much for your response Glen! I missed this. No insulation, very interesting! Do you think Tongue and Groove panelling would still be adequate? With no vapour barrier, I imagine there might be some leakage of hot air. I guess in a warmer climate this might be negligible.

  10. Hi Glenn,

    Just started framing up my outdoor sauna in Mpls – 6×8’ with electric heater. In our excitement we hung the T1-11 siding on the 2×4 studs without installing the housewrap. Planning on a perforated foil thermal paper on the interior of the studs with mineral wool inside the cavity. Do you think it is worth removing the T1-11, putting the wrap on there, and reinstalling? There are only 1 seam on the siding panels on each wall, in addition to the corners which I plan to seal well with silicone and spray foam. Will this be a big problem?

  11. G’day W.Q.burrage and CO. I’m also South East Vic. Built a house and left the sauna room till now, a few years later. I insulated and clad the Sauna room with bathroom wetboard, save dust etc in the interim. I’m thinking of just silver paper and Cedar over that. Western Red Cedar VBoard. I thought silver paper was to stop heat getting into my pine framing. Should I rethink this?
    Thanks

  12. I’d rethink this and maybe “gut” the sauna, down to studs, keeping the insulation, then foil, then western red. A guy could drive himself crazy thinking about possible moisture trapping, heat sink, yadda yadda, and if you do this extra effort, you’ll be no second guessing on the bench, just chilling.

  13. GT, i’d not bother removing the T1-11. I’d be using non perforated foil vapor barrier though. This will provide a moisture barrier.

    Housewrap goes on the outside of your T1-11, and behind your siding.

  14. Hi Glenn, I’ve been reading through your eBook and have really enjoyed it, and found it useful. Right now, I’m on the stage where I’ve installed the first layer of cement boards, and getting ready to install the second layer of boards with a 1″ gap. I got a bit excited after getting the boards, and installed them prior to foil bubble wrapping the hot room.

    In the book you mention that there are two schools of thought with the cement boards, and it seems that the foil barrier isn’t necessarily needed behind them. Is it worth while it to take down the existing boards I have screwed into the wall, to put bubble wrap behind, or just stay the course?

  15. I’d keep going. As long as you have a good seal so that moisture won’t get behind your foil, you are in great shape.

  16. Rockwool wool insulation is great. It has a higher R value per inch than fiberglass batting. Further, it is ok if it gets wet. But this won’t happen as we are foil sealing vapor barriering the walls.

  17. I have just built a barrel sauna and am installing the stove today. The wood is thermally modified Nordic spruce. I will also add a corrugated metal roof secured on 2×2 slats attached to the outside of the sauna. The slats will not run the length of the sauna. There will be gaps to allow airflow between the outside of the sauna and the metal sheets. I would like to insulate the roof under the metal panels and have considered using rockwool or another type of fire and moisture resistant insulation. Would you still use a vapor barrier of some sort? And if yes, could you recommend a specific type and where to place it? Between the wood and the insulation? Or between the metal and the insulation? I have seen it discussed both ways. I am in Southwest Colorado at 7700 ft elevation. In winter we get subzero temps and several feet of snow. It is otherwise a fairly low humidity area the rest of the year. Thank you in advance.

  18. Mark: At first I was going to say “no vapor barrier.” And yet I pause, as I imagine the staves of yoru barrel sauna opening and releasing steam, which, without vapor barrier, would absorb into your rockwool (the right choice!). This won’t be the end of the world, as Rockwool is ok with moisture, but not good with prolonged moisture.

    So maybe we go..
    1. barrel spruce
    2. foil
    3. slats/rockwool
    4. metal sheets.

    I like this plan. It is a bit unorthodox, but it is our best shot, given that your replicating stick frame sauna construction, basically, albeit from the inside out.

    I like it.

  19. Mark,
    Vapor control is a bit more complicated and nuanced by climate. In your zone you’d need several inches– at least 6″ ( R 21)but probably more-of batt insulation behind the spruce to offset the cold. Moisture will condense on the cold surface of a vapor barrier; the colder the climate, the more insulation you need behind (to the outside of) the insulation so that surface remains above the dew point. The IBC building code book has guidelines. Without this, moisture will collect behind the spruce. On the wall, with an air gap, likely no problem, but it may pool in the ceiling and stain the spruce.
    I’d suggest 4″ of polyisocyanurate foam above (it can be bent), it’s the board used on roofs- with an air gap behind the spruce. But really, checking with local engineer is best as the rules are very different for different climate zones.In Humid Australia for example, moisture is moving INTO the walls of the structure when it is cool; VB in the wall behind the cedar will trap moisture behind it- on the outside. This has lead to sometimes toxic mold in places like Texas. In any case always know which way the wall assembly will dry to (inside or out) and never put VB on both inside and out. Building Science Corporation is a great source for info. But also be assurred, you (Mark) are in very dry climate, so moisture that gets into the assembly will likely dry out anyhow.

  20. I’m building a shipping container Sauna and using metal studs and spray foam insulation. Has anyone used Schluter sheets as a vapor barrier? Then I plan on securing #2 Cedar T&G to the Schluter sheets. Thanks for any comments/suggestions

  21. Hi Glenn,
    I was given enough foil-faced polyiso boards to insulate the entirety of my 8×8 wood fired sauna. However I am getting mixed reviews on whether this is a safe product to introduce into a sauna that will experience high temperatures. I wonder what your opinion on the matter is? Is it worth changing course to a less synthetic insulation variety?

  22. I had a conversation with an Engineer at John’s Manville (one of the main manufacturers), with a service temp. rating of 250° F, foil-faced polyisocyanurate board is acceptable in a sauna. My own shop test have assured me that the temp. of the foil behind the cedar (with and air gap) doesn’t get too close to this (about 175° F ). Repairs of older installations have confirmed that it holds up well. However, I avoid its use near the wood burning stove (my walls can get 300°) and use mineral wool and sauna foil (rating: 1000°F). I’m also sensitive to off gassing, so I tape all edges and seams with 3M 3381 tape or better (also rated to 250° F).. BTW, all US products have a data sheet you can download from the manufacturer- so best to go to the source for this info.

  23. Regarding foil faced polyiso: I’ve look at life from both sides now, and still somehow, my opinion is no on this material Yes, the Johns Manville service temp rating for the product is 250°f. but There is the of the argument regarding foil faced polyiso boards and sauna.

    There is the ASTM standards, and then there’s what we think practically to what adds up to longer time frames in a controlled small space environment. I just sleep better at night not using polyiso and sauna. that’s me.

    On the hand of “let’s

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