Some clarity regarding windows in the sauna

From the mailbag:

Hi Glenn,

Can you please give me your take on importance of opening windows vs.  non opening in the hot room? in the change room?  I am getting ready to build my windows……..Thanks for all the help.
Hi Jim:  A sauna hot room is – or should be – a small space.  50 square feet is a good hot room size.  One of the ways to give a hot room a bigger, more open, inviting feel is with well thought out window placement(s).
So you know where you’ve been and where you’re going, I like a 12″x12″ window centered in the sauna door.
simple spring for a sauna door.
The candle window: a window between the hot room and changing room provides some great continuity between the two spaces.  Read more about it here.
A lively open hot room: sauna door, candle window and all cedar tongue and groove.
A lively open hot room: sauna door, candle window and all cedar tongue and groove.
As far as a window from the hot room to the outside, the use of a fixed transom window is a nice feature as it offers light and vistas to the natural world, while providing some relative privacy compared to a conventional size window.  Here’s a good application of a hot room transom window, as viewed from the outside:
8x12 authentic backyard sauna.
8×12 authentic backyard sauna.

Fixed Glass Windows:

The beauty of making your own window, whether an external transom window or a candle window to the changing room, is that you can make it literally to the exact size of your choosing.  I recommend firing up the Googlator and typing in your city and the words “glass” and “window” and ordering your custom size 1/4″ double pane glass from a glass company to fit your window opening(s).  I’ve always been pleasantly surprised by the cost.  $30 for a smaller candle window.  $50 for a good size transom window.  As you’ll be trimming around your doors, etc. you’ll have no problem setting your own window.  And this will be super reasonable compared to buying a new window from Pella or Andersen.

Functioning Windows:

Using windows that open and close can have their place in sauna building.  In the hot room, one can open the window to help air out the sauna, allowing heat and humidity to escape.  This feature is especially useful if the sauna building is being used for other functions, like a guest cabin.  That said, sauna hot rooms without a functioning window can be vented pretty well by just propping open the hot room door.  In the changing room, I find using functioning windows an asset.  Optimal chill out / changing room temperature is about 30-50 degrees f. (my opinion).  The window sill becomes a good spot to store a cold beer between rounds on a cold winter’s night.

Trimming around windows is a great expression of craftsmanship.

During the paneling stage, as the amateur sauna builder becomes more and more proficient working with tongue and groove cedar, it is rewarding to extend the craft by trimming out your windows.  Ripping trim (table saw), cutting lengths (miter saw), and applying trim (Finnish nailer) start to take shape, and create the “wow” you deserve with our authentic sauna build.
Trim around the sauna windows.
Trim around the sauna windows.

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51 thoughts on “Some clarity regarding windows in the sauna”

  1. This may be useful to someone, as I’m currently building my own outdoor sauna, after searching for a well priced piece of tempered glass online, I ended up finding a place in Burnsville, MN that sold me a 1/4 thick tempered glass cut to 24″x36″ for $32 with tax. Check out River City Creative Glass in Burnsville if you need glass. (Again, no association between them and me)

  2. Great topic! Just a few things to add on what Glenn already covered in the above post.

    Fixed windows are probably best and easiest for hot room to provide a pleasant view, etc. Functional windows are harder to incorporate into a hot room but are actually very commonly found in saunas throughout Finland. The challenge is finding a properly constructed functional window so that it holds up to the hot room environment – vinyl is probably the poorest choice because it may deform and deteriorate from sauna environment, metal may hold up but will be very uncomfortable for end users to touch or use, wood is probably best so that it is comfortable for the end user to touch when opening/closing. Fixed and/or smaller functional windows are best for hot room in my opinion. Functioning windows in changing room are recommended and always a nice touch – these can be any type of window since heat will not be a factor.

    Great post!

  3. keep in mind that an operable window never seals up as well as a fixed window, no matter how well they are built. for a hot room application on an exterior wall, this isn’t necessarily the end of the world but if you are looking for the ultimate in trapping heat, a fixed will beat an operable every time. of course, any window between the hot room and the exterior is going to be a massive heat sink. this is why the window between the hot room and changing room is an excellent compromise, especially if the changing room has an exterior window such that you can still see outside from the hot room.

  4. I prefer to have an functional projection window that I can open and air and dry out the sauna & leave open when not in use. It is nice to have a window to the outside so you can watch the seasons or a moon lit night. Stay away from vinyl windows as they will crack. I would also recommend a tempered glass although my own sauna has a standard window glass and has survived temperatures over 250°F. If you have a decent stove I would not be too concerned about heat loss from an exterior window as most wood fired stoves (kiuas) put out 60,000-80,000 BTU’s a few gaps here and there won’t mater. However, adequate insulation on the ceiling and foil wrapped walls are a must to minimize heating time.
    PS. Another US misconception is that you need to have cedar lined walls. Most Finnish sauna interiors are made from pine or spruce. Cedar tends to encourages mold & mildew growth which reduces the air quality and results in a urine like odor. Cedar is good for exterior uses but the heat of the sauna will dry the wood and minimize rot. My sauna is 10 years old my walls & ceiling are pine and the benches aspen and I use it every week and the wood has not rotted yet.

  5. basically a window is not needed at all in the actual sauna room (löylyhuone).
    the fact is that every square meter of glass in the sauna adds one cubic meter of the virtual volume as well…meaning that u need more power in the kiuas – hardly anybody knows this.
    sauna is supposed to be dark, thus allowing saunagoers to concentrate on the real core of the sauna!

  6. We are putting a fixed wood window with tempered, insulated glass in our sauna. We have been trying to figure out what insulation to use around the window. We are thinking of the spray insulation for Windows but then I worry about fumes although the insulation will be covered up with the foil. Do you have any recommendations for insulation around the hot room window?

  7. Yes, Kathy… i’ve used spray foam around windows with very good success. Yes, all gets hermetically sealed via foil bubble wrap.. foil tape well applied.

  8. the classic ‘great stuff’ foam is rated for exposure up to 240 degree F. considering it will be behind trim, actual exposure temp will be much less than in the sauna, no worried using the product. be sure to use a foam designed for windows or doors. ‘regular’ foam has greater expansion characteristics compared to window/door foam and there is a risk that the expansion could warp the window/door. this is a bigger deal for doors or operable windows so with a static window like in a sauna, you could probably use either kind. for peace of mind though, i would go with the window/door foam.

    in my sauna, i just stuffed some fiberglass insulation between the window and frame for the window between the hot room and changing room. hot room side was sealed with foil tape, changing room side had no seal. wood trim covered everything up.

  9. Thanks Glen! We did go ahead an use the Great Stuff product for doors and windows. Should we have used the fire retardant flavor instead? The window is on the opposite side of the room from the wood stove in our layout.

  10. Miller nailed it (as he usually does). I think any flavor Great Stuff works as long as it’s the minimal expanding (great) stuff, so it doesn’t warp the windows. Glad your sauna build is coming together Kathy.

  11. the fire retardant foam is not necessary. the purpose of that type of foam is to slow the progress of a fire from one floor of a structure to another through the various gaps/openings surrounding pipes, wires, ductwork, etc. without the foam, a fire in a basement could easily sneak through unfilled gaps up into the walls of the floor above. ‘regular’ foam doesn’t have the same fire resistance characteristics as the fire rated-stuff. it has to do with how it burns and is not related to temperature exposure. the fire-retardant foam is a building code requirement for residences but doesn’t really apply to a detatched sauna structure. besides, if your sauna is on fire to the point you are worried about fire passing through a window gap, you probably have bigger things to worry about. 🙂

  12. Wondeing if it is safe to have a window in the sauna room when temperatures get down to -20C outside, is there a danger of the glass breaking?

  13. Rob:

    Great question. I’ve had no problems with heat extremes cracking glass window in sauna hot room, and frankly I thought I would. And i’m speaking as a Minnesotan who can stand outside between rounds in -20c, take a sip of beer, and scratch my head trying to figure out why the glass doesn’t crack. And i’ve built hot room glass windows in a bunch of saunas (I especially like a 36″ wide x 18″ tall transom window). I do have one suggestion, though: as you trim out your glass in the hot room, don’t go too crazy with the finish nailer, so if the glass does crack, you can pull off the trim and window jam and replace glass. Oh, and run a bead of silicone where window butts against cedar trim, to keep moisture from getting down in there.

  14. I am Building a sauna in my house with a (not to open) window to the outside 27″ x 71″. But I am in great doubt what kind of glass to use? Should I use a tempered glass alone and in what dimension – or a tempered and insulated glass with a tempered energy glass in the outer/outside turning of the two glasses?
    I would be more than happy, if you could help me!

    John Willumsen

  15. John: Great question. I’ve been brewing with a follow up post on this topic for saunatimes, but i haven’t yet been able to get a definitive word from an expert in the glass industry. Thicker glass to hold up to temp. extremes is good. Tempered is good for a sauna door, in case the glass were to break.

  16. What materials are best for operating window frames (to exterior or interior/changing rooms)? I guessed and have seen written here that vinyl windows will crack and I presume warm madly and quickly become difficult to operate. Wood with an exterior vinyl clad (important for my environment), I expect would have similar problems.

    Full wood frame would rot too quickly in my environment or be ned constant maintenance on the exterior.

    I suppose my choice would be aluminum. Although it would become hot to the touch, it would be durable to the interior and exterior environments for a long life.


  17. Heasley: All logical thinking and rambling (and I have been right there with you on all that you mention).

    Operating windows in changing room:
    Interior wood clad.
    Exterior vinyl.

    Best results with this. I have gone with wood exterior but only to match primary dwelling.

    And while we’re talking, Let’s talk about windows in hot room:
    The way I have advanced is to not go with functional windows in hot room. We are having great results using fix glass and framing around for transom, candle, hot room door windows.

    Need venting? Install a vent.
    Need fresh air? Pop open the door for a few seconds.

    Rationale: Functioning windows just get all messed up over time (as you mention). Way too much stress from excessive moisture, condensation, expansion (as you mention) and it just gets to be a pain in the buttox region (akin to knots on sauna benches).

    So, hope this helps. I like functioning double hung wood clad windows for changing room. Fixed glass for hot room. Oh, and I like a LOT of windows in changing room. Makes the space seem bigger.

  18. I’ve fallen in love with sauna bathing in the last few years, and realized as we were starting a bathroom remodel that I could fit one in….though I have some issues, in that the place I can put it has a lot of glass! I’m scrambling to figure out the details so the contractor can proceed, and I would so appreciate knowing if it’s not going to work, before I build it! I’m having such a hard time finding advice.

    The interior dimensions will be 6’w x 6’d x 7’h. It will be 43% thick, tempered glass (2 full walls and a window). Said another way, besides the roof and ceiling, I’ll have one full wall of wood and another wall with a 5’w x 3.75’h window. The window is an operable Marvin window with unpainted white oak frame.

    And one more issue I see, is that the only place my heater can go is underneath the window. I could bump up the size of the heater, but would that endanger the window?

    Are there any good rules to follow regarding the amount of glass? I’ve loosely heard that up to 50% glass is acceptable, but others that say it’s too much. I really would love to have this sauna, but because of my constraints, I can’t design it with more wood. What do you think?

    Thanks for any help!!

  19. What I am thinking: there is that great line: “constraints are often blessings in disguise, as they get us to think more creatively.” So, in your case, if I were undertaking this project myself, i’d be creatively sensitive to two things:
    1. Temperature of glass above your sauna heater.
    Once you decide on your sauna heater, i’d play “stump the dealer” and ask them what the radiant wall temp is xx inches above the heater. If they start shuddering or stammering, ask them to refer you to someone cool who has recently purchased and installed the same heater, then have them get a laser temperature gun on the wall above their sauna, and then you’ll know. With that info. then you can go to a non-phone-it-in glass supplier and ask them about temperature tolerances of that glass, and see if your numbers are in sync.
    2. percent of glass relative to overall hot room.
    The issue here is OUTSIDE window. Experientially speaking, outside windows are cold, as we know. I’d be calling Marvin (they are a MN company, so you’re sure to find someone nICE up their food chain) and start by asking them what the temperature tolerances are for an exterior window such as yours. These folks get paid to be cautious (much like the building inspector and insurance man), so you may want to qualify your question with some framing up of the situation like, “not holding you to this answer but…” or “if we were hanging out in your backyard having a drink, would you…” to help derail them from the employee soldier marching minimize liability corporate speak.

    Hope this helps,g.

  20. I am building a unique sauna with 3 solid walls and one window wall. The windows I have found came out of a large residential building near the ocean so they should be tough. The glass is 5′ X 6′ with a metal frame, and is double pane. I do not think it is tempered but not sure.

    Was thinking about using 2 of the windows to have an inner and exterior window to make and additional air space from the cold outside. I am worried about the glass cracking and it being unsafe. Do you think this is too risky if it is not tempered glass?

    Any advise would be appreciated! Thank you!


  21. Adam: Love the creative thinking. My only advice with uncharted territory is to cover your path. In other words, i’d advance with what you’re thinking, as it seems plausible and reasonable as a thought or idea. However, if something messes up, like moisture between the glass or it cracks, or something bad happens, think about how, during the construction phase, you can allow for something to mess up, and a plan to fix it.

    Eg. I would install the windows with easily removable trim or not nail in the flanges too much. Test run your sauna for a season or two. Chances are, all will work fine, in which case, you can calk and seal your trim and be done with it. But, if something goes wrong, you can pull the windows and frame in for a smaller window or come up with an alternative plan.

    That’s what I love about sauna building. Once your sauna is functional, you can start using it, and feeling out your unique elements. The journey to good sauna is the destination.

  22. building my own toaster oven box, no changing room, sitting outside on concrete slab.
    to reduce insulation loss-i’m going with a fixed ‘transom’ window that will be approximately 34X10. I’m heading towards double or triple glazed but wondering about application of reflective films on the interior of the glass? what about breather holes or inert gas fillings-anyone with insight or experience on this?

  23. Hello Glenn!

    Thank you for your kind words and advise. You really revitalized my passion to build my sauna. I was getting a little caught up in the details. The suggestion of building in a way to allow me to easily modify key features if something happen to the window makes perfect sense. Thanks so much for your time!


  24. Building an earth-bermed decagon shaped sauna here (6 sides are bermed and 4 are exposed. Thinking of a small functional window on one side (that also has wood pocket shutters)… anyhoo, do you think a small window about 12 or 13” square would warp and need constant repair?? I have a friend (and skilled wood worker) who installed a small functional window in another sauna that seems to have held up well. Any experience?

  25. Hi Jill:

    Sounds like a great project. As a general rule, i like fixed glass windows in the hot room, vs. functional windows, and controlling the ventilation through a couple vents with “chutes”. That said, I do have a bunch of experience: my cabin sauna has a functional window that has held up fairly well (built in 1996). Wood clad is the way to go, so hope this helps!

  26. I am building a sauna in a corner of my basement that has an existing glass block window to the exterior. Will glass block be able to with stand the heat of the sauna — especially considering the cold temperature on the other side of the window ?

  27. Ellen:

    Love the question and valid concern. I helped another person do exactly this, and they said all is well. But this was a couple years ago and either all is still well, or the glass cracked and he ran away from further communication, so his testimonial is not proven.

    Intuitively, my hunch is that you’ll be fine. It’d be nice if this glass block window were to have a window in it, for ventilation, but that’s another matter. If it were me, i’d advance the project and sauna on.

    If the glass block window proves to give you trouble, either cracking due to thermal shock and awe or too cold of a thermal breach, well, i’d deal with that after. (being mindful of how many nails in the trim around your window so you could get after it from the inside if you need to replace).

    Good sauna is the driver here.

    Let us know how it rolls!

  28. Hi there from Lake of the Woods, Canada! We are building a sauna with a large fixed window. Do you have any insight into how far away our wood burning stove should be from our double paned window? We are worried that the heat will compromise the glass I.e. break it, but we haven’t seen any commentary on this. Thank you! – Kay

  29. Hi Kay:

    Amazing timing on this as we just finished a really high end top of the line sauna project here in Minneapolis. We worked with Marvin Windows (close to you!) who didn’t want to take too much ownership to providing a 5’x6′ fixed glass window for our hot room application. We bought it anyway, tempered high end stuff, and are advancing the ball.

    Many saunas are being built with larger glass window features, so we have our fingers crossed. SUGGESTION: as you install the window, for year one, don’t nail completely the exterior trim around the window, or use screws. after a good long Lake of the Woods winter, you’ll know how well your glass holds up, and if it cracks, you can take out the window/glass and replace with a super high temp tempered option.

  30. Hi Ben:

    My view on using push out awning windows in hot room, or any functioning windows for that matter, is not a rosy one.

    In winter, frost and ice can easily form on the windows and it’s a bitch to close them. And the integrity of the functioning windows get lost over time, in other words, windows warp given all the heating and cooling and wet and dry action.

    That said, i’m looking over at my cabin sauna, built in 1996 with a cheap ass vinyl clad window in the hot room, a low end thing bought from a lumber yard as a demo floor unit, and well, this awning window still does the job. The crank is a tad messed up, and you have to push it from the outside to close it completely, but by golly it still provides perfect ventilation and a great view to the lake.

    Would I install this same dumb ass window today, if I were building this sauna again, now 24 years later? No.
    Is it a big deal that I messed up and used it, at a time when I didn’t know jack shit about sauna building? No.

    This is what’s great about sauna.

    I’m reminded of my trip to Tampere, Finland. Rajapporti sauna. about 100 years old. Having a beer with the proprietors. He wrote down the word in Finnish, but the translation is something along the lines of…”everything is right just as it is.”

    Rajapporti produces fantastic heat. It’s a raggamuffin place. Old, built by Russian soldiers, actually before Finnish independence. There’s a window on the way up the stairs to the elevated hot room. It’s cracked and lost its seal a bit. But there is no need to fix it.

    We work too hard in life to try to make things perfect. But there is perfection in imperfection. A good sauna is like this.

    “Everything is right just as it is.”

  31. Glenn. I just contacted a local glass dealer. He said they have 1/4 ” tempered glass bit not insulated. Is it the same? Thanks

  32. Hi Glenn, I wish I read this post before I ordered by sauna. I now have a beautiful fixed window right above the heater. When I pour steam over the rocks, I can see steam escaping out the window.
    Is there a way to seal the window?
    Thanks in advance!

  33. If you can see steam escaping out the window, then you must have a leak in your window. For many, windows are a good source of ventilation, meaning that a vent doesn’t need to be installed.

    So, not sure how to get a good seal on your window. Maybe lock it?

    But to make you feel better, i’ve taken many an awesome sauna in Finland with functional windows that are a tad porous.

  34. Hi,

    I am thinking about buying a sauna with floor to ceiling windows on one wall of a 2x2m sauna. The sauna will be heated by a Harvia wood stove heater. I like the windows because I can capture some amazing views while bathing. I fear though that much glass even though thick, tempered and insulated will cost me heat. However, I’m based in Ireland where tempretures in winter don’t fall too far below 0°c nor do so very often. What do you think? Here’s the link to the sauna I’m thinking about getting (don’t hate cos it’s not cedar 🤣).


    Cheers, John

  35. John:

    When you say:

    “I’m based in Ireland where temperatures in winter don’t fall too far below 0°c nor do so very often”

    You hit upon the magic differentiator.

    We have a thermal engineer on our board of directors, but he is predisposed at the moment and unable to do actual thermal modeling for your exact situation. But as a guy who has taken thousands of saunas, and many in megga sub 0°c, I can tell you that the energy loss gets exponential. So, more temperate climate means that you have a good chance of maintaining good heat with a mega glass sauna hot room.

  36. Hi Glenn,

    I am starting the process of building an indoor suana in my basement. The question I have is this: I have a single pane 1/2″ thick 51″ x 80″ tempered glass panel (it was a frameless shower door that I took out from a bathroom remodel).

    It has been sitting in my garage for a couple months now, and I am wonder if it would work as a very large window for the sauna. My main concern is how well it would insulate.

    I would really appreciate your thoughts.



  37. James:

    Love the thinking.

    What I would do, and what I advise many others to do with similar inquiry: It’s called the “give it a shot” plan. In your case, it’d look something like this:

    1. Frame for this window.
    2. Install this window.
    3. Don’t trim out the window, or if you do, set a couple thin screws to hold trim in place.
    4. Panel all walls except the window wall.
    5. Sauna.
    6. Sauna more.

    If all goes well, finish paneling hot room, and frame in the window for good.

    If your heater struggles to get up to temp., like the little train that couldn’t, well then it’s not the end of the world to take out that large glass panel and frame in for a smaller window.


    Before framing for smaller window, I’d test a temporary solution. Insulate and temporary frame what was this large window opening. Then sauna more to validate that smaller window is your solution.

    James, all this may sound like a lot of f**’ing around. It’s really not. You’re playing with the house’s money on this one, so it’ll be kind of cool for you to test, iterate, sauna, feel it, change if needed, and that’s often how we make our best saunas!

  38. We are looking to build a small indoor sauna on a tight budget. What are your thoughts on using a full glass storm door and plexiglass for transom windows? We thought the storm door could be cladded with cedar.

  39. Instead of plexiglass for transom windows, consider real glass, as plexiglass doesn’t hold up well over heat/time.
    Full glass storm door: Being that you’re talking an indoor sauna, your heat differential is less than the outdoors, so it may work out.

    If the full glass storm door doesn’t hold up, you can replace with your own homemade door. The method detailed in my ebook is not too expensive (sandwiching plywood with t&g).

    Wishing you good sauna,

  40. James (tempered shower door topic above) if you are still around, did you eventually move forward with this project and did this work as expected? I’m about to do the same thing here (24×74 x10mm tempered as main door and two 13×74 tempered on each side as fixed window) and wondering if you could provide feedback.

    Otherwise will follow Glenn’s advice and report back.


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