The definitive word on what kind of glass to use for windows in the sauna (I can really see clearly now)

I have built a bunch of saunas. I help hundreds build their own sauna thanks to the book I wrote, and an unwavering enthusiasm to help folks build their own kick ass saunas (with no compromises).  The question of what kind of glass to use in a sauna comes up frequently.

So, what kind of glass should I use for windows in the sauna?

Well, I finally got through with an expert in the field.  And it took a lot of work.

Folks in the glass industry seem about as temperamental as the product they sell. Industry experts that I have spoken with are generally not the most patient and don’t seem like a lot of questions from DIY’ers. Most are more accustomed to dealing with tradesmen.  I understand how patience can wear thin as ordering the right custom cut glass can be an involved process.  There are many glass options to choose from, and every order involves a careful verification of dimensions.  Mistakes are costly.

However, I have finally been connected to an expert in the field who was patient and friendly, and locked into my barrage of questions.  Michelle is a glass industry expert with 17 years experience.  Even better is that she told me “I never stop learning” which tells me that she is open minded enough to keep expanding her knowledge in the field.  (This, by the way, is exactly my theory of sauna building, which is why my ebook is open to involving with sauna builders’ generated content).

Ok, enough of all that, here’s the scoop:

For windows in your sauna:

  1. Plan your sauna window carefully.  I am a big fan of transom windows up high. – 16″x 30″ or so.  They don’t get in the way with whatever is underneath, and allow privacy.  There is the candle window, and window in the hot room door.
  2. Contact a local glass company.
  3. Order 1/4″ insulated tempered glass, cut 1/8″ to 1/4″ less than your rough opening.
  4. Frame in the glass using the same material as your hot room (cedar paneling ripped to size).
  5. Run a bead of silicone around the inside and outside of window.

Insulated tempered glass is two pieces of glass bonded together.  This creates an insulated unit.  This system reduces moisture on the glass.  This system helps facilitate the temperature extremes of a sauna hot room and a cold winter’s night.

What if the seal of the double pane glass breaks down and moisture gets between the glass?

This shouldn’t happen, but if it does, we are shit out of luck.  We either have to live with it or replace the glass.

What about cost?

a 16″x30″ insulated tempered 1/4″ piece of glass costs $93.00.  That’s about twice the price of single pane.  But this price is a fraction of the price of buying a window.  We build our saunas one time.  Let’s use the best glass option we can find for our sauna windows.

a hot room transom window as viewed from the outside, with saunatimes promotional material underneath.

For more discussion and clarity on windows in the sauna, please read here.

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60 thoughts on “The definitive word on what kind of glass to use for windows in the sauna (I can really see clearly now)”

  1. Thanks a lot for going into all the trouble getting a competent answer to this important issue!

    All the best,

  2. for those not wishing to fool around with framing in a window, sauna supply companies offer complete windows. these are made from tempered glass, available in different sizes/colors (e.g. tinted but costs more than clear), different wood (cedar, aspen, etc.), beveled frames for water runoff and typical come with trim pieces. set in place, a few nails and done. this is what i did with my build. i have a clear window between the hot/changing rooms. window cost me $170 (w/o shipping) and for me, the extra cash was worth it.

  3. Good info, Glenn. I went with a vinyl slider in the hot room. One pane is currently cracked and the frame of the slider warped a bit during an especially hot session. It’s still functioning, but eventually I’ll replace it with a tempered wood frame window.

  4. I acquired a 2×2 double pane window glass. I was worried about breakage as it is just above my 6kw heater…two years now and works like a champ.

  5. My 36″x36″ sauna window developed two top-to-bottom cracks recently, after the sauna temperature reached 210 degrees. It was only about 32 degrees outside at the time. The cracks are only in the inside pane, but there is some displacement of the glass and I’m worried it might fall in if accidentally bumped. Does anyone know of any product I could put on the cracks as a temporary measure? ie. high heat silicone? I’m worried about investing in another window in case the same thing happens again 🙁

  6. i’d go with silicone for sure. And then when it comes time to replace the glass, consider breaking up the 36″x36″ window into multiple (smaller) panes so the chances of cracking will be less and if your window does crack, it’ll be a smaller piece and that much less of a pain.

  7. I’ve been seeing pictures of modern saunas with very large windows and even patio doors, is this ok ? I guess vinyl is out of the question , should they be triple pane glass ?

  8. Alain: You’re right on about the newish trend towards “big glass” windows in our saunas. Thermal pane technology is allowing for some pretty large windows. The inside of a hot room can be 200 degrees f. hotter than the outside, making for a seemingly impossible temperature extreme and tolerance for glass to be able to handle. But this is all possible thanks to the advent and invention of thermal layering, as you point out. I”m not exactly sure if triple pane is the answer. As this is an important, and fairly expensive element to your sauna, my advice is to get with a glass company, explain your application, and have them “spec” the window for your sauna. This way, they are somewhat on the hook to help ensure you get the right glass for your sauna application. Good luck, take pics! send our way.

  9. Hello,

    I just finished a 8×12 ft. outdoor sauna and took it on its maiden voyage tonight and it works great! The Only problem is my large 3×5 ft. window has developed a long crack on the upper part of the inside of the sauna and not sure why? This is what i have

    (1) piece at 60″ x 36″ converts to (1) piece at 59 3/4″ x 35 5/8″ Rough Opening dimension
    (1) piece at 60 1/4″ x 13″ converts to (1) piece at 60″ x 12 5/8″ Rough Opening dimensions
    (3) pieces at 14.5″ x 14.5″ converts to (3) pieces at 14 1/8″ x 14 1/4″ Rough Opening dimensions

    1″ OA and will include low-e

    I had an option of tempered or Annealed and this is what the glass place told me.
    “Annealed is when if something breaks the glass, it breaks into shards. Tempered is when if it breaks it will break into tiny pieces in order not to cut anyone. 🙂 We had chatted about tempered, but I believe you opted for annealed, as tempered isn’t necessarily a stronger glass.”

    Is it possible the window was flawed and they may be a leek? I believe it is warranted but want to order the correct window when replacing. BTW this is just glass i framed it in.

    Thanks for your helpful responses!

  10. Bummer Dennis! Glass cracking may be due to the heat extreme or may be due to some too tight application upon install. I’d suggest bringing up the issue to glass expert in your area. They will probably jump right on the heat issue right away, but you will point out to them that glass is used in hot environments all the time. Some sauna doors are all glass, as example.

    3×5 is quite an opening. Forgive as i didn’t quite follow with your dimensions above, but after taking out the cracked glass, you may want to consider converting that large rough opening from a 3×5 to maybe 2 @ 3×2 1/2 or something like that, thereby 2 pieces of glass vs. one large. I know it will break up the intended big window look but maybe that’s just too big of a piece of glass? Keep in touch, send a pic!

  11. I love the look of the transom window. Is that simply a piece of tempered glass pushed flush up against the exterior trim and sitting on the lower 2×4 of the window opening, and then held in place by pieces of cedar trim on the inside? (with presumably a bead of silicon inside and out).

  12. Chris: you’re right there. Tempered glass cut to fit at least 1/8″ less than your rough opening.Flush up to exterior trim, bead of silicon as you mention, and the only trick is a good measurement to rip your jam to hold the glass in place on the inside. Voila! You got it.

  13. I’ve talked with a couple of glass shops and they say there is no such thing as insulated, tempered glass a quarter of an inch thick. They say they can get laminated glass that thick but not “insulated, tempered” glass – when I requested what you noted above, they looked at me like I was nuts! So – is this quarter inch laminated safety glass? Thanks!

  14. laminated is probably the same and should work great. As long as our glass windows aren’t too close to the sauna stove..

  15. I am thinking of building an interior sauna on an outside wall that has an existing window. Can I use the window? Or should it be replaced with something else? It is a new construction and the window is a Marvin Integrity window.

  16. Beth.. depends upon the window but my inkling is to try to use it. insulate with rigid foam and 2x2s and fir out that outside wall.

  17. Hi,

    Sorry to Hijack this thread, but I have a question about the sauna pictured above.

    Did you have to sheath the exterior in plywood and house wrap, or is the insulation right behind the T&G siding?
    Ebook has been an awesome help, but struggling with this part of the build.


  18. Hi Quinn:

    Great to hear that “Sauna Build: Start to Finnish” ebook has been of awesome help for you. Regarding your question, I built this sauna in photo above, so I know the answer. For sake of clarification to your question, let’s follow the following scenario: say you wanted to put in a hot room vent, and you were standing outside wearing your Troxers, holding a cordless drill with a hole saw, ready to drill into the exterior wall.

    Here’s what you’d be drilling through:
    1. Cedar lap siding.
    2. Tyek house wrap.
    4. plywood sheeting (or maybe it was OSB, I forgot).
    5. R13 batting (between 2×4 joists).
    6. Foil bubble wrap (taped well at the seams).
    7. T&G Cedar paneling.
    8. Your buddies shoulder, sitting on the upper bench. ouch!

    Regarding #2 and #6:
    There are those who are worked up about the potential of trapping moisture in joist cavity if using both an exterior vapor barrier and an interior vapor barrier. I am not of this camp. If we build our sauna building and our sauna correctly there will never be moisture or water getting into our joist cavities. I like house wrap because it goes on in with a staple gun in 20 mins and it protects our interior walls from weather elements. I’m up for rebuttal on this, but this is what has worked for me for many years.

  19. vapor talk, my favorite! ‘vapor barrier’ is a somewhat generic term that is often applied to house wrap, poly sheeting, tar paper, faced insulation, you name it but there are differences between these products, particularly the permeance, or ability for water vapor to pass through the material. bubble insulation or kraft foil paper have very low permeability, very difficult for vapor to pass through. conversely, house wrap has very high permeability. it helps keep liquid water (e.g. rain) away from the sheathing but vapor passes through quite easily. and this is the ideal setup for a sauna. you want to limit moisture transfer from the hot room into the walls and also provide protection on the exterior against rain getting in but that exterior wall also has to breath, which is what house wrap provides. it lets any moisture that finds its way into the walls (and it will, no matter how good the construction is) a way out, keeping the wall assembly dry. so while there is a ‘vapor barrier’ on both sides of the wall, it is fine and a quite standard building technique. it would be a mistake to not have anything on the exterior and just put the siding up against the sheathing.

    vapor ‘barriers’ on both sides of the wall become an issue when both sides have a low permeance barrier, something like bubble wrap on the interior and, say, plastic poly sheeting on the exterior. this is not a good arrangement as any moisture that finds its way into the wall cavity will have a very hard time getting out, can lead to mold/rot issues.

  20. Miller: fabulously detailed. I will enjoy my hot rounds that much more, and sleep even better tonight. Thanks for this excellent info!

  21. Question – is the 1/4″ thickness for EACH pane in the double pane insulated window, or is that the combined thickness?

  22. Hello,

    I am planning to buy an indoor infrared sauna ( which is built from pieces ) and I would like reshape it for outdoor sauna – insulate it. Would you be so kind to help me how to do it? Thanks


  23. It may be overkill but the windows I had made have 1/4 inch panes with a 1/4 inch gap between. Working very well as a transom in the hot room as well as a candle window and door window.

  24. Mira: You’re treading on difficult territory. Infrared cabins are not built to sustain the type of steam and heat produced in a real sauna.

    Suggestion: start over. Invest in my ebook ($20 donation). If you don’t have a very clear path toward your own AUTHENTIC sauna, i’ll be happy to give you your donation back.

    Happy to help you along your journey.

  25. I found 3/16th’s tempered glass on craigslist. jwill that be thick enough? Or maybe I should build a double pane framed window? (not sure if that’s advisable).
    I’m only at the planning stage and your book and blog articles are immensely helpful. I’m sure I’ll be asking more questions. thanks in advance for your knowlegable answers.

  26. Hi Mary,

    Glad the book and this website is helping you with your own sauna build.

    Re: 3/16″, yes, unless you’re talking a huge window, this is adequate thickness. I’m not a fan of double pane as it’s vulnerable to trapping moisture over time, potentially. Such a great time to be building a sauna !

  27. 25 x 10 are the size. Really, how much heat do you lose with single pane tempered glass. I’m deciding between one or two windows side by side. the sauna size is 8 (or 7.4) by 6. Great time to stay home and build. 🙂

  28. If using cedar backrest and benches would using a hard wood like maple on the walls and ceilings still get to hot and ne uncomfortable ? also I was wondering about the vapor barrier I used a home depot product its looks like foil bubble wrap but it says it’s only rated 180 Fahrenheit should that be sufficient?

  29. Ryan:

    I like your thinking!
    1. Depot Foil Bubble wrap:
    Yes, very astute! I arm wrestled with the manufacturer on this issue and can sleep at night with this analysis. If a hot room is, say, rocking at 220f, the t&g paneling is also very hot, yet it does create an insulation and heat break from front to back. As much as 60f. Now, this is good in theory, but in practice, i’ve tested this. I recently fixed a sauna I had built and it was heated by a beast of a stove (Kumma). As I peeled back the boards, I saw no degradation of the foil bubble wrap, so the wood insulation theory is affirmed. And one more thing on this, if it’s still a bit of a question, I recommend sourcing regular foil vapor barrier, not the more readily available foil bubble wrap. This product has no poly or poly bi product in its composition, so it should be rated higher than 180f, but I haven’t checked. Please let us know how you shake out with this.

    2. Hard wood on walls.
    hmmm… Let me sleep on this one. Just because i’ve never seen it, doesn’t mean it’s not doable. If I had the stock laying around, i’d consider the ceiling for sure. Yet the walls? If you were to bump into them or put a hand on them, maybe you’d regret using that material. I just don’t know for sure. Maybe others can chime in on this one?

  30. I have a free 12″x48″ triple pane vinyl transom window. Would you use it on a new sauna build?

  31. Hi Chad:

    You can use this but vinyl will get compromised over time. I have a vinyl window in my cabin sauna hot room – not recommended – but built in 1996 and it’s still basically alive and kicking. And there’s the concern that moisture can access between the panes over time, but I haven’t had this happen, or seen it happen.

    If it were my sauna, i’d probably use it, as it is free and you’re looking at it. I would apply your trim around the flange on the outside so that I could access and pop off the trim without wrecking it, and get at the window for replacing it, if things go south.

  32. Hello:

    I’m wondering about using a single pane of 1/4 inch tempered glass (36”x48”) that I have laying around. (It was intended to be used with a railing unit, but we were delivered the wrong size). Would that work, or is it better to go with a true double pane window. We will be using it in update NY, where it will be very cold outside.


  33. Jonathan.

    My advice for you is “go for it.” Use the glass you have. I have often used 1/4″ tempered for sauna builds. Transom windows, window in hot room doors, etc.

    The one trick, though, is when you flash the outside of your window, don’t set your nails flush, for the first winter. Or use screws. And maybe go easy on any calk. Just feel it out. If your glass is going to crack or not be adequate in terms of double pane thermal insulation, you’ll know before Thanksgiving, and you’ll really know by Christmas.

    Then, come Spring, assuming all is holding up well and your happy, you can come back and calk and set your exterior trim for good. If the window cracks or not hold up to the cold/heat, you can easily come back and replace the glass with double pane tempered.

    My hunch is that you’ll be fine, but hedge your bet by following above. Lemme know how it goes! and please let me know where in Upstate NY you are… i’m going to road trip up your way this Winter and Spring and mebbe we can sauna.

  34. Hi Glenn, great article. I’m looking to source the 1/4 tempered glass for a transom window like you described. I live near Minneapolis, any input on a reliable glass retailer that could help in the area? Your ebook has been very helpful thus far.

  35. For the ” Saunaplex ” I’m building I intend on using 8″ x 4″ glass blocks set in a row with a manufactured ” crawlspace vent ” that is the same size as the block. I’m going with 2 blocks, 16 ” awning vent, then 2 more blocks. They can be siliconed together according to the mfg or just forced tightly together with the provided mouldings…

    You see any issues with my idea?

    Jim Wolfe

  36. Hi Glenn,
    Transom window question…
    My sauna 8 X 12 building (outside shell, walls, gables and roof -we’re building our own insulated floor platform) is in the process of being built by a local shed company. I ordered a tempered glass thermopane window from a local glass company (30″ X 18″) to use as a transom window in the hot room. It isn’t mounted or framed in wood, but two glass pieces are mounted with 5/8″ metal on perimeter. The shed folks want me to frame it in wood ahead of time and they will build it into the wall.
    Any suggestions on how to mount it in wood? I didn’t anticipate that they wouldn’t be willing to frame the window itself. . Or should I just give them the size and location of the window plus 1/8″ and have them just frame in the window opening, and then get a carpenter friend to mount it when we get the shed here?
    Also, after reading the comment about people leaning agains the window, (cleaning it and it gets hot) should I put the location 8 ” down from ceiling height and 20″ in from corner, to avoid that issue? the sauna long wall dimensions are 7′ with an 20″ wide upper bench, lower bench below that.

  37. i like your shed company idea. Get that window on some saw horses and with a table saw and a miter saw, you can build a jam and trim and make that window easy to frame into a stud wall. good stuff!

    I like a transom window about 12″ from ceiling and centered within the hot room wall. But I like more to use cardboard and cut out the window and mock it up. Then frame it in.

  38. What about polycarbonate panels? They’re multi pane, offer insulating qualities and are easy to work with over glass. Are they okay for a sauna?

  39. I have a single pane, 1/2″ thick tempered, glass shower wall that i’d like to use as one of the walls in my sauns. It’s about 4’x8′. ( Yes i know there will be some heat loss there for sure.)
    I live in Canada and am a little concerned about the fact that’s it’ll be well below freezing on one side and 100 degress on the other side. In other saunas i’ve build, I’ve used double pane windows, but i have this glass and it’s free.
    Do you think i’ll crack? Do you think there will be too much condensation that’ll run down and affect the wood trim that i’d trim it with?

  40. Hi Jesse:

    I think using this large shower glass is a great idea. And I hear you about the massive temperature extremes. Here’s how i’d proceed:
    1. Install this window.
    2. Screw exterior trim around the window (not nail).
    3. Don’t calk the trim around the window (just yet).
    4. Use your sauna.

    If things hold up over the winter, then revisit #3, and apply your exterior calk, to seal it up out there.
    If the window fails (cracks), and it should fail early if it’s going to fail at all, then you can take out the exterior trim, pull out your window, and work on plan B, which will be replacing that window with a smaller one or perhaps breaking up the space to install 2 or 3 panel smaller windows in its place.

    Regarding too much condensation, I suggest trimming your glass window with Western Red Cedar. This species does extremely well in sauna, under hot, moist conditions. You’ll want to be mindful towards applying a bead of silicone along the edge where the trim meets the glass. This is the seam where water could run down the glass and sneak behind your trim, creating all kinds of yuckiness, which you’ll avoid with that bead of silicone. Blue tape the glass and the trim. It’s worth it. Clean “Finnish”. :).

  41. Glenn, I have two 18″x32″ red cedar windows on order from a Sauna company in Wisconsin for my to be built 7x7x8 sauna with a change room of similar size. The sauna and change room will be attached/part of the rest of the building which will be a “she-shed” (the sauna/changeroom is the “he-shed”). The she-shed will have black trimmed Andersen 100 series windows (made of fibrex, stronger than vinyl supposedly) on it.

    My dilemma is whether I should/could put a fixed transom Andersen fibrex window in the sauna with a black frame/trim that will match the other windows or should I go with the red cedar windows and perhaps paint/stain the outside of those windows black to somewhat match the rest of the windows? Thank you Glenn

  42. Thanks Glenn, but wondering if I should play it safe and go with the red cedar windows (stained black) made especially for saunas, or take a chance and go with the asthetically more pleasing Andersen window that would better match the main building windows? Bottom line is I’m not sure the Andersen fibrex diuble pane windows are well suited for a sauna environment? Thanks Tom

  43. Hey Glenn,

    im looking to build a sauna 6’x8′ with a fixed window on the 6′ wall facing the lake on my property. Ideally i’d like that window to be as big as possible for optimal sight lines, im hinking 5ftx5ft.

    Can you advise on what that window should be made out of? What kind of glass? My contractor has concerns about it as i am in Canada and temperatures in the north can go up to -30 degrees in the winter.

    please let me know!

  44. I tried warming up the guys at Marvin Windows, Warroad, MN. After some hockey and sauna talk, they still wouldn’t endorse or warranty their windows for sauna. I would try engaging with the glass supplier in your area. Getting their involvement may help with local intel and get them to take ownership.

    1/4″ tempered gets expensive, but is often used in the public sauna domain.

    Hope this helps,

  45. Converting a cistern in our basement to a 5.5′ x 5.5′ sauna. We would like to add a row of 6″ x 6″ glass block on the top of one of the inside walls. Will glass block hold up to the heat?

  46. Glass block can hold up to heat, but you may want to confirm about how much heat by contacting a glass block manufacturer as I don’t know the temperature ratings.

    We want to be sensitive towards heat loss through glass. One row isn’t going to make a big impact, so you should be good there.

  47. Hi, Glenn. Wondering if you have any experience or thoughts on using film treatment on glass to provide privacy? And how that might hold up to temperature extremes inside and outside? We live in Minneapolis and our backyard sauna will have some sightlines to neighbor’s homes. Still want transom windows, but looking for ways to not make it a total fishbowl from the outside at night. Thinking about a tint or mirror treatment, if it’s possible.

    thanks, and love your website.


  48. Right on Ben, I hear where you’re going and though I don’t have experience with film treatment on glass window for sauna, my first inclination is to apply it outside the window where the intense heat is less apt to mess with it.

    I don’t think this stuff is too expensive? Give it a go? Please let us know how it works out.

  49. Hi Glenn. Thanks for writing an article on this! I’ve seen some contradictory info on the right type of glass for a sauna window, so it’s good to get an expert opinion here.

    My question for you – have you ever heard of someone putting a skylight in their sauna? As I go through my own build, I’m wondering if that would be an option vs. a traditional window.

  50. Hi Glenn,
    Does 1/4″ laminated tempered glass hold for sauna doors as well? We are planning on doing a 16″x60″ lite of glass in our sauna door (that will be right next to the sauna heater. The door opens into a shower room area (not outside.) I called some glass folks in town (Portland, Oregon) and I got some mixed messages back including concern about the lamination melting at such high temperatures. Some were resistant to insulated as well because “the spacers could melt.” What are your thoughts? Laminated or single sheet of tempered glass?


  51. Hi Glenn—can you use argon-filled, low-r glass in a sauna window? We’re doing a 4’x4’ window in our new sauna build. Thanks so much for any input!

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