Authentic Sauna Blog

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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25 Comments on This Post

  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.

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