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Building a fixed-pane window into foam

user941025 | Posted in General Questions on

I’m spinning this off from one of the many triple-pane threads rather than derailing it with my question.

Keith Gustafson wrote:

They get better:
I believe that the 1 1/8 triples are their thickest standard, as they did not want to go higher.As it is the ones that I selected end up with a U of .13, [R 7.7] COG.
Since much of the money in a manufactured window is in the opening mechanism and sealing, consider fixed glass. It would be pretty simple to design a frame that buts up to an inch of foam so no thermal bridge. Any half sober carpenter could make a houseful in a few days.

Has this approach been discussed much here? I’m looking to install one fixed high gain window in the next couple of months–yet even though Cardinal HQ is in my town, if I want to buy a decent fixed window, it’s coming down from Canada while I eat both the price and the wait.

So what’s the proper support and sealing process to get a high-performing fixed window if I want to set a pane into polyiso or XPS, sprayfoam or caulk it, and then cover it up? I’m assuming once a pane of glass is set comfortably into its bed of foam, I can just extend a buck out each way on top of the foam both inward and outward from the pane. What approach and materials (caulks, gaskets, foams) make building your own fixed window effective? And, XPS rather than polyiso? Can’t I theoretically come out ahead of (for example) Fibertec if I set Cardinal 180 right into foam under plywood, no sash?

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  1. GBA Editor
    Martin Holladay | | #1

    Many builders have made their own fixed windows; I've done it often.

    My tips:
    Build the frame out of a rot-resistant species of wood. I use 4/4 or 5/4 white cedar, planed smooth.
    Rabbet the corners of the frame, and secure the corners with resorcinol glue and stainless-steel screws.
    Include a sill with ears. Of course, the sill should be angled to drain to the exterior. The sill sits in angled dados in the window jambs. The sill should extend far enough to drip beyond the siding.
    Use butyl glazing stops to set the IGU into the frame. At the sill, the butyl glazing stops should be short lengths -- not continuous -- that hold the IGU off the sill so that the IGU never sits in water or against damp wood. Use 3/4 by 3/4 stops (white cedar) held into place with screws so that the glazing can be replaced when it breaks. There should be no wooden stop on the exterior of the sill; at this location I sometimes use a short length of angle aluminum in the center of the glass as a stop. The angle aluminum is held in place by a single stainless-steel screw. (Don't forget a butyl glazing stop between the aluminum and the IGU.)

    Once you've built the frame, you can "set the frame into foam" if you want.

  2. gusfhb | | #2

    You will also need to select sealants that are approved by the igu manufacturer.

    Cedar is nice, but soft, I don't think VG fir would give you a problem, and is a bit cheaper.

    since you are doing a thick wall, I have a detail in my head that would allow you to insulate right up to the interior of the igu so that the frame is not a weak spot. Maybe I can figure out a way to scribble it. Also with thick walls, if you do innie windows, it may not make sense to ,make a sill that projects proud of the siding, a metal flashing pan should work, then any wood becomes 'trim'

    I have 2 foot overhangs [4 foot downstairs] and a house full of site built[not by me] windows that do not leak and have a wood stop all around,. when we replaced all the fixed glass I ran a bead of silicone at the top of the bottom stop so that water would tend to run over it rather than behind it, but never at the bottom of the bottom stop, if that makes any sense to you.

    If you plan small overhangs[don't] that allow water to cascade across the window surface, use an abundance of caution with details, I certainly get away with much because many of my windows never even get wet. The price is that they are always dirty......

  3. user941025 | | #3

    Excellent. I've got some calls to make during lunch on Monday. Thanks, fellas.

    Yeah, if it's risky or inadvisable to drop in the window into foam without sashes, then fair enough. Keith, if you scribble out your design, let me know.

  4. gusfhb | | #4

    I did say scribble....
    I am showing an extra deep rabet for the window to hide some of the spacer. One could also use an oversized stop to hide the view of the foam from the outside. I also did not show any attachment to the building, I am assuming it is screwed into the framing at appropriate locations, I 'think' it should eliminate bridging

  5. user941025 | | #5

    Hey, thanks! Excellent.
    While you were scribbling up that, I was scribbling up this. I'm going to put my ignorance on full display again--what are the reasons that sashless would be a bad idea? See attached.

  6. user-988403 | | #6

    There is an art to window making and there are a lot of things to consider. I am not saying it is a bad idea to make your own fixed glazed window but to discuss all "engineering" it takes here would take forever (in my opinion). Just a few points:
    Keith design: The problem is that the IGU is covered by more than an inch (that`s what it looks like) of material. This causes extreme temperature differences of the IGU which causes stress on the panes and might cause them to crack.
    Your design: Since you did not include measurements its hard to tell if the same problem occurs!? Also there are wind loads on the glazing and I am not so sure if how you want to fasten the glazing. Depending on the size this could be a problem!

    I think the best idea is to build a frame as Martin described and have someone check your details addressing water management, structure, airtightness and thermal bridging. I am in Minneapolis and have access to a shop and know a lot about windows if you want I might be able to help. Shoot me an e-mail: [email protected]

  7. gusfhb | | #7

    I seriously doubt there would be enough temperature gradient to crack anything, A window is going to crack from mechanical stress[from the building moving etc] rather than thermal.

    I don't know what is represented by the green in Minn. drawing.

    Water sealing is the big deal on the thick wall, which I did not deal with at all

  8. user-988403 | | #8

    Keith, the thermal gradient is a real issue window manufacture know about. This was one limiting factor to passive house window design. If this was not an issue the glass would be embedded in the frame more to get rid of thermal bridging at this connection. I have seen this failure more often then failure caused by the building moving. I guess my biggest point was that windows (fixed or operable) are complex products and many issues have to be addressed. Water management is definitely one of them.

  9. GBA Editor
    Martin Holladay | | #9

    Q. "I'm going to put my ignorance on full display again--what are the reasons that sashless would be a bad idea?"

    A. If I understand you correctly, you are basically suggesting that you build a frame out of rigid foam, and install a big IGU in the foam frame by setting the IGU in a big glob of caulk, all around the perimeter.

    The obvious problem is structural, as others have mentioned. IGUs are heavy, and that isn't a very secure way to hold one in place.

    My main objection is different: the first time that a baseball goes through your fixed window, you'll discover that you haven't planned any way to replace the broken glazing.

    Finally, I don't think your design will be watertight over the long haul.

  10. gusfhb | | #10

    RE: Phillip

    I am certain that thermal breakage is an issue, and a risk in any window. The main gist of my scribble is insulating the frame since we are unable to do the cute tricks a mass manufacturer can do to insulate the frame components.

    If betting on the likelihood of breaking the inside pane of a triple glazed igu with warm edge spacers in a wood frame, I think the odds are on my side.

    I can think of several reasons why a window company would not do this. Cost is first as you are buying a lot of glass that no one is looking through.

    Second, my own operable windows are 5 inches smaller in each dim than my fixed glass, subtracting another 2 inches would make them look like portholes.

    Third, and perhaps most likely, is that is just not useful! I would think that a smaaaat engineer somewhere would have developed an innovative seal system allowing full floating igus within the window system. My only thought is that given the cost and aesthetics, it is not worth the trouble.

    So it is probably a dumb idea, but I may try it if I get the chance, I will let you know if they break....

  11. user-659915 | | #11

    I'll add a further reason to Martin's list why frameless direct-set windows are a bad idea. In our area at least, local inspections departments will simply not accept direct-set glazing - there are far too many failed examples from the 1970's when it was all the rage among greenhorn hippie builders looking to save a buck (ha!). In fact they won't accept a carpenter-built frame either, unless it's accompanied by an engineer's letter confirming that it meets wind load requirements.

  12. GBA Editor
    Martin Holladay | | #12

    We can all keep piling on reasons why Minneapolis's proposed design is bad. Here's another: the most common cause of seal failure in IGUs is damage to the IGU due to moisture accumulating at the seal edge -- something that is most common in poorly designed windows that allow water to accumulate in a channel at the bottom of the IGU. If you can keep the bottom of the IGU slightly elevated and dry (not sitting in water), it will last a lot longer. That's why I recommended using butyl glazing blocks to elevate the IGU above the cedar sill.

    If you set the entire IGU in caulk, the caulk will eventually fail, and the remaining caulk will hold water against the IGU edge seal, leading to seal failure.

  13. wjrobinson | | #13

    Interesting topic. A bit about breaking. Over the last few decades we installed via a log home manufacturer thousands of huge really huge fixed glass units. Glass alone for 8'hx8'w costs $1,000-2,000. Five people climbing scaffold in unison to set. Our biggest fear was breaking one and either being stabbed to death, sliced to death or just having to pay for breaking one while we built, not, having one break, because the frame changed the temperature too much. We did break one on a project of mine with a fastener hitting too deeply framed glass verses frame.

    Never was comfortable near big glass. We would wait till the very end of work on the glass wall side of a home to finally add custom super large glass.

    Be very careful working with unframed glass is my first last and best advice to add to this thread. I now let my glass company install the glass and stops. Small glass that one or two can set are much easier to handle but you still have to be aware of how dangerous the glass can become in an instant.

    Minneapolis, curious as to what program you are drawing details with? Thanks. Sketchup for me.

  14. user941025 | | #14

    all the rage among greenhorn hippie builders looking to save a buck (ha!).

    I'll accept that mantle, but please--in my case, it's "hillbilly," not "hippie." Thank you.

    This is a really informative thread. Thanks, everybody, for weighing in so far. Let me read this over some more and chew on my options.

    Phillip, I'll drop you an email. Thanks.

    AJ: Illustrator CS2. Nothing's to scale. It's the cocktail napkin for people without scanners. And I saw The Omen II when I was a kid, and therefore fear plate glass.

    Also, fwiw, this is not a permit-required structure (zoning only), but that doesn't mean I'm fine with the window failing in one way or another, so James, your point is taken--thanks.

  15. user941025 | | #15


    I'm still interested in the fixed pane options question. (I've got my fixed fiberglass-framed window for the thing I'm working on right now, but this still perturbs me.)

    Variation on the above question, after a discussion a couple of weeks ago with a spec writer and last night with another GBA visitor:

    What's wrong with affixing an unframed IGU with continuous VHB tape to framing, then outsulating (probably with a polyiso layer lapped over the IGU edges), and taping/flashing?

    Depending upon detailing, this could still basically be in the insulation center.

    (Also, don't most highly-insulated fixed windows run afoul of Martin's foul ball objection?)

  16. GBA Editor
    Martin Holladay | | #16

    "VHB" had me confused, but I looked it up: "very high bond." Okey doke.

    A well-made fixed window is designed to allow for the glazing to be replaced. I've built many fixed windows over the years, and all of them were designed (a) to elevate the bottom on butyl setting blocks so that water doesn't sit around the bottom of the IGU, and (b) in such a way as to permit the glazing to be replaced if the glazing is broken or the seals fail.

  17. user941025 | | #17

    Yes, apparently it holds facades to commercial buildings.

    Bearing in mind that the frame is the thermal-performance weak spot compared to a decent IGU, it seems like this question is worth poking at some more.

    (a) ok. Can't that water be diverted through sealing?

    (b) Fair enough. However, visiting that recent retrofit in Minneapolis that a bunch of people here worked on, it was hard to miss that those fancy Optiwins don't look like they're coming out. Perhaps they were all operable and those sashes are all removable. I'm not personally sure. I don't think so.

  18. GBA Editor
    Martin Holladay | | #18

    Q. "Can't that water be diverted through sealing?"

    A. No. I don't know of any way to prevent water from reaching the bottom of an IGU over a long period of time by the use of sealants alone. Remember, there is thermal expansion from summer to winter. You need drainage, or at least an air gap under the IGU.

    Q. "Those fancy Optiwins don't look like they're coming out."

    A. Everybody gets to decide how to build their own house. But I think it's nuts to buy a $1,000 window and build the house around it so the glazing can never come out. I've had IGUs with failed seals, and I've appreciated details that allow fast and easy glazing replacement.

  19. gusfhb | | #19

    You simply cannot have truly unframed glass in any building you care about.

    3 reasons:

    1] Water:

    There is no way to ensure that water will not penetrate your wall assembly at some point. The assembly I scribbled won't leak water[assuming it is installed correctly] even if it has no sealant. well, it won't leak into the wall.

    2] Structure: glass wants to be carefully supported or it will break. Maybe VHB is good enough, but there is risk if it is not.

    3] Water: did I mention it would leak?

    I believe you are overthinking. Trace the shortest thermal path at the edge of the window, and it quickly becomes across the surface of the glass through the spacer and back across the glass, even if a frame was R100

  20. user941025 | | #20

    OK. Thanks again to both of you.

    MH: "Remember, there is thermal expansion from summer to winter."

    For what it's worth: here's the spec sheet for the family of tapes, typically used for curtain walls. It's a double-sided tape so this would be face-mounted; they're viscoelastic, prepared to handle expansion and contraction, and can take quite a lot of weight.

  21. GBA Editor
    Martin Holladay | | #21

    You still need a frame to protect the edge of the IGU. Would you use this tape (instead of conventional glazing tape) to prevent rain running down the face of the glass from reaching the frame at the bottom of the IGU?

  22. user941025 | | #22

    With regards to glass face-mounted to the exterior of the frame, a forum member did point me to walchfenster--so here's a detail (attachment) from walchfenster for their fixed window. I wonder if there are any answers to be found here, as pertain to viability.

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