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Shims and/or spray foam around vinyl windows

user-6356169 | Posted in General Questions on

Windows were installed on our new build while I was out of town. These are Jeld-Wen vinyl casement windows, and the installation instructions say to shim at various locations on the sill and jambs. Well, this wasn’t done at all. I’m insisting that this still needs to be done, thinking that Jeld-Wen probably thinks that some support is required in these areas. Builder says it won’t matter because the window will be foamed all around and it’s not going anywhere. So I’m wondering if he has a good point (ignoring for now the warranty implications of not following manufacturer instructions), at least on the jambs.

On the sills, which are sloped, I understand that we’ll want to limit the foam to a bead along the interior edge of the window, so that the sill drains freely. But I’m wondering if you could go most of the way out to the exterior edge of the window with the foam in a few spots along the sill, in effect creating foam shims. Does cured spray foam have that kind of strength? Reason why I think this might be preferable is that the sloped sill and the bottom of the window are not in parallel planes, and I’m afraid that the shimming might not be careful enough to bring the top surface of the shims up to the plane of the bottom of the window.

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  1. Expert Member
    Dana Dorsett | | #1

    Some foams are more rigid & structural than others. Zero-expansion latex foam sealants (best practice for most vinyl windows) don't have the same structural rigidity or adhesion capacity as polyurethane foam or low-expansion always-flexible foams.

    It's a judgement call- if the builder seems competent and will warranty the Jeld-Wen compliant work in writing you're probably going to be OK.

  2. user-6356169 | | #2

    Dana, I hadn't heard before that latex foam sealants would be best for vinyl. Even Jeld-Wen's installation instructions recommend low expansion polyurethane foam. What's the problem with polyurethane on vinyl?

    Can you make any recommendations on some good brands of latex foam sealant?

  3. Expert Member
    Dana Dorsett | | #3

    With many vinyl windows some polyurethane foams can apply enough pressure to distort the vinyl frame enough to interfere with opening/closing. The low expansion always flexible foams can sometimes too if the installer overfills it.

    DAP's DapTex Plus latex foam sealant (available at most box stores at about $5-5.50 for a 12 oz ca) is pretty good, but there are others. The stuff expands a tiny bit, but not enough to create much pressure.

    Latex foam sealants are a bit more expensive than polyurethane can-foam, but it's not a ridiculous up-charge by any means.

    Fine Homebuilding did this short comparison piece a half-dozen or more years ago:

    A comparison of Dow's Window & Door version of polyurethane can foam and DapTex and Touch'n'Foam No Warp low-rise polyurethane can be found here:

  4. Expert Member


    Vinyl windows need shims under the sill if the rough-opening isn't completely flat, but I'm having trouble understanding what purpose they serve at the jambs. As Dana says, even low-expansion foam can distort the frames. The best practice is to put spots of foam around the perimeter gap and let them harden. You can then fill the rest in without worrying.

  5. user-6356169 | | #5

    Thanks for all the info, Dana, will delve into it later.

    Malcom, my assumption was that the shims at the jamb supply some extra support near the hinges and locking mechanism (casements). But I don't know. And the windows sits up off of the sills - they've been nailed by the flanges and given a gap at the sill. I assumed that the shims under the sill were for addtional support and to prevent bowing and warping. You're supposed to add additional shims on the mulled units. Maybe I don't understand ... If they've already been nailed and are plumb and level, do you think there's no need for shimming at all?

    Also, if we put spots of foam around perimeter and let them harden, how exactly does that protect the rest of the frame once it's filled in? Are you counting on adhesion between frame, foam, and r.o. lumber in these spots, so that any expansion of the foam you later apply would be in a path of less resistance, in other words, further to the interior or exterior rather than laterally in towards frame?

  6. severaltypesofnerd | | #6

    Could you post an excerpt from the Jeld-Wen instructions?
    You might not be too late to shim from the outside. But if the sill is flat, the rear edge is probably great without shims.

    Do watch that they don't foam that front edge: many many builders block drainage in this way.

    Foam as a shim may not work out well in the long term, as it will compress. Be sure no foam shows after trimming or painting, as all foams degrade in sunlight. The builder should be using low expansion foam to avoid excessive pressure. If they're using single use "great stuff" cans that's a sign of a less professional company: the guns work way better and leave less waste.

  7. Expert Member


    The flange is really only for horizontal loading and keeping things square. You need to take the weight of the glazing and frame on the RO or shims. I've never foamed at the sill. It does leave a week spot in your insulation, but keeping a drainage plane there more important.

    Yes, the small spots of foam act as adhesive to keep the jambs and head square when the continuous foaming occurs. Alternately you can foam in two stages with a small continuous layer and the fill in the rest after it cures.

    If Jeld-Wen wants shims at the jambs, I'd provide them.

  8. user-6356169 | | #8

    Thanks Malcom, think I might also just have them leave out the foam at the sill. Was planning on having them just run a bead along the interior edge but it might be safer to just leave out entirely. We'll be air sealing with tape from interior, so as you say we'll just lose a bit of insulation.

    Bryce, here's a link to the Jeld Wen instructions:

    See part 4, step 9 for the sill shims and part 5, step 4 for the jamb shims. Actually, I'm just noticing that they say in part 5 to shim the head as well.

  9. Expert Member


    I confess I find how to deal with the sill quite puzzling. Traditionally we sat the window on a flat rough sill and that was that. Then (quite logically) sill-pans and slopes were introduced, as that's where the moisture damage almost always occurs.

    Our code was further amended to say you needed a drainage path from the sill to outside the WRB. This was by either mounting the window outboard of your rain-screen strips or drilling holes in the bottom flange.

    If you look at a section you see that, unlike at the jambs and head, this moves the air-sealing into the caulked-joint at the inside face of the window frame, and there is no insulation in the gap. That's where we are now, and to me it isn't a very satisfactory solution. Maybe some other posters have encountered better ways to do this.

    The shims at the head are another novelty to me. For decades window installation instructions precluded even nailing the top flange - the idea being that the framing might shrink and place a load on the window. Then a few years ago they admitted this wasn't a problem and we merrily nailed the hell out of it.

    Now we are supposed to shim it too? Like at the jambs i don't see the point. It isn't analogous to the situation with doors where you are securing the shim right through the frame. I side with your builder, but as always be mindful that ignoring their instructions might come back to bite you. Again I'd be interested to hear from others on this.

  10. severaltypesofnerd | | #10

    It's a nail fin window, right? Looks like you can add shims later.

  11. GBA Editor
    Martin Holladay | | #11

    Your link to the instructions didn't work for me. I found Jeld Wen installation instructions elsewhere -- here is the link, and the relevant illustrations:
    Jeld Wen window installation instructions.

    I'm not sure, of course, whether you have the same type of window. Perhaps your window requires different installation instructions.

    My default instinct is to follow manufacturers' installation instructions.


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