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Suggestions for insulating/sealing ~10″ wide solid stud walls (back-to-back studs with no air gap) between windows.

DIY_2016 | Posted in GBA Pro Help on

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  1. Expert Member
    Dana Dorsett | | #1

    It looks like part of it was insulated with 1" or 1.5" XPS, and the rest was not?

    What does the wall stackup look like on the interior side?

  2. DIY_2016 | | #2


    Yes, 1" XPS rigid foam was attached to the studs and slid underneath the window flanges. A 1" board was then nailed over the foam and window flanges. Finally aluminum flashing was nailed on top of the board and caulked (poorly) to the windows.

    From what I can gather on the inside it appears drywall was simply nailed directly to the other side of those studs. Not sure it matters but there's also a thin hardcoat layer over the drywall.

  3. DIY_2016 | | #3

    It was all completed as I mentioned above. The first picture is showing what I've partially taken apart. Here's a better view of how it originally looked (top part at least is untouched).

    How do I embed pictures instead of using links?

  4. DIY_2016 | | #4

    Pictures attached.

  5. GBA Editor
    Martin Holladay | | #5

    Considering the limitations, the solution used by the original builder -- a layer of rigid foam -- looks good to me. Why are you second-guessing this approach? There's no way to improve the R-value unless you are willing to make the wall fatter.

  6. DIY_2016 | | #6


    I started this project because I had a couple of spots in my house (on the other side of these windows) where the hardcoat fell off because water was working it's way through the drywall during heavy storms. I realize there is no practical way to increase the R-value but instead my goal is to properly seal it from any moisture.

    (As a side note the old, worn, discolored flashing with nails all through it and 3 to 4 layers of different caulking attempts is a major eyesore that you can't really see in the section in the photo I provided. So I not only want to properly seal it but also make it look better in the process.)

    The original approach relies entirely on the caulking between the flashing and windows to keep water out. There are a total of 12 windows and 2 doors on this side of the house which totals well over 200 linear feet of caulked joints. There is no housewrap or barrier behind it as a backup. In theory the rigid foam should act as a barrier but it was installed poorly with random missing sections and seams everywhere (as shown in photo) because small scrap pieces seem to have been used. Another issue is they used 1" foam when the gap between the studs and window flange is actually only 3/4". You can't see it but the foam is actually shaved down somewhat behind the flange to fit or not even placed completely under the flange.

    My plan is the following:

    1 - Remove all of the existing flashing, boards, and foam between the windows.

    2 - Fill in the gaps between the windows and studs with spray foam. You can't see it from the photos but I can reach in behind the window flanges and pull out the small strips of fiberglass (sometime not even present) that were tucked in originally and then fill it with spray foam.

    3 - Attach 3/4" foam board to the studs behind the window flanges. Plan is to put a layer of caulking between the back of the window flanges (over top of the nails holes) and the foam board. Only nails going through the foam will be the ones going through the window flanges (which will be going through the layer of caulking). The differences between my job and the original is that there will be minimal joints in the foam board, no random nails through the foam board, and caulking between the foam board and window flange.

    4- Apply 9" wide flashing tape over the window flanges and foam board on the vertical studs and 6" flashing tape on the horizontals. The flashing tape would span the entire gap and therefore no house wrap would be needed. Is that a problem, not having the breathable quality of housewrap? Also what would be the best flashing? A Dupont StraightFlash type of product or a thinner alternative like 3M 8067? Seems like Straightflash might be the better choice.

    5 - Cover everything with painted PVC trim board and optionally caulk it.

    My thoughts are that this approach doesn't rely on or need caulking between the PVC trim board and windows. It can be added for aesthetics but is not critical.

    Will this work? Is there a better approach?

  7. GBA Editor
    Martin Holladay | | #7

    You are correct that you don't want to depend on caulk for any of the waterproofing.

    You can use vapor-impermeable materials for this repair work, as long as the studs can dry to the interior (which they can).

    Peel-and-stick flashing (or tape) is relatively inexpensive. You might want to use two layers of flashing: One over the studs, from window to window, before the rigid foam is installed; and another over the window flanges, after the rigid foam is installed.

    All of the usual rules apply: when possible, lap the joints shingle-style, so that the upper layer laps over the lower layer. The bottom flanges of the windows shouldn't be taped -- they should allow drainage.

  8. DIY_2016 | | #8

    Thanks for the feedback Martin. I like the idea of two layers of protection. I had to put this project on hold for the last couple of weeks so I haven't started. Before I do I have a couple of more questions please.

    - Would it be better to use house wrap (Tyvek) instead of only vapor-impermeable flashing tape for the two layers. The 1" layer of XPS is semi-permeable and I don't intend to completely seal the outer layer of PVC trim board. So the stack-up from the inside out would be paint, hardcoat, drywall, solid wall of 2x4 studs, housewrap, 1" of XPS, housewrap from window flange to window flange, 3/4" PVC trim board (installed with no caulking and some type of 1/4" thick furring strips behind it to build it out so it's closer to being flush with the windows). Could this setup dry to the outside or would the XPS foam and PVC trim board (again with small gaps on the side and 1/4" furring strips behind it) not allow it?

    - I'm installing the 1/4" furring strips behind the PVC trim boards to ultimately make the finished project look better. However, from a functional standpoint it can serve as a rain screen. Does this make sense and do you see any reason not to do this?


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