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Spray foam and drainage space at sill pan cavity

elyk22 | Posted in General Questions on

Hello GBA,

The gap of the sloped sill pan and window was spray foamed to the back of window flange.

I’ve removed foam from some windows. Is it worth removing SPF from other windows and re-spraying with less foam.. or use backer rod/sealant for interior seal?

Or should I leave the rest like they are and have the blocked drainage gap.

I’m wondering if it is more important to insulate under window frame to improve performance of the outie windows.

I called the window company and they said insulation of this space is more important then drainage potential. Reason being, if you allow air into this drainage space, it could find a way into the window casing (if there was a crack) and leak into the window and possibly living space?


I apologize for asking too many questions in my last post. It does not promote clarity and no one wants to read a novel, I should know better 😉 Won’t happen again!


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  1. GBA Editor
    Martin Holladay | | #1

    The usual method for installing a window is to leave an exterior drainage path at the sill. It sounds like you did that. (For example, if your windows have flanges -- I don't know whether yours do -- it's OK to install caulk under the window flanges on the jambs and head, but it isn't a good idea to install caulk under the bottom flange.)

    After the exterior water management and flashing details are complete, the air sealing work (between the window frame and the window rough opening) is done from the interior side. The most common way to do this is with canned spray foam, as you did. (Some builders use European tape to air seal these areas -- that's a good approach, but it isn't cheap.)

    In general, you want to use a light touch when foaming the window sill on the interior, to create a bead on the interior side of the window, without adding so much foam that you completely fill the entire gap all the way to the siding.

    That said, I agree with the window company you consulted. I think you are overthinking this. If you flashed the rough opening before the windows were installed, and your flashing method includes a sloped sill pan, you shouldn't worry. You won't get much (or any) liquid water on your window sill, and any moisture that gets there should evaporate before it causes any problems. It's not going to reach your stud cavities in any case.

  2. elyk22 | | #2

    Thanks Martin,

    I'll refoam the gap.. but try not to go all the way back tight to the flange.

    Is it ok to use ridgid foam (type 2 EPS) supporting shims at the sill? Would that provide a thermal break to window frame? Or stick with the cedar shims.

    overthinking probably ;)


  3. GBA Editor
    Martin Holladay | | #3

    I would use cedar shims rather than EPS. But other factors matter -- the width of the window, how well the flanges are secured, and whether or not you have enough spray foam to provide some structural support.

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