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I removed the window stool trim to replace it and discovered the following.

  • No window flashing - is this a big problem? Should I try to add it?
  • Gap under window is poorly sealed - remove the old foam and then add new foam... or?

The outside of the window is vinyl siding. Easy to access.

Instead of replacing the foam, should I do something different (backer rod, caulk, etc)?

Should I insulate the wood with 1/2 foam board?

Anything else I should consider before installing the new trim?

Additional context: This is a photo of the window from the interior. At the bottom you will see the drywall. Above the drywall is the wood framing. Between the wood frame and the window is a gap that is somewhat filled with foam.

interior window stool

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If your home was built to modern standards it has flashing of some sort on the outer portion of the window opening--a pre-formed plastic tray, rubber flashing tape, or a combination of those. You wouldn't necessarily see it from the inside. Water is supposed to be handled on the outer face of the wall.

I would simply procure a can of "low expansion" (window and door) spray foam and overlay what's already there. You're seeking to accomplish two things: improve insulation (R) value, and create an air seal (assuming the flashing hasn't already done that). If your plan is to install a new extension jamb, do that first. Then insulate with the foam, then install casing.

Note that spray foam is extremely messy. Consider it permanent on anything it touches. Take appropriate precautions such as masking the drywall and floor below the window. Once it's sprayed, let it cure (about 20 minutes), then shave off excess with a knife.

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There was no foam behind your window stool trim, so moisture probably got in there and rotted the wood. I would replace the wood with either batt insulation or 1/2" foam board. If you choose to use batt insulation (probably the easiest to install), you should also caulk the gap between the window and the vinyl siding. A dab of paint will prevent water from entering again.

Its just my view!

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  • There was no mention of rot, and I don't see any indication of water damage in that photo. Also, vinyl siding shouldn't be caulked. It needs to move, and caulk rarely works in that situation. It'll also look terrible.
    – isherwood
    Dec 30, 2022 at 13:50

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