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I have four double windows where moisture appears in the center or corners of the interior window sills when it rains. Even a light rain will show some moisture/softness on the interior window sill. I never see any water leaking though, just moisture appearing. It is not condensation — happens in all temperatures when it rains. Also, the windows seem to be draining properly - I don’t see water collecting between the pane and the screen.

Windows are on three different sides of the building.

left corner of window 1

center of window 1

right corner of window 1

enter image description here

right side and corner of window 3

My downstairs neighbor has issues with one of the windows directly below mine. No one else in our building has reported any issues with their windows.

Our building has gotten a new roof (thought to be unrelated and seems to be), and we’ve had many attempts to fix the window leaking from the outside (resealing multiple times) and inside (recaulking around windows). Nothing seems to help.

The photos show 3 of the 4 windows (one has dark paint on the interior sill, so it’s hard to photograph). Two of the windows have had interior repairs started, which is why you see plaster (?) instead of paint and discoloration of the plaster. The third window also has some issues with the bottom corner of the window frame, and the third window just leaks a bit different than the other two pictured.

We had a window/roofing company do a water test after the most recent exterior resealing and new roof, and they were hardly able to get the windows to leak at all. The only time a little moisture showed up was when water was directed on the top trim of the window, even though we never see any moisture at the top. They said the water test ruled out the siding, etc., though.

That company proposes the following solution: “Our solution for this is to remove the PVC trim surrounding the leaking windows, install tyvek siding tape on the wall area behind the removed PVC trim, install new PVC trim surrounding the leaking windows, and then seal with caulk.”

Another company says that we need to replace all of the windows entirely, and the indicator of that is that there’s moisture/wetness on the sill and not at the top of the windows.

We’ve had the outsides of the windows resealed with caulk so many times and seen zero improvement, so we’re just not sure that fixing only the trim is the answer, but new windows are so expensive.

Has anyone else seen this happen? Any ideas?

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  • Unless the window frames are rotted away or the windows are busted, replacement will only empty your pockets. It might come down to the windows having to be removed and put back in right.
    – crip659
    Sep 24, 2023 at 15:40
  • Bottom of window frames likely has clogged-up drainage channels. They can clog up just from accumulated dust, spider webs, fly "specks", etc. The mesh some window makers stuff inside the channels to exclude bugs makes the problem worse. The water backs up and leaks to the interior thru imperfect frame joints.
    – kreemoweet
    Sep 25, 2023 at 1:27

2 Answers 2

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It is entirely plausible that rain is entering the area between the wall and the windows from the top. gravity is pulling the moisture down and it is pooling at the bottom. That is manifesting itself as the moisture spots that you are seeing.

The windows need to be sealed. The plan of removing the trim and sealing with window and door flashing tape, reinstalling the trim and caulking is a sound idea.

The previous caulking most likely was ineffective because it was directed at sealing the trim and not the window to the wall.

The idea of replacing the windows is an attempt to sell you windows.

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Proposed option 1:

"remove the PVC trim surrounding the leaking windows, install tyvek siding tape on the wall area behind the removed PVC trim, install new PVC trim surrounding the leaking windows, and then seal with caulk."

This is the least intrusive intervention that might work, and would be my minimum first attempt. If the siding extends under the PVC trim, it will also need to be removed. I assume that inspection/investigation of possible sources of rain penetration farther up on the wall and on the roof above have already been looked into...

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