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Community and Q&A

Windows Leaking at Sill

Shane_McCafferty | Posted in General Questions on

I thought I did everything right…but my window is leaking at the sill and I don’t know why…or how it’s even possible under the circumstances.  I hope you guys can help.

I wrapped the house with HydroGap WRB, cut the window opening flush at the top and bottom, wrapped the sides into the jambs, angle-cut the WRB above the window and folded the flap up,  used flexible acrylic flashing tape on the sill and 6” up the jambs (after first taping the sill corners), shimmed and installed the windows by screwing through the nail flanges, spray foamed, flashed the nail flanges with HydroFlash tape in shingle fashion (starting at the bottom, then sides, then top), folded the top flap of the WRB down and taped the flaps.

I’ve come to realize that I shouldn’t have taped the bottom flange. Is this simple error the problem?  If so, how would the water get past the top and side flashing tape and then the spray foam to leak onto the interior side of the sill?

I’m baffled.  Hopefully there’s a simple fix and I don’t need to completely  remove all of these windows and start over.

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  1. Expert Member
    NICK KEENAN | | #1

    Leaks like this are a pain. The leak could be anywhere in the wall above the window, if the water gets behind the WRB it comes out along the bottom of the window.

    The best possible case is if you can reproduce it at will by spraying the wall with a hose. Then you can start figuring out where the leak is. If you can't reproduce it you'll never know if you've fixed it.

    1. Shane_McCafferty | | #2

      So it’s not a huge issue that I’ve taped the bottom flange?
      I stuck my hand under the top flap over the window and it wasn’t wet under there. But I suppose It could be coming from the left or right of where I checked.
      It’s definitely possible that water could be getting behind the WRB where it meets the fascia board at the top of the wall. This window is installed in a flat-roofed wall. None of the siding, eaves, or aluminum work has been done yet so it’s definitely seeing a lot of water in tonight’s rain storm.

  2. Expert Member
    Michael Maines | | #3

    Can you describe more about your leak? Have you noticed it happening in some rain events and not others? Is the WRB sealed to the roof plane? What is the window's DP rating? What is your environment like--flat suburban land, exposed coastal hilltop or something in between? How airtight is your house, and do you know if it operates under neutral, positive or negative pressure?

    1. Shane_McCafferty | | #6

      This was the first rainstorm since installing the windows. The WRB isn't sealed to the roof plane yet. It simply terminates under the fascia board. I was concerned that water was entering from this point and getting behind the WRB but everything seemed dry when I investigated. I live in a rural area - surrounded by corn fields on 3 sides and trees on the 4th. The house isn't remotely airtight because we're still in mid-renovation...but it will be when we're done!!

  3. creativedestruction | | #4

    The taped bottom flange is the issue. Cut holes in it to ensure drainage at the sill. Water always finds a way in. You just need to give it a way out.

    See this article, and don't miss footnote #2:

  4. maine_tyler | | #5

    Could the windows be leaking?
    side flashing tape laying nice and flat?

  5. Shane_McCafferty | | #7

    Thank you all for your comments. I was definitely stressed last night when water started leaking in my brand new windows...but in the light of day I've managed to solve the issue (I think). Jason S. was right...the taped bottom flange was definitely part of the problem. I cut the tape below the window this afternoon and water trickled out. I had a good look around the top of the leakiest window and spotted a small gap at the top corners where the side nail flanges meets the top nail flange. I should've done a better job of wrapping the tape onto the sides of the windows instead of just covering the nail flanges. They're tiny holes...but probably enough to allow water to leak into the rough opening and get trapped behind my taped bottom flange.
    We're supposed to get more rain this weekend so I'll be able to put my repair to the test. Fingers crossed!!!
    Thanks again GBA community!!!

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