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Wind-driven rain is penetrating through the sill during construction

Dreamhouse67 | Posted in Green Building Techniques on

Our windows have been installed with sills wrapped in tar paper and peel and stick, flange caulked and corners sealed and entire flange tuck taped. Exterior building wrap not yet installed as our sheathing inspection has been delayed. Over the past couple of days of strong wind/rain storms, water is penetrating through to the interior sills. Before we go any further, is this likely an issue with how the windows have been installed or will proper exterior rain screen solve this issue???

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  1. GBA Editor
    Martin Holladay | | #1

    It's really hard to answer your question without access to your job site (or good photos). There are many components of a house that help keep rain away from the rough sills of your window openings; these include roof overhangs (how wide are yours?), siding, the rainscreen gap, and the WRB, in addition to the details that were used to flash the window rough opening.

    It's highly likely (but not certain) that when your house is complete, your rough sills will be dry.

    The best mantra to remember is, "Flash the rough opening, not the window." Before you install your windows, your rough opening should have a sill pan that drains to the exterior, and flashing on the rough jambs that direct water to the sill pan. Do your window rough openings have those features?

    It's hard to imagine a good way to keep your rough sills dry without a WRB (plastic housewrap, asphalt felt, or a similar product). But from your description, I'm not sure that your sill pan details are robust enough.

  2. Dreamhouse67 | | #2

    While sill pans were used for the all the deck/French doors, I can guarantee none were used for the windows. Overhangs are 18-24" and you could see on the outside that they had been doing their job as all was dry above the windows.
    These were done 2 months ago while we were still trusting and not asking enough questions or informing ourselves prior to things being done. Now trying to keep a step ahead, research, ask questions and get what we need for this house. with the next thing being our WRB and rain screen, I want this done right!!!

  3. JC72 | | #3


    Your GC should explain why he allowed the windows to be installed prior to the WRB and provide a remedy.

  4. Expert Member

    That's the sequence many WRB manufacturers recommend.

  5. JC72 | | #5


    I wouldn't call it a recommended sequence but a less than ideal workaround. I've seen flanged windows installed w/out the WRB first where they don't include the lower (lapped) piece of WRB as seen in beginning of the video. They just tape it the bottom course to the flange..

  6. Expert Member
    Michael Maines | | #6

    Frances, what kind of wind zone are you in, and what is your topography?

    I was once involved with a project in a very punishing location, on a high bluff facing open ocean on one side and a cove on the other side, and learned the hard way that constant 30+ knot winds would not allow the high-DP rated windows to self-drain as designed. Roof overhangs did not matter in our case because the rain usually comes in sideways or upside-down. We had excellent WRB and flashing details but the windows we ended up using were just not designed for that situation.

    Most builders and many residential architects are not familiar with the options for that kind of exposure. Feel fortunate that you caught this problem early. If you allow construction to continue without addressing it, potential solutions become much more expensive. We ended up stripping the entire face and adding a rain screen, and even that did not fix the problem. It probably would have worked with a different window, but not the ones we installed.

  7. Expert Member

    Taping the the flange at the sill? Ouch! Apart from the sill though the only difference is whether the WRB laps the flange at the jambs or is underneath it. Both ways should end up with flashing tape on the joint. I don't see much difference as long as all the bases get covered.

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