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Community and Q&A

A couple window flashing questions

joenorm | Posted in General Questions on

I’m using Tyvek as a WRB. FlexWrap and Straightflash for the pan and jamb flashings.

I remember Malcom saying that a metal head flashing should also be incorporated into the mix…..but I cannot remember under/over which layer he suggested it placed?

Also. I am leaving caulking and tape off of the front of the sill as suggested here and in the Tyvek instructional video. But I am wondering about this point at a means for air intrusion?

How do you properly air seal the sill of the window if you’re leaving it open for drainage?


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  1. Expert Member


    The head-flashing would be installed just before the Tyvek in this sequence:

    Just after the Straight-Flash around the windows in this sequence.

    In both cases bed the flashing in caulking on the top of the window frame.

    Air-sealing around the window should be at the interior between the window frame and the rough-opening. Tape, caulk or foam.

  2. BrianPontolilo | | #2

    Hi Joe.

    Malcolm gave you good advice. Also, if you are leaving the bottom window flange open for drainage, you should install a sloped sill beneath the window on top of the rough sill. A piece of beveled siding is often used. Air seal near the interior edge of the window. Here's a drawing of the head flashing detail and another showing the sloped sill that you may find helpful.

  3. joenorm | | #3

    With the window flange screwed tight to the FlexWrap at the sill, I really don't see how water is going to want to "drain" here. Evaporate through over time, maybe.

    Thanks for the videos and diagrams, very helpful.

    1. Expert Member
      MALCOLM TAYLOR | | #5


      What actually happens at the sill is a bit of a mystery. We know in the past it was the most vulnerable part of a window assembly that often rotted. In reaction we now slope the sill, and provide a membrane so bulk water can drain. When these details first came out it was suggested the the window flange be mounted on top of the run-screen battens, and that gaps be left at the sill to provide a direct path. Do either rain-screen cavities or window-sills actually see sufficient water to drain, rather than evaporate? Probably not, but it doesn't do any harm to detail them as though they would.

  4. Expert Member
    Peter Engle | | #4

    Flexwrap has wrinkles that allow for weeping and you should not be screwing the sill flange "tight." "Snug" is about right. Remember that we're usually not talking about a lot of water here. If the rest of the installation is done right, the window sill pan only sees water coming from major storms and the (unlikely, we hope) leakage through the window sill itself.

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