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Community and Q&A

Flashing windows in extreme exposure category

Smc1900 | Posted in Energy Efficiency and Durability on


I’m in zone 6, 4800 HDD, Extreme wind and rain category. I’ve reviewed the details on GBA for window flashing, flanged window, water managed, housewrap —  the complete 14 step installation.

I’m questioning the wisdom in step 6 where they fold the house wrap into the sides of the opening, and then subsequently placing a bead of sealant over the housewrap, installing the window, and then flashing over the flange and the housewrap. If we assume that housewrap will leak eventually, have we not allowed the water to get into the rough opening? Would it not be better to place sealant on the plywood sheathing, install the window, install flashing tape over the flange and against the plywood, then install the housewrap over the flashing tape and use either another piece of flashing tape or perhaps just sheathing tape (such as Tuck Tape in Canada) to secure the housewrap against the window flange?

Or maybe I’ve misunderstood step 6 and the subsequent steps?

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  1. GBA Editor
    Martin Holladay | | #1

    There is more than one way to flash and install windows.

    We install siding to keep rain away from our sheathing. Then we install housewrap to handle the very small amount of rain that gets past the siding.

    You are asking whether we should worry about water that gets behind the housewrap. We can play this game infinitely, can't we? If siding stops 99% of the rain, and housewrap stops 99% of the rain that gets past the siding, should we worry about the 1% of the 1% that gets past the housewrap?

    I think the basic answer to your question is to include a rainscreen gap. The rainscreen gap provides all kinds of redundancy. It allows drainage, it provides a capillary break, and it facilitates ventilation drying. You should include the gap, and then sleep well at night, without worrying about the water that gets behind your housewrap.

  2. Expert Member
    Michael Maines | | #2

    Steve, wrapping the WRB into the rough opening is for the occasional drop of water that finds its way through, not for bulk water. It's probably not really necessary if other details are done well, but it's standard practice for builders and there's no harm in using that detail.

    Having designed and built homes in high-wind, rainy locations, I agree with Martin that a rainscreen gap is very important. Another detail that is often overlooked is how to the windows keep water out; if it's a self-draining system, designed to take on water and release it through weep holes, you might want to choose a different system, as those can be overwhelmed with non-stop wind, unless they drain into the rain screen cavity.

    1. Smc1900 | | #6

      I'm using Kohltech casement and picture windows, in their supreme line, so no weep holes.

      Looking at what most contractors around here do, they don't wrap it into the opening.

  3. Expert Member

    I find wrapping the WRB into the rough-opening makes sealing between the framing and window jambs a lot more difficult.

  4. Expert Member
    Peter Engle | | #4

    I agree with Malcolm. I like to cut the paper flush to the outside edge of the rough opening, then flash the opening jambs and sill with sticky tape of whatever flavor you like. Apply sealant to the WRB (sheathing at the top), install the window, then tape the sides and top, flip the WRB down over the top fin and tape the top and cut corners. Lo-rise spray foam between the frame and the sash all around and you've got a pretty bullet proof opening.

  5. Expert Member

    I install windows pretty close to this method - although for some reason they inexplicably don't flash the top.

    1. Smc1900 | | #7

      I like that approach. Should they not tape the top of aluminum flashing (I assume it's aluminum) over the foundation? Also, I'm not sure what you mean when they don't flash the top - maybe they updated the video after originally posted?

  6. Expert Member

    All our walls have rain-screens. With them I make the assumption that some water may get into the cavity, but none behind the WRB, so I would be happy relying on the lap over the base flashing, rather than taping it.

    We include (our code requires) a metal head-flashing with end-dams on all windows, rather than just the Flash Tape they show in the video. Bedding the head-flashing in caulking insures no wind-blown rain can push up between the it and the window frame.

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