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Window Flashing – Housewrap over ZIP

user-6651407 | Posted in General Questions on

We will be installing Housewrap over our ZIP system sheathing.

The Housewrap will act as the WRB. Windows already installed and flashed with ZIP tape.

How can we best flash the windows and doors with the addition of the housewrap, since the windows are already installed?

Also, are there moisture issues associated with using two layers of WRB against each other?

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  1. GBA Editor
    Martin Holladay | | #1

    Using housewrap over Zip sheathing doesn't create any moisture issues, as long as the windows and doors are properly flashed.

    Your biggest problem will be at the rough sills. Presumably, the rough sill have sill pans -- probably site-built sill pans made of peel-and-stick flashing. These sill pans must direct the water to the WRB -- in your case, the housewrap. So you will need to lift the peel-and-stick flashing at the window sills -- assuming it is liftable -- so that it laps over the housewrap.

  2. JC72 | | #2


    Not to be an @#!$ but hasn't the OP asked this question in various forms over the past two months?

    I'm beginning to question the sincerity of the OP.

  3. user-6651407 | | #3

    I was trying to find my previous post. I didn't mean to post the same question twice. I finally stumbled upon my prior thread. I do apologize for that. I am sincere. Sorry for any confusion or frustration.

  4. user-6651407 | | #4

    Martin, so I will not need to have the builder remove and re-install the windows? That is the part I was trying to wrap my head around.

  5. GBA Editor
    Martin Holladay | | #5

    The answer depends on what type of flashing your builder installed on the rough sills, as well as on the type of windows.

    We need either a more detailed explanation, a photo, or both.

  6. GBA Editor
    Martin Holladay | | #6

    If any GBA readers (including Bridget) want to review the earlier thread on this topic, here is the link:
    ZIP sheathing with asphalt felt: Window and door flashing.

  7. GBA Editor
    Martin Holladay | | #7

    Are these flanged windows or unflanged windows?

  8. user-6651407 | | #8

    Windows, Doors, including rough sills, are presently flashed using ZIP tape. I am unsure if your question above was asking about the brand of windows, but they are ReliaBilt. I am getting differing opinions on if they should be removed in order to properly re-flash.

  9. user-6651407 | | #9

    They are flanged.

  10. GBA Editor
    Martin Holladay | | #10

    First of all, I'm not sure that it's all that important to move your WRB to a new layer.

    But if you really want to do it... you may have to back off the fasteners on the bottom of the window, and investigate where the Zip System tape terminates. You need to lift this Zip System tape, so that it can lap over either (a) the housewrap, or (b) a new piece of flashing tape that laps over the housewrap.

  11. walta100 | | #11


    Can you please help us understand why you have lost faith in your Zip system choice? It would be a rare bird that would suggest that housewrap is more likely to be water proof than Zip.

    Zip system is arguably the best water/ air barrier on the market, you have invested a lot of time and money in the Zip system.

    If your Zip system was so poorly installed that it must be replaced why do you think the same contractor can correctly install housewrap?

    From here your question sounds like “We will be installing a blue tarp over our standing seam copper roof. “

    If you do not trust your WRB aim a garden sprinkler at the Zip for an hour and test it. If your hunch that you have leaks is correct, make the contractor fix them.

    The one thing I now understand about building a house once you have made a choice do not look back, second guessing will only make you crazy. There are a thousand choices ahead of you, embrace the decision you have made and concentrate on the next choice you must make.


  12. user-6651407 | | #12

    We have almost reached our 180 day exposure. Siding is on order, but we want to make sure we follow ZIP system instructions. Also, I have read quite a few articles, on this site even, where builders still install housewrap over ZIP.

    Martin, I am still waiting on Huber to respond, but can you help elaborate on keeping the WRB with the ZIP, installing housewrap over it, how can the windows be properly flashed? I hope that question makes sense.

  13. GBA Editor
    Martin Holladay | | #13

    Are you the builder? Or are you a homeowner who is second-guessing the decisions of your designer and builder?

    Either Zip sheathing or housewrap can be used as a WRB. For more information, see All About Water-Resistive Barriers.

    No matter which layer is chosen as your WRB, every window rough opening needs to be flashed before the windows are installed. The rough opening needs sill pan flashing and jamb flashing -- as I said, before the window is installed. After the windows are installed, the windows need head flashing.

    The sill pan flashing needs to direct water from the sill pan to the exterior side of the WRB.

    For more information on flashing windows, see this video series: Video series: Flashing windows.

    In order to follow the advice I'm just given, you need to know which layer -- the Zip sheathing or the housewrap -- is your WRB. Choose a WRB, and flash to that layer.

  14. user-6651407 | | #14

    I am a homeowner second-guessing.

    I do understand either ZIP sheathing or housewrap can be used as the WRB.

    The windows are flashed right now with the ZIP as the WRB.

    What I am having a hard time understanding, is how can you keep the ZIP as the WRB, add housewrap to satisfy the 180 day exposure requirement, and properly flash the windows. My thinking leans towards having to make the added layer of housewrap as the new WRB. You had mentioned before, it might not be that important to move our WRB. But we would still want to make sure the windows incorporate the added housewrap.

  15. GBA Editor
    Martin Holladay | | #15

    You are correct: Zip System sheathing should be covered with siding no later than 180 days after it is first exposed to the weather. The fact that you are exceeding that limit is a sign of poor planning, but I imagine that you and your builder already know that.

    Installing housewrap over the Zip sheathing is inexpensive and can't hurt. It makes sense to do that.

    If I were you, I would probably leave the window flashing as it is (assuming, of course, that your research shows that your builder followed the recommended flashing steps). My guess is that the Zip sheathing will stay watertight.

    Installing housewrap over the Zip sheathing is good insurance, even if the housewrap is not used as a WRB.

  16. walta100 | | #16

    If you are sure the Zip system is install correctly I think you should leave it alone.

    Adding the second WRB is much more likely to create a leak than over exposed zip.

    I say do not sweat the 180 day limit, undoubtedly Zip picked t a conservative number under the worst case conditions. Are you in a location that gets a lot of UV light say Utah or New Mexico? Understand your exposure days were short winter ones. Also understand building material warranties are close to worthless as they never cover labor, and they never admit fault before declaring bankruptcy.

    That blue tarp I mentioned before may not be a bad way to temporary cover your Zip and keep your exposure under 180 days.


  17. Chaubenee | | #17

    Walter gives good advice. But if you are an excessive worrier than put some tyvek on, it can't hurt. I think you are second guessing yourself and your build a bit too hard and panicking. A hundred and eighty days or a hundred and ninety. Will it matter?

  18. Expert Member
    MALCOLM TAYLOR | | #18

    It's probably worth remembering that the clock starts ticking on your WRB once it is exposed to sunlight too.

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