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Window installation in wood buck

CanadianExpy | Posted in Green Building Techniques on


Just installing outie windows in a new build. The question I have is the air and water sealing method, I have my WRB running inside the sides and bevel sill wrapped in stretch tape. The gap is 3/4 on either side of the the window, which have no flange. I have seen either spray foam the whole gap  and depth or  backer rod with a silcon ( adhesive). I also seen backer rod on the interior and exterior with nothing in between.  At the sill I know not to close it off , but should just put a interior seal some how. 

Thanks for any help.
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  1. GBA Editor
    Martin Holladay | | #1

    Use a high-quality European air sealing tape at the interior (especially at the bottom -- the stool).

  2. CanadianExpy | | #2

    Not sure I can get that, at least not anytime soon. Any preference on brand?


  3. CanadianExpy | | #3

    When you are talking about tapping the interior , are you saying just tape from window frame to the wood.?
    Anything recommend on the exterior side?

    Thanks again

  4. Expert Member
    Peter Yost | | #4

    Hi Dave -

    In order to keep your sill pan flashing free to drain, you need to airseal the interior of the window unit to something in the rough opening. If you have a back dam as part of your sill flashing, you use a tape to air seal between the backdam and the window unit.

    Frankly, you could use the Huber Stretch tape to accomplish this air seal: you apparently already have it and it has a very aggressive and durable acrylic adhesive.

    I have to say though, in your third photo, pretty scary looking connection of your sill flashing to your torn WRB...


  5. CanadianExpy | | #5

    Thanks Peter
    yeah the picture makes it look bad, I am going to be redoing that one. I have had to install the WRB (tyvek) around the windows before the 6" of exterior insulation.
    I don't have back dams, I thought a slope sill was better.
    I am trying to get my hands on more Zip tape but its been an issue, I have lots of 3M tape would this work? I may be able to get the ZIP tape.
    I assume taping from the window frame to the wood on all sides and not the Tyvek is best.

    If you have any other comments, criticisms , suggestions I appreciate them.

  6. Expert Member
    Akos | | #6


    For my windows with sloped sill, I had the peel and stick come all the way inside the rough opening. I only had the house wrap come near the rough openings, the rest was all peel and stick.

    For sealing, I went with backer rod and caulk on the inside (tape is easier, the 3M stuff seems to work well, super sticky to most surfaces) and spray foam the rest of the bottom. Canned foam is hard to get liquid tight and will drain.

  7. gawdzira | | #7

    WTF, Walmart sells it? This is the type of tape that is your buddy.

    Siga Wigluv tape from Walmart

    1. CanadianExpy | | #8

      Unfortunately not in Canada
      Thanks though.

    2. GBA Editor
      Martin Holladay | | #9

      Thanks for the link. I must admit that I never thought I'd see the day that Walmart would stock Siga Wigluv tape...

      1. mackstann | | #11

        Just like Amazon and Ebay, Walmart allows other merchants to list and sell items on If you look closely you'll see that Siga tape is actually sold by some random merchant called UnbeatableSale who just happens to list things on

  8. CanadianExpy | | #10

    I have been searching online for ideas on how to flash the front of windows and I am not sure I should I should have my Tyvek wrapping all the way inside past the front of the window?
    Any opinion on this...

    1. Aedi | | #12

      This website has a good step-by-step guide for window installation using a lumber buck:
      To my knowledge, how far into the window the tyvek goes does not matter. What is important is that it extends underneath any flashing you use.

  9. natesc | | #13

    These diagrams should be helpful. Looks like they use backer rod on the inside and outside, and leaving weep holes at the exterior sill backer/sealant. That makes a lot more sense to me than leaving the gap exposed on the exterior.

    I'm not a big fan of how you're using the house wrap. I would use 6" zip tape. I would just cut squares of the stretch tape and use it on all the corners, it actually makes it easier to get the corners nice and tight and it saves a lot of money.

    The reason they don't like using spray foam is that when the window leaks there is no clear path for the water to run outside. I personally have mixed feelings on that. The nice thing about spray foam is that it's more bulletproof than caulk and when the windows need to be replaced it is easy to cut through though though foam. Also cured foam can be peeled off of vinyl without leaving a trace.

    A sloped sill WITH a backdam is really the way to go. In a perfect world I would almost like to run the flashing all the way back and lap it over the drywall, that way you can see the damage right away. I have seen water stains on flashing tape indicating that water migrated up the sloped sill.

  10. CanadianExpy | | #14

    Thanks Nathan for the link very helpful.
    So in my case WRB is my Tyvek which I always understood had to be on the exterior of the foam.
    So that is why I have ran it into the rough opening. My sheathing is my air barrier, the 2x wood buck has been sealed and taped.
    Few questions , why are they running a membrane into the head of the window opening? what is its purpose?
    As for the sill what size, how many weeping holes would be recommend? Have never seen this actually done.
    As for the 3/4" gap between RO and window, I thought maybe Roxul as this would allow water to escape down to the sill? but would give some insulation value or sure if this is worth it?
    I like the idea of backer rod and sealant for interior and exterior sealing.

    1. natesc | | #15

      In diagrams 7a,7b,7c, the WRB is behind the insulation, on the face of sheathing. There's nothing wrong with that as long as you don't spray foam the cavity insulation or put up plastic on the inside of the studs. Also depends on climates and all that, YMMV disclaimer.

      They are using liquid flashing in this example to connect the WRB on the sheathing to the window. The window leaks so you want the WRB to extend far enough inside the buck and to drain outward so that it doesn't rot anything.

      It is fine to put your WRB on the outside of the foam, and it some ways makes things simpler. The main way it would be simpler is if you wait to install the window until the insulation and WRB is installed, and then you can install your windows by the book instead of trying to cobble the WRB together with strips of tyvek. In my opinion there is more likelihood of errors occuring by installing windows before the WRB is installed.

      I think every 12" or so for weep holes would work. You could also backer rod and then low expansion spray foam as it might be a little faster/less work filling a 3/4" gap. Low expansion should not push the back rod around.

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