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Community and Q&A

Butyl or Rubberized Asphalt Window Flashing

[email protected] | Posted in General Questions on

Hello everyone,

I have window installers coming to install vinyl replacement windows in my existing 1940’s house with brick veneer, over tar paper over wood fiberboard sheathing. Climate zone 6. The existing windows are vinyl from the mid-90’s.

I am assuming there is no window flashing around the rough opening, so I planned to add some while the windows are out. While most of the window openings seem fine, there is a small indication of water infiltration (bubbling of the paint/plaster) below the living room window (the largest window, although not facing our predominant winds). I made an opening in the plaster at this location and it seemed dry now but might be in for some surprises when we see the rough opening.

There is almost a 2″ air space between the brick and the sheathing, so I am hopeful I might be able install something and apply reasonable pressure…

My first thought was to use rubberized-asphalt flashing, because of possible issues with butyl bonding to the tar paper. From what I read, rubberized-asphalt has 2 problems: vinyl and sealants. I believe the vinyl in the windows is not an issue with the rubberized asphalt (I sure hope not…) but I have no idea what kind of sealants the installers will used (or frankly, where they will use them). The windows are flangeless replacement windows. The gap to the brick will be bridged with custom metal flashing, similar to the existing.

So, options:

1. Use butyl flashing and hope it works ok with the tar paper.
2. Use rubberized-asphalt flashing and hope it doesn’t get messed up by the caulking.
3. Don’t add anything, it’s been (mostly) fine for 80 years…

Thanks in advance!

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  1. bongo30 | | #1

    What did you end up using? My installer used asphalt-based flashing tape on window flanges and now I have to research my options.

  2. [email protected] | | #2

    I used butyl flashing but probably could have used rubberized asphalt because I don’t believe any caulking was applied directly to the flashing anyway.

    Unfortunately I don’t know how the flashing is holding up because I can’t see it but time will tell.

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