Flashing/Tyvek sequence for windows

Old 06-15-05, 11:37 AM
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Flashing/Tyvek sequence for windows

I've rebuilt two sides of my duplex, replacing everything (including the mudsill). The plywood sheathing is up, the windows are ready. I'm now studying flashing procedures for the windows. They seem to indicate that the Tyvek housewrap goes on first (slitted at the top for window head flashing), followed by 9" flashing at the windows over the Tyvek (bottom sill first, and overlapped top to bottom).

Most of the 9" window flashing rolls I've seen have self-adhesive backing on them. I can see how this would be useful if the flashing was applied directly to the sheathing. However, if this flashing is to be applied over the Tyvek, it would seem that through-fasteners would be needed too -- and the flashing adhesive would then be of limited use.

Am I missing something here? And: Is there something "better" to use for window flashing than the SBB (asphalt) based flashing rolls? I read on one label that this stuff doesn't like PVC. Does that mean that it shouldn't be used with vinyl windows?
Old 06-15-05, 05:35 PM
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It's recommended that you use Tyvek peel and stick products with Tyvek housewrap. If you purchase Tyvek Straightflash, you can follow the instructions on the box. The June 2005 issue of Journal of Light Construction also has a nice article on this very topic. In a nutshell:

Window rough openings must be cut out before the housewrap is installed. The tyvek is then applied to the sheathing. Then you cut the tyvek in a special way: first make a horizontal cut along the top of the R.O. At the top corners, cut 9" slits at a 45 degree angle, and fold the top tyvek up and out of the way, temporarily holding it in place with a piece of contractors tape. Next, make a vertical cut that extends 1/2 way down the middle of the R.O. From the middle of the opening, cut at 45 degree angles down to the bottom corners of the R.O. (at this point, the JLC article recommends installing a piece of 4" cedar siding in the bottom of the R.O. to provide a sloped sill.) Fold the Tyvek flaps (left, right & bottom) into the opening, cutting them off even with the interior framing, and taping the edges with contractors tape.

Next comes the sill flashing, if so desired. (Tyvek Flexwrap works well to form a flexible "pan flashing" in the bottom of the R.O. before installing the window.) Caulk the top and sides of the R.O, then install the window. Once the window is shimmed and nailed off, apply Tyvek Straightflash over the side nailing fins. Then apply Tyvek Straightflash over the top nailing fin (this piece will seal directly to the sheathing, since you've lifted that top flap of tyvek up and out of the way.) Then, fold that flap of tyvek back down over the Straightflash, cut it about 1" above the window (don't cut the peel & stick!) and then tape the edges of that flap with contractors tape.

This is pretty much the recommended way to install windows nowdays. You might find a little variation here and there, but you can't go wrong if you follow these directions. If no water gets behind the tyvek from above, you should have a leak-proof window.

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