Leak-free Windows Series: Counter-Flashing The Head of a Window
Cut a reglet into foam wall sheathing and slip counterflashing above your window or door.
When you go the extra energy-efficiency mile by sheathing with foam, it can complicate window installation. Because there is no housewrap, there is nothing to tuck the head flashing under. You can use flashing tape, but…
If you don’t have a Z-flash lap that you can lap a piece of housewrap under to get a mechanical lap flashing over the top edge of the flashing tape for the head of the window, what you can do is put a piece of aluminum or other metal flashing that’s grooved into the foam itself. What I like to do is take a piece of metal flashing about 4 to 6 inches wide and just bend a 45-degree angle in it about ½ to ¾ inch deep. Then use that metal as a straightedge to guide the knife blade.
Now, these knives work a little better than an ordinary utility knife because you can extend it a little bit further and get a nice steep angle on it. You only want to go about a third to halfway through the thickness of the foam; you don’t want to cut all the way through the foam. And that groove is not quite wide enough to fit my flashing up into. So I use a little flat bar that just opens that groove up enough so I can slip the metal in there. It may take a couple of passes to open the groove up. A few pieces of housewrap tape will hold that in place.
Water that does get down into this foam sheathing gets drained onto the flashing, onto the head flashing of the window, and then down and over the top of the window. This cap flashing isn’t that important because it’s just there as a sun shield to the flashing tape itself over the head of the window.
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