Flashing for Old Windows
How do you get a good weathertight seal on old windows that don’t have a flange?
I’m stripping the siding off of my house, and have added housewrap to replace the old brittle building paper. I won’t be replacing the windows or casing (which seems to be part of the window itself), but I would like to weatherize them while I have the chance. Everything I see these days is about flashing flanged windows. How do you get a good weathertight seal on old windows that don’t have a flange?
—Tim, via email
Editorial advisor Mike Guertin: It’s not uncommon for old windows to have the exterior casing keyed into the jamb with a lap joint and glue. In this case, they probably can’t be removed without damaging the jamb and/or the casing. In the past, I’ve had good luck with a workaround, though.
Start by using a thin flat bar or stiff putty knife and pry the side casings away from the wall about 1/8 in.—just enough to fit the blade of an oscillating multitool or reciprocating saw to cut any nails within 1-1/2 in. of the casing edge without damaging the casing-to-jamb joint. With the nails out of the way, slide in a 4-in.-wide strip of self-adhering flashing tape with the sticky side facing out. To make the strips manageable, put them in a cooler or freezer for 30 minutes before installing so the adhesive is less aggressive.
The strips should extend a couple inches above the top of the head casing and a couple inches below the bottom of the sill. You may have to back-cut the sill a little to permit the tape to slide behind it. The portion of the side strips below the sill will need to overlap the new housewrap later, so don’t fasten them to the wall. Slide a putty knife behind the flashing tape and use it to press the adhesive side of the tape onto the back side of the casing.
Next, apply strips of 4-in. flashing tape over the exposed face of the installed strips—sticky side to sticky side—and also to the sheathing. Leave the release sheet on the bottom corners of these strips of tape that extend below the sill so that the housewrap can be slid beneath later. Now apply a strip of self-adhering membrane—6 in. to 9 in. wide—along the top of the head trim, with about 1⁄4 in. of the tape wrapping over onto the face of the trim. This piece should extend past the 4-in. flashing tape on both sides.
Finish up by installing a sloped metal or plastic cap flashing over the head casing with a 1/2-in. or greater drip leg on the front and end dams on each end.
Unfortunately, this flashing method won’t do a great job of air-sealing. In order to get there, or at least get closer, you will likely need to remove the exterior casing.