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Flashing A Flanged Window - Continued

I like to use a J-roller to apply strong, even pressure to all flashing membranes and tapes. Although the tool is designed for countertops, it works great for this application.

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I then apply flashing membrane over the window head flange, lapping it over the tops of the jamb flashings.

Applying the head flashing directly to the sheathing provides another level of protection. If any moisture were to find its way behind the housewrap and seep down to the window, it would encounter the head flashing and be directed over the window head flange, not behind it.

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Once I've applied the head flashing, I fold the housewrap flap down over it, tape the 45-degree corners, and "skip-tape" (apply short pieces of tape with gaps in between) the lower edge of the flap in place. I also skip-tape the bottom (uncaulked) sill flange. These skip-taped gaps at the lower edges act as weeps, giving any moisture that gets to the window head a way to escape. Skip-taping also limits air leaks, though it's not entirely airtight. (For my money, I'd take an air leak over a moisture problem any day.)

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Because I build in a cold climate, I apply a bead of low-expanding foam in the gaps between the window unit and the rough framing, close to the face of the interior wall. I apply only enough foam to create a seal, not to fill the entire gap: Too much foam can distort the window unit.

Working in the Cold and Wet

The DuPont flashing membranes are meant to be installed at temperatures of 45 degrees or above. In cold-weather situations, I keep the flashing in a warm area and cut pieces as I need them. As the temperature drops, it sometimes becomes necessary to heat the area that will receive the flashing (I use a heat gun). In wet conditions, I get rid of any standing water and wipe down wet surfaces with a dry rag before applying the flashing membrane. Again, when in doubt, I'll dry things off with a heat gun.

In cold or wet conditions, I'll also use a hammer stapler along the edges of the flashing membrane to hold the material in place so the adhesive remains in contact with the sheathing or housewrap.

Just Do It

Flashing windows properly isn't difficult, and adds only about 15 minutes in labor and $15 in materials per unit. Some builders substitute less-expensive contractor's tape for flashing tape. The problem with this is that when the siding or trim is fastened at the window, every fastener becomes a potential leak, whereas the flashing membrane seals around the fastener.

As a builder, there's nothing I do in one day that puts more dollars in a house than installing the windows. Spending a little extra time getting things right sure makes sense to me.

Carl Hagstrom installs windows in Montrose, Pa. He is a JLC contributing editor and a speaker at JLC Live.