It’s important to consider fastidious window-flashing details before you raise the walls of any house. If you’re building a house with rigid-foam sheathing and a rain-screen gap behind the siding, you’ll need to incorporate some new practices.
The details shown here were developed for walls sheathed with 1-1/2-in.-thick rigid-foam insulation applied directly to the studs. The foam is taped and acts as an air barrier and drainage plane. To provide solid nailing for the window flanges, the rough opening is picture-framed with 2x2 furring. The furring also needs to be air-tight and watertight, so it is flashed before the foam is installed and caulked to the framing.
The sill pan can be built on siteAll windows leak, so rough openings need to be designed so that they can handle water entry. Although the illustration on the facing page shows a peel-and-stick membrane used to form a site-built pan flashing, some builders prefer to use ready-made sill pans.
If you build sill pans on site, install a 1/2-in.-tall back dam made from scrap lumber on the rough sill to prevent water from entering the interior. Lap the peel-and-stick membrane over the back dam, rough sill, 2x2 furring, and foam sheathing. Be careful when folding the membrane up the sides of the rough opening. It’s important to avoid accidental punctures at this vulnerable location because the bottom corners of window openings are a common path for wind-driven rain to enter a wall.
Because there is no housewrap in this installation, the next step is to wrap peel-and-stick membrane up the sides of the rough opening, again lapping over the 2x2 furring and onto the rigid foam.
Vent the rain screen below the windowRain-screen siding is vented at the top and bottom of the wall. You don’t necessarily have to vent the siding where it meets the windowsill. It will speed drying, but it isn’t strictly necessary. If this detail is important to you, there are two ways to do it.
You can hold down the vertical furring strips from the window-sill, leaving enough of a gap to allow air to circulate horizontally to vented rain-screen spaces on each side of the window. Or you can shim the windowsill trim board with spacers so that air can loop around the trim board and make its way out. If you take this approach, be sure the window is proud of the trim board below to keep the trim board dry.