Four rules for window installation
Sill flashing as a trim detail. Galvanized steel or copper can protect the framing from water intrusion while adding character to the exterior.
We don’t install commercial curtain walls or European windows in our typical residential projects, so we’ve had to develop installation details that adapt proper pressure-management principles to U.S. flanged windows. For these details, we follow four rules.
1. Protect the head of the window from rain by tucking the window in from the face of the wall. Roof overhangs don’t help much on the coast—water blows sideways, up, and down—and an 18-in. overhang on a taller building doesn’t help the ground-floor windows at all. On a building with 12-in.-thick well-insulated walls, we slide the windows back 3-1⁄2 in. so that every window has an individual built-in overhang right above the window head. That means we install most windows as “in-betweenies” instead of “innies” or “outies.”
2. Treat the window flange as an installation aid only, not as a water or air seal. Expect that water will be able to enter behind the flange during extreme conditions, and always provide a pathway out. Follow the manufacturer’s instructions; your warranty depends on it.
3. Make sure water can’t pool on bare wood and rot the wall. Wrap rough openings properly, with tape or flashing that is lapped top over bottom, and install a sloped sill pan below the window. It’s now part of most window manufacturers’ instructions to place a clapboard or sloped shim at the rough sill, but we still rarely see this recommendation followed until we ask for it. The critical and often-over-looked air-sealing area is at the interior of the sill. If
the window is not shimmed at its sill—the usual situation we see—there needs to be a bead of caulk or a gasket seal at the inside face of the window to prevent water from blowing up under the open exterior window flange, right under the window frame, and into the building.
4. Establish a long-lasting air-and-water seal at the inside face of the window unit, and connect that seal to the rough opening. The best material for creating this seal is a flexible and long-lasting construction tape such as Dow Weathermate or 3M 8067 Flashing Tape, or a European tape such as Siga Rissan or Pro Clima Tescon. The tape will experience consistent temperatures on the interior side of the window; it won’t, however, go through extreme freeze/thaw cycles like exterior tape. It should also remain flexible longer than spray foam. However, this tedious, nonstandard technique for most U.S. builders will likely lead to an upcharge, so we usually have to accept spray foam as our interior air seal. (Fiberglass scraps do not belong in a rough opening.) We insist on a blower-door test to ensure that the air seal around the window was installed properly, and we often make builders come back with caulk to close up the holes they are surprised to find in the spray foam. Our typical non-Passive House airtightness standard for residential work is 1.5 ACH50, but we always give builders the cfm50 number that would allow them to beat the 0.4 ACH50 achieved by one of their competitors. It never hurts to let them know where the bar really sits.