A THICK SKIN THAT KEEPS A HOME WARM AND DRY
What to think about when considering exterior insulation
BY GARY BERGERON
Sheathing with foam-board insulation is a great way to get more R-value with a conventionally framed house. It can work just as well in remodeling projects as in new construction. Applying rigid foam takes some getting used to, but the basic tasks are not that much different from installing typical plywood or OSB sheathing—and the lightweight foam is easier on your back. The biggest differences are in how you detail windows and doors and how you fasten and seal the foam boards to the wall. Rigid foam can be the insulation, air barrier, and drainage plane all rolled into one. Pay attention to the details and seal every possible air leak, and you will end up with a more durable and energy-efficient home.
Some things to think about when planning to use exterior foam insulation:
For more information: Calculating the Minimum Thickness of Rigid Foam Sheathing Using Rigid Foam As a WRB
Foam panels don’t provide shear strength
Rigid foam can replace plywood wall sheathing, but it can’t replace the strength associated with it. You still need to include some sort of bracing for this. Your engineer can specify where the shear panels or diagonal bracing needs to be. If plywood panels are used for shear strength, cover them with thinner layers of foam; this will keep the overall wall thickness consistent.
Extend the rough openings with a plywood box
Window and door rough openings get plywood boxes that extend from the inside edge of the wall to the outer edge of the foam sheathing. This allows you to mount the windows and doors flush to the outside of the wall. Be sure to compensate for the thickness of the plywood when you frame rough openings. Flash the windows to the foam with flexible flashing tape or some other window flashing system.
The right tool for the right cut
Straight, long cuts can be made easily with a tablesaw or circular saw. Long-bladed utility knives and small handsaws are great for tighter cuts and for adjusting the fit after the foam board is in place.
Effective air-sealing begins with a tight fit
Staggering the seams (two sheets of 2-inch foam, in this case) and taping the joints are important, but where foam butts windows, doors, and corners, the fit should be snug—tape shouldn’t span cracks—so it’s important to measure and cut carefully. Before installing the foam, run a bead of caulk where it will contact the top and bottom plates and the edges of walls. Be sure to tape all seams on each layer of foam, and flash to all windows and doors.
Tape sticks better on a clean surface
When cutting with a power saw, you will get a lot of dust on the boards. A paintbrush or a small dust brush works well to wipe dust off seams so that the tape adheres perfectly.
Continuous insulation is simpler and better
In the case where there’s a first-floor bump-out with a roof that intersects a second-floor wall, run the foam sheathing down to the first-floor ceiling plane. Then fasten rafters and ledgers through the foam to the adjacent wall framing, and build the roof. This way the wall maintains the full R-value right up to where it contacts the insulation in the roof or ceiling plane.
Fasteners, flashing, and furring will be covered in future videos in this series.
—Gary Bergeron is a partner at Synergy Companies Construction. The home in this video was designed by Building Science Corporation in partnership with the U.S. Department of Energy’s Building America Program.
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