GET THESE FIVE TRICKY SPOTS RIGHT, AND YOU’LL GET A SUPERINSULATED, ENERGY-EFFICIENT HOME
An experienced builder explains the ins and outs of airtight exterior insulation
BY DAVID JOYCE
Installing rigid foam on the outside of a house is a great way to double the R-value and eliminate air leaks. Large sheets of foam insulation, screwed and sealed to the walls or roof, can cover or replace conventional sheathing and create a continuous barrier to heat loss or gain—something you don’t get with cavity insulation alone. We wrapped the house in this video with 4 inches of polyisocyanurate in two staggered 2-inch layers. This thickness strikes a balance between ease of installation and maximum R-value. If the foam is any thicker, it’s hard to hit studs when driving screws. Plus, combined with 5-1/2 inches of cellulose in the stud bays, 4 inches of rigid foam brings the wall up to a respectable R-40.
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Foam sheathing needs to be tight and sturdy
No matter what type of house you’re insulating, how well the rigid foam is connected to the rest of the wall makes all the difference in performance and durability. Strong screws should extend from wood strapping on the outer surface through the foam sheathing and into the framing below. The foam boards should fit tight, and all edges and seams must be well sealed to keep out air and water. The key is to identify all gaps and choose the best way to seal them; a few key details should cover most of the bases for a job well done.
1. Use bug screens and flashing below the foam
Many builders protect foam sheathing from insects by wrapping the bottom edge with strips of window screen. We take a belt-and-suspenders approach to air-sealing by backing up the screen with a sheet-metal drip edge. This directs water away from the foundation surface and adds extra protection from bugs and wind. My crew and I bend aluminum or galvanized steel sheeting so that it extends up behind the foam and out past the bottom edge far enough to nail to the bottom of the wood strapping. Then we nail a strip of window screen several inches wider than the flashing to the vertical part of the drip edge, leaving enough length to wrap under the foam and over the bottom of the strapping that holds the foam in place.
2. Build plywood boxes to span deep window openings
To compensate for the thick foam, we extend all rough openings with site-built boxes made of 3/8-inch plywood. The boxes are not structural; their main purpose is to provide a straight, solid surface to air-seal to and to align the windows. Strips of plywood nailed to all four surfaces of the rough opening extend from the inner edge out to where the outer edge of the foam will be. Don’t forget to leave room for the plywood when framing rough openings.
3. Fasten the foam with plywood scraps before the strapping goes on
It’s virtually impossible to install the strapping that supports the foam sheathing as the foam is going up. Instead we drive long screws through a couple of small scraps of plywood to temporarily support each piece of foam. The screws go through the foam into the framing below, which comes in handy when laying out the permanent strapping. Solid-wood strapping screwed through the foam into every stud will pull the sheathing tight to the framing and provide a solid nailing surface for siding and trim. The 3/4-inch space this leaves behind the clapboards does two things: it leaves plenty of room for any water that gets in to escape; and it allows the foil facing on the surface of the polyisocyanurate foam to act as a radiant barrier, boosting the insulating value of the wall.
4. Stagger all layers of foam, caulk all outer edges, tape all seams
Another belt-and-suspenders approach of ours is using two layers of 2-inch foam rather than one 4-inch layer of foam. We offset the pieces so that nowhere is there a continuous seam penetrating straight through to the framing. Even the corners are “woven” together with alternating pieces. We install one layer at a time, taping or caulking all gaps as we go. We use construction adhesive or silicone sealant where the foam laps wall plates and outside corners. Housewrap tape goes over all seams and inside corners, and where foam meets windows and doors.
5. Use the right tape for the job
Different tapes stick well to different materials. Some are designed for a single purpose, while others are good multitaskers. After trying several kinds of tape, we’ve been happy with Dow Weathermate for most foam seams; it sticks to everything and always stays put. For wider gaps and outside corners, we use strips of ice-and-water or peel-and-stick window flashing.
—David Joyce 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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