Installing the Rigid Foam
We took a number of steps to improve this home’s thermal performance, such as air-sealing and insulating the attic and rim joists with spray foam and adding 2 inches of rigid foam to the basement walls. But for exterior walls, we settled for just one inch of foam. This would beef up the wall’s R-value from about R-10 to R-15, yet still allow us to nail the shingles through the foam and into the sheathing, rather than to cleats installed over the foam.
Although Foamular 250 (800/438-7465, owenscorning.com) has higher compressive strength than Foamular 150 (25 pounds per square inch vs. 15 pounds per square inch), it’s only slightly more expensive in our area, making it a better choice under nailed-up cedar shingles. On this job, the rigid foam functions as both an air barrier and a drainage plane, so we were careful to seal the edges of the foam panels to the sheathing and their butt ends to each other. We’ve experimented with several different caulks and sealants but have gotten the best results with GE’s Silicone II paintable silicone; other caulks, including regular silicone, simply peel right off the foam after they’ve cured. We held the rigid foam in place with a few roofing nails while the adhesive set.
Around the flashed window openings, we applied continuous beads of silicone caulk on top of the flashing tape before installing the foam. The foam was bedded in the caulk and held back far enough to leave room for the casings. To complete the semipermeable air barrier, we sealed the seams and edges of the foam with Owens-Corning Bild-R-Tape, designed specifically for XPS insulation (Figure 3). Along with the flashings and caulk, this belt-and-suspenders approach provides an extra layer of protection against air and water infiltration. And with no interior vapor retarder and a perm rating of 1.1 for the rigid foam, these walls can dry both to the interior and the exterior.
We preassembled the casings in our shop from 1-inch-by-5-inch Versatex PVC stock (724/857-1111, versatex.com) and special-profile PVC back-band moldings and 2x2 sills (877/822-7745, advancedtrimwright.com). We used stainless steel fasteners and Bond&Fill structural adhesive. After gluing and pocket-screwing the head and side casings together, we glued and screwed 2x2 PVC sills to the casings from underneath. The back band — which matches the original moldings — was glued with Bond&Fill’s FastCure, an adhesive with an eight-minute curing time, and then clamped to the casings.
Next we padded out the assembled casings with 5/4 x 1 1/2-inch PVC extensions (see illustration). The sill extensions have a 15-degree bevel to match the existing sills, and — like the jamb extensions — they’re completely bonded to the trim with adhesive and tacked in place with 16-gauge stainless steel finish nails. Even in a production setting, building retrofit PVC casings is expensive. The materials to trim five mulled and 12 single units cost about $3,500, plus another $2,000 to fabricate and assemble the frames.
Before fastening the casings to the jambs, we dry fit them and marked their outside edges on the foam. We then applied heavy beads of Bond&Fill Flex to account for the uneven walls and ensure waterproof joints between the rigid foam and the back of the casings. We also applied sealant between the casings and the jambs — enough to cover the edge of the flashing tape, but not so much that it would squeeze out and bond to the upper sash (which would make the sash inoperable). Taping the top of the sills made it easy to clean up any sealant that squeezed out there.
The most critical part of the installation was mating the sloped sill extension to the original sill (Figure 4). We aligned these two parts with our fingers and shot a couple of 2 1/2-inch 16-gauge stainless steel finish nails through the side casings and into the jambs just above the sill to tack the casings in place. Then we adjusted the fit: Though old windows are often out of square, PVC is flexible, so bending the trim to fit was easy. Next, we used Cortex screws to pull the casings tight to the jambs. These screws are self-drilling and cut a clean hole in the PVC that’s the exact size of the supplied plugs (800/518-3569, fastenmaster.com).
We also used stainless steel trim-head screws through the sill extension and into the original sill to pull this critical joint tightly together. Afterwards, we cleaned up the squeeze-out at the joints and filled in any holes with Bond&Fill in preparation for priming and painting.
After installing the trim, we sealed the sides and tops of the casings to the foam with more Bond&Fill Flex. Then we added a 1 5/8-inch metal drip cap over the top of the casing, giving it a slight slope to help drain water away, and flashed the drip cap to the foam with StraightFlash butyl tape.
To provide a drying air space between the foam and the back of the shingles, we installed the red-cedar shingle siding over Benjamin Obdyke Home Slicker (Figure 5). Because red cedar is fairly rugged and we had added only an inch of foam, we were able to use a siding nailer to fasten the shingles directly to the sheathing underneath with 2 1/2-inch stainless steel ring-shank nails.
Performance and Cost
The upcharge for adding the layer of foam to this siding job was about $9,000 to $10,000. With all of the air-sealing and insulation work we did, we were able to improve the home’s airtightness from 8.01 air changes per hour at 50 pascals (ACH50) to 3.24 ACH50 — almost a 60 percent decrease — without visibly altering the home’s appearance.
The owners will be happy with their lower energy bills, but already they’re noticing elevated humidity levels in their home. We plan to address that issue by installing a heat recovery ventilation system, or HRV — an additional expense they’re not happy about. We’ve also noticed that moisture from the dryer exhaust and side-vented gas boiler is getting up behind the siding, wetting the shingles, and causing some tannin-bleed. We’ve tried blocking off the air space in those locations; however, I suspect that we might have to extend both vents well clear of the siding to solve the problem.
Joe Cracco is a remodeler in Cumberland, R.I.