Installing the Foam
Once my crew starting stripping off the old roof, we found more areas of damaged sheathing, with the worst damage on the north-facing side of the roof where the skylights were located. Interestingly, the sheathing over the heated garage — which was conventionally insulated with cellulose blown into the attic rather than spray foam on the sheathing — looked as new as the day the shingles were installed. It’s possible that humidity levels were lower in the garage attic, since a sheathed and spray-foam-insulated attic partition wall isolated it from the rest of the house.
After patching the remaining areas of damaged sheathing, we began installing foam, working from the ridge down. We quickly fastened the rigid sheets to the sheathing with 2 1/2-inch-long Duro-Last HD screws and washers (800/248-0280, duro-last.com), using four to six fasteners per sheet. Because the sleepers would hold the foam sheets securely in place once they were installed, we needed only enough fasteners to keep the foam from slipping while we walked on it. To ensure that the blanket of rigid foam was as continuous as possible, we carefully sealed all of the joints between the sheets and around penetrations with low-expansion canned spray foam.
The trickiest part of installing the foam board was fitting it around a half-octagon roof framed over a bump-out entry on the south-facing wall of the house. That roof had been framed California-style on top of the existing roof deck, but the lower-level roof sheathing wasn’t continuous (as it typically would be with overlay framing). With the upper framing partially open to the lower framing and perhaps not enough sheathing to spray the foam against, it seemed possible that there was an air pathway up into the upper cavity. This would have allowed water vapor to collect and condense here, and could explain why there was considerable rot on the sheathing and framing at the octagon roof peak. To block off convective pathways into this cavity, we removed the rotted sheathing, cut rigid foam to fit around the repaired framing, then carefully sealed the foam joints with a portable two-part spray foam kit (800/321-5585, fomo.com) before resheathing the roof.
Ventilating the Roof
The original roof trusses had been blocked off at the wall plates with plywood baffles before the Icynene was sprayed, but the eaves themselves were uninsulated and finished with vented metal soffits. By cutting away a 4-inch-wide section of the old roof sheathing along the eaves and outside the wall plate line, we were able to use the original vented soffits to provide an air path up into the 2x4 sleepers installed on top of the rigid foam. The sleepers are fastened through the foam to the trusses underneath with 5 1/2-inch-long Duro-Last fasteners. We first tried 7-inch screws but found that the shorter screws had plenty of bite and took much less of a toll on the battery life of our cordless impact drivers.
Since condensation didn’t appear to be an issue on the hip roof over the garage, we saved a little money by not installing continuous foam there. Instead, we cut the foam into 3 1/2-inch-wide strips and padded the sleepers out with these rippings before installing our new sheathing, to avoid visual problems at the eaves and ridge. When we installed the sleepers, we cut them a little short wherever there was an obstruction — at the hips and around skylights, for example — to provide ventilation pathways.
To accommodate the thicker roof assembly at the eaves, we extended the existing subfascia with 2x6s cut with a beveled top edge to match the 6/12 roof pitch. Later, after felting the roof and installing the new architectural shingles, we covered the subfascia with new metal fascia material, and installed new gutters to finish the eaves.
There was no way around it: Installing exterior foam this way also meant remounting the skylights. In this case, the wood skylight frames were so distressed from interior condensation they had to be replaced anyway. At the ridge, we installed a LOR-30 Lo-OmniRoll ridge vent (800/643-5596, lomanco.com), a continuous roll-out design that has performed well for us.
In the end, the total cost for repairing this roof was about $32,000, which included the two new Velux skylights. That’s a lot to pay for repairing a roof leak, but with this belt-and-suspenders approach the new roof assembly will be better equipped to handle excess moisture than the original one was.
Builder Mark Parlee specializes in exterior renovations and building-envelope problems in Urbandale, Iowa.