Gregg is renovating his 50-year-old house in Wisconsin and trying to devise the best way of insulating exterior walls from the outside. The house was built conventionally, with 2×4 walls, fiberglass batt insulation, fiberboard sheathing, and hardboard siding.
He plans to tear off both siding and sheathing and remove the batt insulation, then apply 3 in. of spray polyurethane foam insulation into the stud bays. The existing kraft paper vapor barrier on the interior side of the wall will stay in place.
Several local contractors have recommended the addition of between 3/4 in. and 1 in. of rigid foam insulation over the studs before applying 1/2-in. OSB sheathing and lap siding. But the recommendation is giving Gregg pause for thought.
“While I like the idea of eliminating thermal bridging through the studs,” Gregg writes in this Q&A post at GreenBuildingAdvisor, my concern is that there will be an air gap between the two layers of foam insulation (both of which have limited permeability). In this scenario, wouldn’t I be creating an ideal situation for condensation to form between the two layers? We obviously live in a very cold climate and winter temperatures regularly reach -20°F to -30°F.”
One concern, voiced by GBA senior editor Martin Holladay, is that when the sheathing is applied over the foam, and not directly to wall framing, it can’t provide much structural rigidity. In that case, he’d have to install metal strapping for bracing before the foam is applied.