Fact: insulation cannot be effective unless something stops air from moving through or around the insulation and carrying heat with it. Yes, spray foam or T&G foam boards, for example, are natural air barriers, as well as being effective at slowing heat transfer by conduction. But that’s not the whole story.
A homeowner in northern New York wanted to eliminate ice forming at the edge of his roof and causing problems. The entire house has cathedral ceilings with ceiling cavities insulated with R-30 batts. Since there was no access to the cavities from inside and the roof was old and damaged by ice buildup, the contractor recommended removing the roof and batts, filling the entire cavity with spray foam, then installing a new roof.
It should have worked. So the homeowner was horrified the following winter to find that the ice on the roof was far worse than in the past. There was ice on all four sides of the house!
The reason: Heat from inside the home was getting to the underside of the roof. But how? Baseboard radiators on the exterior walls heated the home. There were ventilated soffits all around the exterior, and the contractor chose not to fill them with foam. The sidewalls, like the original cathedral ceilings, were insulated with fiberglass batts.
When an area of the new roof was removed, it turned out that the foam insulation didn’t fully fill the roof cavity; there was a moderate gap between the foam and the roof deck, and the exterior wall top plate wasn’t sealed with foam.
Heat from the radiators entered the exterior wall cavities where it traveled upward as warm air. It then leaked around the top plate into the soffit and continued up the gap between insulation and roof deck, warming the roof deck enough to create ice dams on all sides of the house.
The boundaries weren’t aligned.
More REMODELING articles about home performance:
Snowball’s Chance: Ice Dams and Home Performance