A GBA reader named 88Clayton is insulating the attic in his house with spray foam and has run into a problem: the level of insulation recommended by contractors is at odds with what the building inspector wants.
“I’m on the southern edge of [Climate Zone] 4,” 88Clayton writes in a Q&A post. “My spray foam contractor recommended R-20 of open cell. That’s what is currently sprayed on my roof deck. The code inspector came by today and is insisting on R-38, as if it [were] a conventional vented attic with loose blown fibers.”
With an air leakage rate of 1.7 ach50, the house is tight when compared with typical construction. Even so, the R-20 recommendation 88Clayton got from all four foam contractors is well below code required insulation levels for his climate zone (R-49 in the International Residential Code) and less than what he has in the walls of his house—R-25.
“It’s a bit strange to have walls that are better insulated than [the] roof,” he says. “Where is the realistic point of diminishing returns? I know the encapsulation itself has benefits that go beyond just raw R value, even though the inspector doesn’t quite get this, but perhaps I do need more foam thickness before the diminishing returns kick in.”
That’s where we begin this Q&A Spotlight.
It’s closed-cell foam you really want
When foam is sprayed directly on the underside of the roof deck, it should be closed-cell, not open-cell, writes Zephyr7. “Open-cell has moisture issues here!” he writes.
Open-cell foam is an effective air barrier, but it’s vapor permeable. Closed-cell foam is both an air barrier and a vapor barrier, so moisture is not able to migrate through the foam and collect in the roof sheathing.