Sanity Check Please: Roofer guys says….

A homeowner in upstate New York asked other builders to check something a roofer told a friend: that when the company adds rigid foam insulation on top of the roof deck during a reroofing job, they only install about 2 inches of polyiso foam board, which comes to roughly R-13.

The question raised was whether that's enough, based on a widely used building code rule for roofs that combine foam on top of the sheathing with regular insulation (like fiberglass or cellulose) between the rafters. Under that rule, when you mix the two types of insulation, the foam on top generally needs to equal at least half of the roof's total insulation value, with the rest between the rafters. If a roof has no insulation between the rafters at all, the foam layer on top has to meet the full code-minimum R-value on its own. Another contributor confirmed this understanding is correct, pointing to an article explaining the rule in more detail.

For a homeowner, the practical point is this: if you're getting a new roof and considering adding exterior rigid foam at the same time, 2 inches may not be enough on its own to meet code, depending on how much insulation already exists between your rafters and what your climate zone requires. The right amount depends on your specific roof assembly, so it's worth checking the numbers against your local code before assuming a roofer's standard foam thickness is sufficient for your house.