Spray foam insulation

A homeowner building a new house in Tyler, Texas ran into a disagreement between the builder and inspector over spray foam insulation sprayed on the underside of the roof deck instead of the attic floor. This approach seals the attic inside the house's "thermal envelope," meaning the attic is no longer meant to be a vented, outdoor-like space but part of the conditioned home.

A building science expert weighing in on the question explained that once foam goes on the underside of the roof deck, the attic becomes a semi-conditioned space and does need some connection to the air-conditioning system, siding with the inspector on that point. But the builder isn't entirely wrong either: the space doesn't need to be fully heated and cooled like the living areas below. Small supply and return ducts running into the attic are usually enough to exchange air and prevent humidity from building up in that sealed space, which can lead to moisture problems if ignored.

The expert also noted that the type of spray foam matters and depends on climate. Open-cell foam, which is what's installed in this case, is meant for warmer climates, while closed-cell foam is used in colder or mixed climates. For a house in East Texas, open-cell foam is a reasonable choice, but the exchange highlights why anyone getting a foam-sealed roof and attic should confirm with their HVAC design that the newly conditioned attic space is getting enough airflow to manage moisture.