- Open-cell
- About R-3.6 per inch
- Closed-cell
- About R-6.5 per inch
- 2 inches closed-cell
- An air seal, about R-13
- 5.5-inch bay, open-cell
- About R-20, not R-38
Why the roof deck, not the floor
Foam sprayed on the underside of the roof deck, with the soffit and ridge vents closed, makes an unvented attic. The attic air then moves toward the temperature of the house. Ducts that used to bake above the insulation now sit inside the thermal envelope.
That is the building-science reason to use foam. If the ducts are not up there, and the attic is a simple vented volume over a flat ceiling, blown-in on the floor is the simpler and usual job.
Do not foam the deck and leave the attic vented. That is a mixed assembly. The deck stays cold relative to humid attic air in some seasons, and warm and tight against moisture in others. Pick one boundary.
Open-cell and closed-cell
Open-cell foam is about R-3.6 per inch. It is an air barrier at the thickness the product is listed for, often a few inches, and it is vapor-open. It fills a deep bay. A 2×6 bay is 5.5 inches, about R-20. That is a wall or a roof bay, not an R-38 attic floor.
Closed-cell foam is about R-6.5 per inch in the numbers we plan with. It is an air barrier, and at roughly 1.5 to 2 inches it is also a vapor retarder, depending on the product. Two inches is about R-13. We call that an air-seal coat. We do not call it a finished attic.
Closed-cell resists water better if a roof leaks onto it. Open-cell can hold water like a sponge. Neither one is a roof repair. A leaking roof gets fixed before foam goes on the deck.
The code minimum on the deck is not the energy code
Florida Residential Code R806.5 allows an unvented attic when the roof assembly is built as that section requires. In Climate Zones 1 and 2A the condensation-control table lists R-5 of air-impermeable insulation. That R-5 keeps the sheathing on the right side of the dew point. It does not satisfy the energy code by itself.
The energy code still wants a real thermal resistance at the roof. R-5, or two inches of closed-cell, is not Table R402.1.2. If we foam a roof deck, the thickness is the thickness that meets the assembly, not the thinnest coat that technically counts as air-impermeable.
Fire, smell, and who has to leave
Spray foam in an attic has to meet the ignition-barrier rule, or be a system that is listed for attic and crawl-space exposure. Bare foam left because the bucket ran out is not a finished job.
Foam smells while it cures. Blown fiberglass and cellulose do not. People and pets leave for a foam job when we say so. We ventilate. We do not treat a foam day like a blown-in day.
If anyone in the house is chemically sensitive, tell us before we schedule. Foam is the wrong product for some houses even when the roof deck is the right assembly.