Best by project · Written September 8, 2026 · Based on the 2021 IRC; your local code may differ
Vented rafters with baffles, or an unvented assembly with foam against the sheathing?
A cathedral ceiling has no attic to put insulation in; the rafter bay is the whole assembly. There are two compliant ways to fill it. Vented: an air channel under the sheathing from soffit to ridge, then air-permeable insulation below it. Unvented: air-impermeable insulation applied directly to the underside of the sheathing, then more insulation beneath if the bay is deep enough. Air impermeability is a tested property of a specific product, and the products marketed for this layer are the closed-cell ones.
Foam is the unvented route. The code requires the foam layer to be thick enough for the climate zone to keep the sheathing above dew point, and that minimum runs from R-5 in zones 1 to 3 up to R-35 in zone 8. Zone 5 asks for R-20, which at a claimed R-5.66 per inch is about 3.6 in of foam against the deck before a batt goes under it. Spraying that thick, overhead, across a whole roof is not a job for cans.
The assembly decides the product
Table R806.5 sets the minimum for the air-impermeable layer when air-permeable insulation is added beneath it: R-5 in zones 1, 2 and 3; R-10 in zone 4C; R-15 in zones 4A and 4B; R-20 in zone 5; R-25 in zone 6; R-30 in zone 7; R-35 in zone 8. Verify against the edition your jurisdiction adopted.
Every pass is sprayed upward. The single-can makers rate one layer at 1 in; a 3.5 in layer is four passes overhead. Canister kits spray thicker per pass, up to 2 in for Froth-Pak.
The ceiling finish is the thermal barrier. Foam left exposed on a vaulted ceiling in a living space is exposed foam plastic.
What the code says
Section numbers are from the 2021 International Residential Code. Your city or county adopts one edition of the code and changes parts of it, and your local building official has the final word. A thermal barrier and an ignition barrier are different things, and the glossary explains both.
Products that document this job
Listed by product family. Each line says why the product is on this list, always from something the maker published. Follow the link for the documents, the questions still open, and dated prices.
Quantity, worked once
Board feet are square feet times inches. The kit figures are the makers’ theoretical yields, which every maker that defines the term says are laboratory numbers; plan on less.
Plan on two canister kits for the foam layer alone, sprayed overhead in two passes. Fibrous insulation goes under it, and the ceiling finish is the thermal barrier.
Run your own numbers in the calculatorQuestions
Enough for the climate zone under Table R806.5 when fibrous insulation goes beneath it: R-20 in zone 5 works out at 3.53 in at a claimed R-5.66 per inch, so 3.6 in in practice, and more if the maker’s figure is an unaged one. If the whole bay is foam, the total has to reach the ceiling target for the zone.
R806.5 asks for air-impermeable insulation against the sheathing. Whether a given product meets that is something the maker has to test and print, so read what it publishes. HandiFoam does list its open cell for cathedral ceilings, but as the fill below; the closed cell is what touches the sheathing.
Depth and direction. A 3.6 in layer is four overhead passes at the makers’ 1 in per layer, and a 300 sq ft ceiling is about 1,080 board feet, which is more than two 24-can kits at theoretical yield. Canisters spray thicker per pass and cost less per stated board foot.
Step-by-step how-to and the full safety picture live on our sister site: Choosing the foam family by job, at Spray Foam Guides. This page connects the job to specific products and to what their makers publish. We have not sprayed or tested any of it, and none of it stands in for the label, the safety data sheet or what your local inspector tells you.