Cathedral & roofline insulation calculator

Sloped ceilings are depth-limited by the rafters. Enter the cavity depth and the tool tells you the R that fits, the board-feet of foam and the kits to buy.

Confirm coverage-per-bag, R-per-inch and set yield against the exact product you buy and order a little extra (~5–10%) for framing, gaps and settling. Coverage, R/inch and set yields vary by product and brand — read the bag/kit and the data sheet.

Calculator

ft²
Sloped area of the cathedral ceiling you are filling.
in
Actual depth: 2×10 = 9.25", 2×12 = 11.25". Leave a vent gap if required.
/in
Closed-cell ~6.0–7.0, open-cell ~3.5–3.7 (labeled).
bd-ft
Off the kit: DIY kits ~200 or ~600 bd-ft.
Result
Cavity R-value (9.25")R-60.13
Foam board-feet3,700 bd-ft
Kits / sets7 kits

A 9.25" rafter cavity at 6.50 R/inch caps out near R-60.13; filling 400 ft² is about 3,700 board-feet (7 kits). Leave a vent gap where required; cavity depth caps the achievable R — a labeled geometry note, not a roof-design or ventilation-code verdict.

Formula

cavity_R = cavity_depth_in × foam_R_per_inch

board_feet = roofline_area_ft² × cavity_depth_in

kits = ceil( board_feet ÷ yield_per_set )

The rafter depth caps the achievable R — you cannot install more inches than the cavity holds (minus any required vent gap). Board-feet is area × the depth you fill (1 board-foot = 1 ft² at 1 inch), and kits round up off the yield printed on the set.

Worked example

A 400 ft² cathedral ceiling framed with 2×10 rafters (a 9.25 in cavity), filled with closed-cell foam at R-6.5/in, from 600 bd-ft kits:

Cavity R: 9.25 × 6.5 ≈ R-60. Board-feet: 400 × 9.25 = 3,700 bd-ft. Kits: ceil(3,700 ÷ 600) = ceil(6.17) = 7 kits.

That fills the cavity solid. If code requires a 1–2 inch vent channel above the foam, subtract it from the depth first — a 9.25 in rafter with a 1.5 in baffle only gives you 7.75 in of foam (about R-50 at 6.5/in).

Depth, vent gap & foam choice

Depth is the hard limit. Unlike an open attic, a sloped roof cannot just get deeper — the rafter dictates it. If R-60 will not fit in the cavity you have two moves: use a higher-R/inch foam (closed-cell) to get more R per inch, or add a layer of rigid board under the rafters to build the assembly up. This tool sizes what fits in the cavity.

Vent gap first, foam second. Vented roof assemblies need a clear air channel from soffit to ridge above the insulation — typically 1 to 2 inches held by baffles. Deduct that from the cavity depth before you compute the fill, or you will over-order and choke the ventilation. Unvented (hot-roof) assemblies are a different detail with its own code and moisture rules.

Closed-cell earns its place here. At ~6.0–7.0 R/inch it packs the most R into a shallow rafter and adds rigidity and a low perm rating, which is why it is common on rooflines. Open-cell (~3.5–3.7/in) needs nearly double the depth for the same R and rarely fits a standard rafter to a high target. Kit yields drop with cold temperatures and waste, so keep a spare set.

What this is not. It is a geometry and quantity tool. It does not design the roof ventilation, the ignition/thermal barrier over the foam, or the moisture/condensation control — those are set by the foam data sheet and local code, and roof covering/sheathing is a separate trade. Confirm the assembly with a pro before you spray.

Reference table

Rafter cavity depth and the R it holds at your chosen 6.50 R/inch foam (fill solid, before any vent gap).

RafterCavity depthCavity R
2×65.50 inR-35.75
2×87.25 inR-47.13
2×109.25 inR-60.13
2×1211.25 inR-73.13

Depth caps the R — subtract a 1–2 in vent gap in vented assemblies. Labeled typicals; confirm on the data sheet.

Frequently asked questions

How much spray foam do I need for a cathedral ceiling?
Board-feet = area × the fill depth. A 400 ft² cathedral ceiling with a 9.25-inch cavity filled solid is 3,700 board-feet, or about 7 kits at 600 bd-ft each. Deduct any required vent gap from the depth first.
What R-value can a 2×10 rafter cavity hold?
A 2×10 has a 9.25-inch cavity. Filled with closed-cell foam at ~6.5/in that is about R-60; with open-cell at ~3.6/in only about R-33. The rafter depth caps the R — you cannot fit more inches than the cavity.
Do I need a vent gap above the insulation?
In a vented roof assembly, yes — usually a 1–2 inch channel from soffit to ridge, held by baffles. Subtract it from the cavity depth before sizing the foam. Unvented (hot-roof) assemblies skip the gap but follow separate code and moisture rules — check with a pro.
Why closed-cell instead of open-cell on a roofline?
Closed-cell delivers ~6.0–7.0 R/inch, so it hits a high R in a shallow rafter and adds rigidity and a low perm rating. Open-cell at ~3.5–3.7/in needs almost twice the depth for the same R and often will not fit a standard rafter to target.
Can I just add more foam to boost the R?
Not beyond the cavity depth. To go higher, either use a higher-R/inch foam or add rigid foam board under the rafters to build the assembly up. This tool sizes what fits inside the existing cavity.

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