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R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
A cathedral (vaulted) ceiling has no attic, so all the insulation has to fit in the rafter depth — and code still asks for R-49 to R-60 in most of the country. Enter the rafter size and climate zone to see what each approach achieves, whether it meets the target, what the code requires for an unvented roof, and what it costs.
With these inputs:
| Result | Value |
|---|---|
| Code target for your zone | R-60 |
| Highest R in this rafter | R-60 — Closed-cell spray foam, full depth |
| Unvented roof: min. foam R against the deck | R-20 |
| High-density fiberglass batts (vented) | R-31 · $990 – $1,485 |
| Mineral wool batts (vented) | R-35 · $1,155 – $1,815 |
| Closed-cell spray foam, full depth (unvented) | R-60 · $3,700 – $7,400 |
| Flash-and-batt: 3.08 in closed-cell + batts (unvented) | R-43 · $1,972 – $3,575 |
| Open-cell spray foam, full depth (unvented) | R-34 · $1,480 – $2,775 |
Vented or unvented?. A vented cathedral ceiling keeps a continuous 1 in air channel between the insulation and the roof deck, running from soffit vents to a ridge vent in every rafter bay. It works with batts and is the traditional approach, but it costs an inch of depth and needs a clear path in every bay — skylights, hips and valleys often block it. An unvented (hot) roof has no channel: foam goes directly against the underside of the deck so warm, moist indoor air can never reach it. The building code (IRC R806.5) allows this when enough of the insulation is air-impermeable — closed-cell spray foam or rigid board above the deck — to keep the deck above the dew point. That minimum rises with climate zone, from R-5 in the South to R-35 in zone 8. When the rafters are too shallow. A 2×6 or 2×8 rafter cannot hold R-49 with any material except closed-cell foam, and even that falls short in a 2×6. The fixes are to fur the rafters down (2×3 strips add 2.5 in of depth), add continuous rigid foam above the deck when the roof is replaced, or use closed-cell foam. The IECC allows R-38 in ceilings without attic spaces for up to 500 sq ft or 20% of the total ceiling area, which covers many single vaulted rooms.
Vented: fill depth = rafter depth − 1 in air channel; R = depth × 3.7 (HD batts) or 4.2 (mineral wool) · Closed-cell: R = depth × 6.5 · Open-cell: R = depth × 3.7 · Flash-and-batt: closed-cell to the IRC R806.5 minimum, batts at R-3.7/in fill the rest · Cost = area × depth × $/sq ft-inch (batts $0.30–$0.55, open-cell $0.40–$0.75, closed-cell $1.00–$2.00)
| Rafter | HD batts, vented | Mineral wool, vented | Flash-and-batt (zone 5) | Closed-cell, full |
|---|---|---|---|---|
| 5.5 in | R-17 | R-19 | R-29 | R-36 |
| 7.25 in | R-23 | R-26 | R-35 | R-47 |
| 9.25 in | R-31 | R-35 | R-43 | R-60 |
| 11.25 in | R-38 | R-43 | R-50 | R-73 |
| 14 in | R-48 | R-55 | R-60 | R-91 |
Computed with this calculator. Deeper rafters, furring strips below, or foam above the deck close the gap to R-49/R-60.
| Climate zone | Minimum air-impermeable R |
|---|---|
| Zone 1 | R-5 |
| Zone 2 | R-5 |
| Zone 3 | R-5 |
| Zone 4 | R-15 |
| Zone 5 | R-20 |
| Zone 6 | R-25 |
| Zone 7 | R-30 |
| Zone 8 | R-35 |
Zones 1–3 R-5; zone 4 R-15 (4C marine R-10); 5 R-20; 6 R-25; 7 R-30; 8 R-35.
Either vented — a baffle in every rafter bay keeps a 1 in air channel from soffit to ridge, with batts below it — or unvented, with closed-cell foam (or rigid foam above the deck) against the roof deck and batts or open-cell filling the rest. Unvented roofs must meet the foam minimums in IRC R806.5.
About R-30 with high-density fiberglass batts or R-35 with mineral wool in a vented 2×10 (8.25 in after the air channel), about R-43 with zone-5 flash-and-batt, and about R-60 with full-depth closed-cell foam.
In climate zones 1–3, yes, on its own. In zones 4 and colder, code requires either a class II vapor retarder on the interior (where allowed) or enough rigid foam above the deck to meet IRC R806.5, because open-cell foam lets moisture reach the cold deck.
The 2021 IECC asks R-49 for ceilings in zones 2–3 and R-60 in zones 4–8 (R-30 in zone 1), but allows R-38 for ceilings without attic spaces up to 500 sq ft or 20% of total ceiling area. Local codes may differ; check what your jurisdiction adopted.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.