How the roof calculation works
A cathedral ceiling or a finished slope is insulated at the rafters, not on an attic floor. The R-value you need is the same one ENERGY STAR recommends for attics. What changes is that the insulation has to fit in the bay:
R to add = recommended R for your zone − R of what's there now
and the new depth cannot be deeper than the rafter, unless you choose Not limited under More options. That option is for a second layer fastened under the rafters.
- Find your target. R-30 in climate zone 1, R-49 in zones 2 and 3, R-60 in zones 4 through 8. Enter your ZIP code and the calculator picks the zone.
- Set the rafter size. The page opens on a 2×10, which is 9¼ in of real depth. A 2×4 is 3½ in, a 2×6 is 5½ in, a 2×8 is 7¼ in and a 2×12 is 11¼ in.
- Choose a material. Fiberglass batts (24 in wide), mineral wool batts (15¼ in wide, the better fit for rafters 16 in apart), polyiso boards, or open-cell and closed-cell spray foam.
The US targets are the R-value of the insulation itself. The calculator does not subtract the wood of the rafters from that R-value. Where a batt is thicker than the bay, it is left out, and the result says the target will not fit.
Worked example: 1,000 sq ft of rafters in zone 5A
Zone 5A calls for R-60. The roof area is 1,000 sq ft, nothing is there now, and 10% is added for waste (1,100 sq ft). With the rafters set to 2×10:
| Material | What fits | Result | To buy | Cost |
|---|---|---|---|---|
| Fiberglass batts, 24 in wide | R-19, 6¼ in | Short of R-60 | 10 bags | $1,190 |
| Mineral wool batts | R-23 + R-15, 9 in | R-38, still short | 28 + 19 bags | $4,134 |
| Open-cell spray foam | Fills the 9¼ in | About R-34 | Installed | $3,977–$9,250 |
| Closed-cell spray foam | 9.23 in | R-60 | Installed | $13,846–$23,077 |
The R-30 fiberglass batt is 9½ in, a quarter-inch too thick for a 2×10, so the calculator steps down to R-19. A 2×12 (11¼ in) does take that R-30 batt: 13 bags, $1,690, and you are still 30 short of R-60.
Choose Not limited and fiberglass can hit R-60 with two layers of R-30, 19 in in total, 13 bags plus 13 bags, $3,379 in materials. That second layer has to sit under the rafters, so you lose headroom. Polyiso in the calculator tops out at three layers of 2 in board, which is R-39 and $5,857 for this roof. The sheets in the catalog are no thicker than that.
Batt and board costs are materials only, at typical retail prices from October 2026. Spray foam costs are typical installed prices. Installers often add a minimum charge of about $1,000–$2,000 on top of a small job.
Attic floor or rafters?
If nobody lives in the space under the roof, insulate the attic floor instead. There is no bay to fill, and the same R-60 of blown cellulose is a much smaller job. Use the attic insulation calculator.
Insulate the rafters when the ceiling follows the roof: a cathedral ceiling, a finished bonus room, or a conditioned attic. Keep a drainage plane and, on a vented roof, the air channel. Seal the ceiling plane so warm air cannot leak past the insulation into the bay.
The full zone table is on insulation R-values by climate zone. Depths for each material are on the R-value chart.