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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.
R-30 fiberglass batts are 10.25 inches thick; the high-density R-30C made for cathedral ceilings is 8.25 inches, so it fits a 2×10 rafter with an inch left for ventilation. A bag of 16 × 48 in batts covers 58.66 sq ft, so a 1,000 sq ft attic with 10% waste takes 19 bags. Enter the area and your climate zone to see whether R-30 is enough.
With these inputs:
| Result | Value |
|---|---|
| Bags to buy | 19 |
| Batts | 207 |
| Thickness | 10.25 in |
| Fits without compressing | 2×12 (11.25 in) or deeper |
| 2021 IECC attic level (zone 5) | R-60: R-30 short |
Is R-30 enough for an attic?. Only in the warmest part of the country. The 2021 IECC sets ceiling insulation at R-30 in climate zone 1, R-49 in zones 2–3 and R-60 in zones 4–8. So R-30 alone leaves a zone 5 attic R-30 short — about 12 more inches of blown fiberglass, or a second layer of unfaced batts laid across the joists. R-30 batts are 10.25 in thick, more than a 2×10 joist (9.25 in), so they stand proud of the joists in an attic floor. That is fine — don't squash them under boards. If you need to store things up there, raise the floor on taller sleepers rather than compressing the insulation. R-30C for cathedral ceilings. In a vented cathedral ceiling the batt has to leave an air channel under the roof deck. R-30C is denser and 8.25 in thick, so in a 9.25 in 2×10 rafter it leaves an inch for a baffle and still reaches R-30.
Coverage = area × (1 + waste %) · Bags = ceil(coverage ÷ sq ft per bag); batts = ceil(coverage ÷ (width × length ÷ 144)) · R per inch = R-value ÷ thickness · Attic shortfall = 2021 IECC ceiling R for the zone − batt R; blown depth = shortfall ÷ R-2.5 per inch
| Product | Thickness | Pieces per bag | Sq ft per bag |
|---|---|---|---|
| 16" × 48" batts (16" o.c.) | 10.25 in | 11 | 58.66 |
| 24" × 48" batts (24" o.c.) | 10.25 in | 11 | 88 |
| R-30C 15.5" × 48" (cathedral, 16" o.c.) | 8.25 in | 11 | 56.83 |
| R-30C 23.625" × 48" (cathedral, 24" o.c.) | 8.25 in | 11 | 86.62 |
Johns Manville Fiberglass Manufacturers Fact Sheet (wood framing, kraft-faced and unfaced). Other brands are close but not identical — use the square feet printed on your bag.
| Area | 16" × 48" batts | 24" × 48" batts |
|---|---|---|
| 100 sq ft | 2 bags | 2 bags |
| 250 sq ft | 5 bags | 4 bags |
| 500 sq ft | 10 bags | 7 bags |
| 1,000 sq ft | 19 bags | 13 bags |
| 1,500 sq ft | 29 bags | 19 bags |
Bags = area × 1.10 ÷ sq ft per bag, rounded up.
| R-value | Thickness | R per inch | Fits |
|---|---|---|---|
| R-11 | 3.5 in | 3.14 | 2×4 wall |
| R-13 | 3.5 in | 3.71 | 2×4 wall |
| R-15 | 3.5 in | 4.29 | 2×4 wall (high density) |
| R-19 | 6.5 in | 2.92 | 2×8 floor/ceiling; compresses in 2×6 |
| R-21 | 5.5 in | 3.82 | 2×6 wall (high density) |
| R-30 | 10.25 in | 2.93 | 2×12 or attic floor |
| R-38 | 12.5 in | 3.04 | Attic floor |
| R-49 | 13.5 in | 3.63 | Attic floor |
Johns Manville fact sheet thicknesses; lumber depths 2×4 = 3.5 in, 2×6 = 5.5 in, 2×8 = 7.25 in, 2×10 = 9.25 in, 2×12 = 11.25 in.
10.25 inches for standard fiberglass batts and 8.25 inches for high-density R-30C (Johns Manville). An unfaced R-30 roll is 9.25 in. Blown fiberglass at about R-2.5 per inch needs around 12 inches to reach R-30.
It meets the 2021 IECC attic level only in climate zone 1. Zones 2–3 call for R-49 and zones 4–8 for R-60, so most attics need R-30 plus a second layer or blown insulation on top.
Johns Manville lists 58.66 sq ft for eleven 16 × 48 in batts and 88 sq ft for 24 × 48 in. R-30C bags cover 56.83 and 86.62 sq ft.
Faced only for the first layer between the joists, paper down toward the heated ceiling in cold climates. Any layer added on top of existing insulation should be unfaced.
R-38 (12–12.5 in) gets closer to the R-49 or R-60 most attics need. If you are laying one layer of batts, R-38 leaves a smaller gap to top up; R-30 suits 2×12 rafters and zone 1 attics.
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.