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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-38 fiberglass batts are 12 to 12.5 inches thick; R-38C for cathedral ceilings is 10.25 inches and fits a 2×12 rafter with an inch to spare for venting. A bag of 16 × 48 in batts covers 42.66 sq ft, so a 1,000 sq ft attic with 10% waste takes 26 bags. Enter your area and climate zone to see what it takes to reach the code level.
With these inputs:
| Result | Value |
|---|---|
| Bags to buy | 26 |
| Batts | 207 |
| Thickness | 12.5 in |
| Fits without compressing | Deeper than a 2×12 — attic floor |
| 2021 IECC attic level (zone 5) | R-60: R-22 short |
R-38 in an attic. The 2021 IECC asks for R-49 in zones 2–3 and R-60 in zones 4–8, so R-38 is short almost everywhere except zone 1 — R-11 short of R-49 and R-22 short of R-60. Laying R-38 unfaced batts across existing insulation is a common way to get there; blown fiberglass at about R-2.5 per inch needs another 4.4 in for R-49 or 8.8 in for R-60. Older codes asked for R-38 in many zones, so it is also the level many homes already have. If yours is at R-38, the extra R-11 to R-22 has a smaller payback than the first R-30 — air sealing the ceiling first usually pays faster. R-38C for cathedral ceilings. R-38C is a high-density batt 10.25 in thick. In a 2×12 rafter (11.25 in) it leaves an inch for a vent baffle under the roof deck. Use it where standard R-38 at 12–12.5 in won't fit.
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, 12.5" (16" o.c.) | 12.5 in | 8 | 42.66 |
| 24" × 48" batts, 12.5" (24" o.c.) | 12.5 in | 8 | 64 |
| 16" × 48" batts, 12" (9 per bag) | 12 in | 9 | 48 |
| R-38C 15.5" × 48" (cathedral, 16" o.c.) | 10.25 in | 8 | 41.33 |
| R-38C 23.75" × 48" (cathedral, 24" o.c.) | 10.25 in | 8 | 63.33 |
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, 12.5" | 24" × 48" batts, 12.5" |
|---|---|---|
| 100 sq ft | 3 bags | 2 bags |
| 250 sq ft | 7 bags | 5 bags |
| 500 sq ft | 13 bags | 9 bags |
| 1,000 sq ft | 26 bags | 18 bags |
| 1,500 sq ft | 39 bags | 26 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.
12 to 12.5 inches for fiberglass batts and 10.25 inches for high-density R-38C (Johns Manville). Blown fiberglass at about R-2.5 per inch needs roughly 15 inches.
In climate zone 1, yes (the 2021 IECC asks for R-30). Zones 2–3 call for R-49 and zones 4–8 for R-60, so R-38 falls R-11 to R-22 short there.
Johns Manville lists 42.66 sq ft for eight 16 × 48 in batts (12.5 in), 48 sq ft for nine 12 in batts and 64 sq ft for eight 24 × 48 in batts. R-38C bags cover 41.33 and 63.33 sq ft.
About an inch of batt and R-11. R-49 batts are 13.5 in thick; in zones 2–3 R-49 is the 2021 IECC attic level, so R-38 needs topping up there.
Yes, if the old insulation is dry. Use unfaced R-38 and lay it at right angles across the joists so it covers their tops — a paper facing between layers can trap moisture.
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.