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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.
ROCKWOOL Comfortbatt is a stone wool (mineral wool) batt sized to friction-fit between studs and joists. Enter the area, the framing and the R-value you want; the calculator gives the batt thickness from ROCKWOOL's data sheet, the R-value per inch, how many batts to buy and — for the common 16 in wood-stud sizes — how many bags.
With these inputs:
| Result | Value |
|---|---|
| Batts | 85 |
| Bags | 11 (8 batts, 39.8 sq ft per bag) |
| R-value per inch | R-4.18 |
| Fits | 2×6 wall (5.5 in) |
Rockwool vs fiberglass R-value. Stone wool is denser than standard fiberglass batts, so it reaches a higher R-value in the same cavity: R-15 in a 3.5 in 2×4 wall against R-13 for a standard fiberglass batt, and R-23 in a 5.5 in 2×6 wall. ROCKWOOL's data sheet also lists it as noncombustible (ASTM E136), moisture-resistant and good at absorbing sound. Comfortbatt is cut slightly wider than the stud bay (15.25 in for 16 in wood framing) with one flexible edge, so it friction-fits without staples. Cut it with a serrated knife and do not compress it — a squeezed batt loses R-value.
Batt area = batt width × length ÷ 144 (15.25 × 47 in = 4.98 sq ft) · Batts = ceil(area × (1 + waste) ÷ batt area) · Bags = ceil(batts ÷ batts per bag); R per inch = R-value ÷ thickness
| Framing | R-value | Thickness | R per inch |
|---|---|---|---|
| Wood stud | R-13 | 3.5 in | R-3.71 |
| Wood stud | R-15 | 3.5 in | R-4.29 |
| Wood stud | R-21 | 5.5 in | R-3.82 |
| Wood stud | R-23 | 5.5 in | R-4.18 |
| Wood stud | R-30 | 7.25 in | R-4.14 |
| Wood stud | R-38 | 9.5 in | R-4 |
| Steel stud | R-10 | 2.5 in | R-4 |
| Steel stud | R-15 | 3.5 in | R-4.29 |
| Steel stud | R-24 | 6 in | R-4 |
| Steel stud | R-30 | 7.25 in | R-4.14 |
| Steel stud | R-32 | 8 in | R-4 |
ROCKWOOL Comfortbatt technical data sheet (USA), thermal resistance per ASTM C518.
| Wall area | R-15 (12/bag) | R-23 (8/bag) | R-30 (6/bag) |
|---|---|---|---|
| 100 sq ft | 2 bags | 3 bags | 4 bags |
| 200 sq ft | 4 bags | 6 bags | 8 bags |
| 400 sq ft | 8 bags | 11 bags | 15 bags |
| 800 sq ft | 15 bags | 22 bags | 29 bags |
| 1200 sq ft | 22 bags | 32 bags | 43 bags |
Bag counts as packed for retail (e.g. R-23 RXCB551525: 8 batts, 39.8 sq ft). Pack sizes can change — check the label.
Comfortbatt for wood studs is R-13 or R-15 at 3.5 in, R-21 or R-23 at 5.5 in, R-30 at 7.25 in and R-38 at 9.5 in — about R-4.2 per inch. Steel-stud batts are R-10, 15, 24, 30 and 32.
It depends on the R-value: a bag of R-15 Comfortbatt for 16 in wood studs holds 12 batts (59.7 sq ft), R-23 holds 8 (39.8 sq ft) and R-30 holds 6 (29.9 sq ft).
Use R-23 Comfortbatt (5.5 in). A 400 sq ft wall at 16 in o.c. with 5% waste needs 85 batts — 11 bags of 8.
It gives more R per inch than standard fiberglass batts, doesn't burn, holds its shape and absorbs sound well. It usually costs more per square foot. Both work when installed without gaps or compression.
Yes, between joists or rafters — ROCKWOOL lists R-30 and R-38 Comfortbatt for attics and cathedral ceilings. For an open attic floor, blown insulation is usually cheaper per R.
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.