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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-13 is the standard fiberglass batt for 2×4 walls: 3.5 inches thick, so it fills the stud cavity exactly. A bag of 15 × 93 in batts covers 106.56 sq ft, so a 500 sq ft wall with 10% waste takes 6 bags. Pick the batt or roll size you are buying and enter the area for bags, batts and the cavity it fits.
With these inputs:
| Result | Value |
|---|---|
| Bags to buy | 6 |
| Batts | 57 |
| Thickness | 3.5 in |
| Fits without compressing | 2×4 (3.5 in) or deeper |
Where R-13 belongs. R-13 is made for 2×4 exterior walls and interior walls you want quieter. In the 2021 IECC it meets the wood-frame wall minimum only in climate zones 1 and 2; colder zones need a denser R-15, a 2×6 wall with R-21, or R-13 plus continuous foam sheathing. It is also the 2021 IECC floor minimum in zones 1 and 2. Kraft-faced R-13 has a paper vapor retarder that goes toward the heated side in cold climates, stapled to the face or sides of the studs. Unfaced R-13 is for walls where the vapor control is handled another way, for interior walls, and for layering over existing insulation. R-13 vs R-15 vs R-19. R-13 and R-15 are both 3.5 in thick — R-15 is simply denser fiberglass, so it adds R-2 in the same 2×4 wall for a higher price per bag. R-19 is 6.5 in thick: it does not fit a 2×4 at all and is compressed to about R-17–18 in a 5.5 in 2×6 cavity, which is why 2×6 walls usually get R-21 (5.5 in, high density) instead.
Coverage = area × (1 + waste %) · Bags = ceil(coverage ÷ sq ft per bag); batts = ceil(coverage ÷ (width × length ÷ 144)) · R per inch = R-value ÷ thickness
| Product | Thickness | Pieces per bag | Sq ft per bag |
|---|---|---|---|
| 15" × 93" batts (16" o.c. walls) | 3.5 in | 11 | 106.56 |
| 15" × 105" batts (9 ft walls) | 3.5 in | 11 | 120.31 |
| 23" × 93" batts (24" o.c.) | 3.5 in | 11 | 163.39 |
| 15" × 32 ft roll | 3.5 in | 1 | 40 |
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 | 15" × 93" batts | 15" × 105" batts |
|---|---|---|
| 100 sq ft | 2 bags | 1 bags |
| 250 sq ft | 3 bags | 3 bags |
| 500 sq ft | 6 bags | 5 bags |
| 1,000 sq ft | 11 bags | 10 bags |
| 1,500 sq ft | 16 bags | 14 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.
3.5 inches — the depth of a 2×4 stud — for fiberglass batts and rolls. That works out to about R-3.7 per inch.
Johns Manville lists 106.56 sq ft for a bag of eleven 15 × 93 in batts, 120.31 sq ft for 15 × 105 in and 163.39 sq ft for 23 × 93 in. A 15 in × 32 ft roll covers 40 sq ft. Check the label on your brand.
Both are 3.5 in thick for 2×4 walls; R-15 is a denser, high-performance batt that gives R-2 more in the same cavity and costs more per square foot.
R-13 for 2×4 walls. R-19 is 6.5 in thick, so it is for 2×8 floors and ceilings or deeper cavities; squeezed into a 2×6 wall it gives about R-17–18.
It meets the 2021 IECC minimum for wood-frame walls only in climate zones 1 and 2. Elsewhere the code asks for more — R-20-class cavity insulation or R-13 plus continuous insulation sheathing, depending on the zone.
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.