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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-19 fiberglass is 6.5 inches thick, so it fits a 2×8 floor or ceiling cavity but is squeezed in a 5.5 in 2×6 wall — where the manufacturer rates it at R-17 to R-18. A bag of 15 × 93 in batts covers 87.18 sq ft, so bags = area × 1.10 ÷ 87.18: a 500 sq ft area with 10% waste takes 7 bags. Pick the product size and enter your area.
With these inputs:
| Result | Value |
|---|---|
| Bags to buy | 7 |
| Batts | 57 |
| Thickness | 6.5 in |
| Fits without compressing | 2×8 (7.25 in) or deeper |
Where R-19 belongs. R-19 is the classic batt for floors over crawl spaces and unheated garages and for 2×8 ceilings. It is the 2021 IECC floor minimum in climate zones 3 and 4 (except Marine 4); zones 1–2 need R-13 and zones 5 and up need R-30 or more. In a floor, fit the batt tight against the subfloor and hold it with wire supports; if it is kraft-faced, the paper goes up toward the heated floor. R-19 is also sold for 2×6 walls, and it works there, but compressed: Johns Manville's fact sheet notes R-19 in a 5.5 in cavity gives R-17 to R-18. A high-density R-21 batt is made to fill the same 2×6 cavity at full rating. Faced or unfaced R-19. Faced R-19 carries a kraft paper vapor retarder and is used where it faces the warm side — exterior walls and floors in cold climates. Unfaced R-19 goes where a vapor retarder isn't wanted or already exists, and when adding a layer over existing attic insulation (a facing between layers can trap moisture).
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) | 6.5 in | 9 | 87.18 |
| 23" × 93" batts (24" o.c. walls) | 6.5 in | 9 | 133.68 |
| 15" × 48" batts (16" o.c. floors/ceilings) | 6.5 in | 18 | 90 |
| 23" × 48" batts (24" o.c. floors/ceilings) | 6.5 in | 18 | 138 |
| 15" × 39 ft 2 in roll | 6.5 in | 1 | 48.96 |
| 23" × 39 ft 2 in roll | 6.5 in | 1 | 75.07 |
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 | 23" × 93" batts |
|---|---|---|
| 100 sq ft | 2 bags | 1 bags |
| 250 sq ft | 4 bags | 3 bags |
| 500 sq ft | 7 bags | 5 bags |
| 1,000 sq ft | 13 bags | 9 bags |
| 1,500 sq ft | 19 bags | 13 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.
6.5 inches for fiberglass batts and rolls (Johns Manville), about R-2.9 per inch. Some brands list 6.25 in; check the bag.
You can, but a 2×6 cavity is only 5.5 in deep, so the batt is compressed and performs at about R-17 to R-18. R-21 high-density batts are designed to fill a 2×6 at full rating.
Johns Manville lists 87.18 sq ft for nine 15 × 93 in batts, 133.68 for 23 × 93 in, 90 for 15 × 48 in and 138 for 23 × 48 in. A 15 in × 39 ft 2 in roll covers 48.96 sq ft.
Floors over crawl spaces and garages, 2×8 ceilings and knee walls, and 2×6 walls where a slightly compressed batt is acceptable. It is too thin on its own for an attic floor in most of the US.
Thickness and use: R-19 is 6.5 in for floors and 2×8 cavities, R-30 is 10.25 in for attics and 2×12 rafters (or 8.25 in as high-density R-30C for 2×10 cathedral ceilings).
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.