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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.
Bags of batt insulation = area × 1.10 for waste ÷ the square feet printed on the bag: 800 sq ft of wall with R-13 bags covering 88 sq ft needs 10 bags. Enter your area, batt size and bag coverage for pieces, bags and total spend. Buying continuous rolls instead? Use the roll insulation calculator — the cutting maths is different.
With these inputs:
| Result | Value |
|---|---|
| Coverage to buy | 880 sq ft |
| Individual batts | 91 pieces |
| Bags to buy | 10 bags |
| Estimated cost | $620 |
| Cost per sq ft | $0.7 |
Coverage to buy = area × (1 + waste %) · Pieces = coverage ÷ (batt width × batt length) · Bags = coverage ÷ coverage per bag, rounded up
It varies by R-value: an R-13 bag typically covers 88 to 106 sq ft, while an R-30 bag covers around 40 to 58 sq ft. The number is printed on the front of the bag.
Yes — split batts around wiring rather than compressing them, and trim narrow bays about half an inch wide so the batt friction-fits without folding.
Batts are sold to fit the cavity, not the spacing: a 15-inch batt fits 16-inch on-centre framing and a 23-inch batt fits 24-inch, because the stud itself takes up the difference. Buying by spacing rather than by cavity width is a common ordering error and leaves you compressing or trimming every piece.
Faced batts carry a kraft or foil vapour retarder that goes toward the warm-in-winter side, so they suit a first installation in a wall or floor. Unfaced is what you want when topping up existing insulation, or anywhere a vapour retarder already exists — two retarders in one assembly traps moisture between them.
It reduces its R-value, though not proportionally — a batt squeezed into a shallower cavity loses less than you might expect but still loses. The bigger problem is gaps: a batt cut short, stuffed around a pipe or left bulging around wiring leaves air paths that cost far more than the compression does. Fit matters more than thickness.
Divide the area plus 10% waste by the coverage printed on the bag. At R-13, where a bag typically covers 88 to 106 sq ft, 1,000 sq ft of wall needs 11 to 13 bags. At R-30, with 40 to 58 sq ft per bag, the same area takes 19 to 28 bags.
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.