Batt Insulation Calculator

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.