Batt insulation coverage & how many bundles
Batts are sold by the bundle, and each bundle covers a set number of square feet at a given R and width. Divide, round up, add for waste. The trap is coverage — it shrinks as the R goes up.
The formula
bundles = ceil(area ÷ coverage-per-bundle), then bump it for waste. Coverage is printed on the bag and depends on both R-value (more material per ft² at higher R) and width (16" vs 24" on-center). Do it in the batt bundle calculator.
Coverage drops as R rises
| R-value | Coverage per bundle | Bundles per 1,000 ft² |
|---|---|---|
| R-13 | ~88 ft² | 12 |
| R-15 | ~67 ft² | 15 |
| R-21 | ~58 ft² | 18 |
| R-30 | ~58 ft² | 18 |
| R-38 | ~40 ft² | 25 |
Read the exact number off the bag — these are labeled typicals. Full grid in the batt coverage table.
Worked example: a wall
A 40-ft wall, 8 ft tall, is 320 ft² gross. Knock off ~40 ft² of windows and doors and you have 280 ft² net. With R-13 batts at 88 ft²/bundle: bundles = ceil(280 ÷ 88) = ceil(3.18) = 4 bundles. That 3.5" cavity, filled with an R-13 batt, gives about R-11.2 through the insulation (a genuine R-13 batt is the labeled cavity value). Run your own wall in the wall insulation calculator.
Add for waste — batts generate a lot of it
Unlike loose-fill, batts leave offcuts at every window, outlet, pipe and short bay. Add ~10% and round up. A 500 ft² wall at 88 ft²/bundle is ceil(500 ÷ 88) = 6 bundles bare, but ceil(500 × 1.10 ÷ 88) = 7 bundles with waste — that extra bundle is the one you'd otherwise drive back for.
Match the width to your framing
- 16" on-center studs → 15" batts. 24" on-center → 23" batts. Buy the wrong width and every bay either gaps or bunches.
- Faced vs unfaced: faced batts have a kraft or foil vapor retarder — face it toward the heated side, and never add a second vapor retarder over existing insulation.
- Cavity depth: a 2×4 (3.5") takes R-13/R-15; a 2×6 (5.5") takes R-19/R-21. Don't stuff a thick batt into a thin cavity — compression kills R.
Friction fit, don't compress
A batt hits its rated R only at full loft. Compress an R-19 into a 3.5" cavity and you get ~R-13. Cut batts a hair oversize for a friction fit, split them around wires and boxes instead of crushing them, and fill the corners. Gaps and voids do more damage than a slightly low R — even 5% missed coverage can cut effective performance sharply.
What to measure first
- Net wall area (gross minus openings).
- Stud spacing (16" vs 24" o.c.) and cavity depth.
- The coverage-per-bundle for your exact R and width.
- Whether you need faced batts (and which way the facing goes).
Worked example: a whole floor of walls
Scale the wall example up. A 24×40 single-story has ~128 lf of exterior wall × 8 ft = 1,024 ft² gross. Knock off ~140 ft² for windows and two doors and you're at ~884 ft² net. In R-13 batts at 88 ft²/bundle with 10% waste: ceil(884 × 1.10 ÷ 88) = ceil(11.05) = 12 bundles. Step up to R-21 for a 2×6 wall (~58 ft²/bundle) and the same net area is ceil(884 × 1.10 ÷ 58) = 17 bundles — the higher R costs you five more bundles because each one covers less. That's the coverage-shrinks-with-R rule doing your budgeting for you.
Faced, unfaced, and the vapor-retarder rule
Facing isn't decoration. A kraft or foil facing is a vapor retarder and goes toward the heated side of the assembly — interior in most US climates. Two hard rules from the field: never install a second vapor retarder over insulation that already has one (you'll trap moisture between them), and don't leave faced kraft exposed in living space where code wants a fire cover. For adding over existing attic batts, buy unfaced and lay it perpendicular to the first layer to cross the joist gaps. Where you need fire resistance or moisture tolerance, mineral wool batts friction-fit and shed water better than kraft-faced fiberglass.
Cutting clean: the offcut math
Batts waste material at every interruption, and a messy install loses more R than a low nominal number. Cut on a scrap of plywood with a sharp utility knife and a straightedge, compress-and-slice, and cut batts ~1/2" wide of the bay for a friction fit. Split, don't stuff, around wires and boxes — peel the batt to half thickness, run the back half behind the wire and the front half over it, so the cavity stays full with no crushed spots. Fill the thin gap behind pipes and at the top and bottom plates. A wall that's R-13 everywhere beats a wall that's R-19 with a scatter of gaps, every time.
Quick numbers to leave with
- Formula: bundles = ceil(area × (1 + waste) ÷ coverage-per-bundle).
- Coverage falls with R: R-13 ~88 ft², R-15 ~67, R-21/R-30 ~58, R-38 ~40 ft²/bundle.
- Width to framing: 15" batts for 16" o.c.; 23" batts for 24" o.c.
- Facing toward the heated side; never two vapor retarders; unfaced for add-over.
- Friction-fit, don't compress — split around wires; gaps cost more R than a low nominal.
Get width and R right, divide, add 10%, round up — and you'll fill the wall without a mid-job hardware run.