Insulation calculator

Insulation is bought by square feet of coverage but installed by the stud bay, and those are different numbers. A 40 foot wall at 16 inches on center has 30 bays. Get the bay count right and the batt count follows.

An 8 foot wall is 96 inches tall, but the cavity is not, and this is where estimates go wrong: subtract the 1.5 inch bottom plate and the 3 inch double top plate and you're left with 91.5 inches. That is exactly why insulation is sold in a 93 inch precut — one batt fills one bay of a standard 8 foot wall. Estimate against the full 96 inches instead and you'll order two batts per bay and buy twice the insulation you need.

So that 40 by 8 foot wall is 320 square feet, 30 bays and 30 batts. Buy the batt width that matches your framing: 15 inch batts for 16 inch centers, 23 inch for 24 inch centers. They are sized to friction-fit into the actual cavity, which is narrower than the spacing by one stud width.

Faced or unfaced is the other decision, and it is a climate question rather than a quantity one. Kraft-faced batts carry a vapor retarder that faces the warm side of the wall in winter, which in most of the US means the inside. Unfaced batts go where a separate air or vapor control layer is already handled, or in interior walls where you are insulating for sound rather than heat. Getting the facing on the wrong side traps moisture inside the wall, so check what your climate zone and code call for before you buy.

Bags list coverage in square feet rather than batt count, and the coverage varies by R-value and manufacturer — a bag of R-13 covers far more wall than a bag of R-21. Take the square footage above and divide by the number printed on the bag. And compressing a thick batt into a shallow cavity doesn't give you its rated R-value: match the batt depth to the framing, 2x4 or 2x6.