Insulation calculator
Bags of batts for a framed wall, floor or attic, allowing for the studs the batts sit between — or the depth and the bags of blown-in it takes to reach an R-value over an attic floor. The coverage on the bag is the figure to trust, so it is yours to enter.
Length × height of the wall, or the floor or ceiling, less windows and doors
Takes 15" wide batts
Printed on the bag. 106.56 is eleven 15" × 93" batts
Cutting around boxes and short bays
Bags of batts
3bags
Batt width
15"
Batt area
300 sq ft
Bags, exact
2.96
Where the number comes from
- Framed area
- 320 sq ft
- Taken by the studsA 15" batt serves every 16" of wall
- 20 sq ft
- Batt face area
- 300 sq ft
- With wasteDivided by 106.56 sq ft a bag
- 315 sq ft
Only the stud strip is taken off. Plates, headers and corners take more of a real wall, so this errs toward a bag to spare — keep the receipt and return it unopened.
Where to buy this
- 3 bags of 15" battsHome DepotLowe's
This searches the material, not the R-value or bag size worked out here — match those on the bag before you buy.
Plain search links, nothing sponsored. Prices and stock are the shop's, and a local lumberyard will usually beat both on solid stock.
About insulation
- Why does the calculator buy less batt than my wall's area?
- Because the studs take some of it. A 15" batt fills the bay between studs 16" on centre, so each row of batts serves 16" of wall with only 15" of batt — 100 sq ft of framed wall takes 93.75 sq ft of batt, and the 23" batt for 24" framing takes 23/24 of the wall. The coverage on a batt bag is the batts' own face area, so dividing the whole wall by it would buy for a wall with no studs in it. Plates, headers and corners take more still, which the calculator does not subtract, so it errs toward a bag to spare.
- How deep does blown-in insulation need to be for R-49?
- It is the R-value divided by the material's R per inch. At a typical R-3.5 per inch, cellulose reaches R-49 at about 14" settled; at R-2.5 per inch, loose-fill fiberglass needs about 19 5/8". Both are typical figures rather than any one product's — the chart on the bag gives the installed and settled thickness for that product, and blowing to the installed figure is what leaves the right depth once it settles.
- What R-value should an attic have?
- ENERGY STAR and the U.S. Department of Energy recommend R30 to R60 for an attic, depending on climate zone — the low end in the warmest parts of the country and R49 to R60 across most of the colder ones. Your local energy code sets the minimum for new work, so check with the building department rather than treating a recommendation as the law.
- Where do I find the coverage per bag?
- On the bag. A batt bag prints its square footage, which is the batts' width times their length times how many are in it — eleven 15" × 93" batts is 106.56 sq ft. A bag of loose fill prints a coverage chart instead, with the square feet one bag covers at each R-value, because the deeper you blow it the less floor a bag reaches. Read the line for your target R-value and enter that figure; it is the authority, and the calculator only does the division.
You might also need
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- Roofing calculatorSquares, bundles, starter strip, ridge cap and underlayment from a building footprint and a pitch.
- Paint and finish coverageGallons from an area and a coat count, with the cans to buy for brush, roller or spray.