Insulation Calculator
Exterior wall and attic floor area from the room size, less openings, turned into bags of batts using the coverage printed on the bag.
- Walls and attic
- Layers for higher R
- Bag coverage editable
- Embeddable
Your estimate
Batt insulation, bags rounded upwalls and attic
Area
Bags to buy
Bag coverage is the area of the batts in the bag. Measuring the whole wall, studs included, slightly overcounts, which leaves a small margin. Check the R-value your climate zone requires before you buy.
Batt insulation is sold by the bag, and every bag lists the square feet it covers. The work is in getting the area right: only exterior walls need insulating, openings come out, and an attic often takes two layers to reach the R-value your code calls for. This calculator handles each of those, starting from the same room measurements as the drywall calculator.
How the bag count is worked out
- Net wall area = perimeter x wall height, less doors, windows and custom openings.
- Exterior wall area = net wall area x the share of walls that face outside. A corner bedroom with two outside walls is about 50%.
- Wall bags = exterior wall area / sq ft per bag, rounded up.
- Attic bags = ceiling area x layers / sq ft per bag, rounded up.
The defaults are Johns Manville kraft-faced batts from its published fact sheet: R-13 at 15 x 93 in covers 106.56 sq ft per bag for 2x4 walls, and R-19 at 23 x 93 in covers 133.68 sq ft per bag for the attic. Bag sizes change by brand and by store, so type the figure from the bag you are buying.
What R-value does the code ask for?
The International Energy Conservation Code sets minimum insulation by climate zone. Under the 2021 edition, ceilings need R-30 in zones 0 and 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8. Wood-frame walls need R-13 in zones 0 to 2, R-20 or R-13 plus R-5 continuous insulation in zone 3, and R-30 or an equivalent combination in zones 4 to 8.
The 2024 edition lowered the ceiling minimums, to R-38 in zones 2 and 3 and R-49 in zones 4 to 8, in exchange for more required efficiency credits elsewhere in the house. Which edition applies depends on what your state or city has adopted, often with amendments, so confirm with your building department before you buy.
A 2x4 wall cavity only fits R-13 to R-15 batts. Reaching R-20 or R-30 on a wall takes 2x6 framing, continuous foam on the outside, or both, which is a design decision rather than a shopping one.
Insulate before the drywall goes up
Batts go in after the rough electrical and plumbing inspection and before the board. Fit them around wires and boxes rather than compressing them behind, since a compressed batt loses R-value. Kraft facing faces the warm side in most climates; check the vapor retarder rules for your zone before stapling.
Frequently asked questions
Primary references
Manufacturer and industry guidance used to check this article.
- Fiberglass insulation manufacturers fact sheetJohns Manville
- 2009 to 2021 IECC and IRC minimum insulation requirements for new homesPNNL Building America Solution Center
- IECC residential committee action report, Table R402.1.3International Code Council
- Recommended home insulation R-valuesENERGY STAR
Product instructions, tested assemblies and local code take priority over general guidance.