How this insulation calculator works
Insulation sells by the bag, and the bag states its coverage — so the whole estimate is the surface area done honestly, then whole packages over it:
bags = area × (1 + waste%) ÷ ft² per bag, rounded up
Worked example: insulating 30 × 24 exterior walls
The perimeter is 108 ft; at 8 ft high that's 864 ft² of wall, and 10% openings leave 778 ft² to fill. At 100 ft² a bag with 5% waste: 9 bags, about $495. Switch to attic mode and the same footprint is 720 ft² — 8 bags.
What R-value insulation by assembly?
| Assembly | R-value |
|---|---|
| Attic | R-38–60 |
| 2×4 walls | R-13–15 |
| 2×6 walls | R-19–21 |
| Floors (unconditioned below) | R-25–30 |
| Product | ft²/bag |
|---|---|
| R-13 wall batts | ≈ 100–160 |
| R-19 batts | ≈ 75–110 |
| R-30 attic batts | ≈ 55–90 |
| R-38 attic batts | ≈ 40–65 |
Insulation FAQ
How much insulation do I need?
Wall mode: perimeter × height minus openings. A 30 × 24 ft building at 8 ft is 864 ft² of wall, 778 ft² after 10% doors and windows — 9 bags at 100 ft² per bag with 5% waste, about $495. Attic mode is the flat area straight up.
What R-value insulation do I need?
By assembly, broadly: attics R-38 to R-60 (colder zones higher), 2×4 walls R-13 to R-15, 2×6 walls R-19 to R-21, floors over unconditioned space R-25 to R-30. Your climate zone’s energy code sets the exact floor — check locally.
How many square feet does a bag of insulation cover?
It is printed on the package and swings hard with R-value — from ~40 ft² a bag for thick R-38 attic batts to 150+ for thin wall rolls. Type the printed figure in and the bag count is exact for your product.
Should I use batts or blown-in insulation?
Open wall cavities and accessible framing suit batts — what this calculator counts. Attics are often cheaper to blow: machine rental plus bags, with the coverage-per-bag chart at your target depth printed on every blown bag.
How much of a wall is doors and windows?
Around 10% on a typical house wall, more on glassy living rooms and garage walls with big doors — hence the editable openings figure. Insulation stops at the rough openings; the headers get what fits.
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