Free tool
Insulation Calculator
Batt bags for the walls, blown-in bags for the attic — figured from manufacturer bag charts, with the math shown, not hidden.
Field measurements
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Printed from build-your-house.com/calculators/insulation — estimate only.
Methodology
How this takeoff is figured
SHEET TO-07
Wall area. Perimeter × wall height × stories, minus 15% for windows and doors — the standard estimating deduction for a typical opening count. If you already know your net wall area, switch to the wall-area input and the calculator uses your number as-is.
Wall batts. Wall area ÷ coverage per bag, plus a 5% cut allowance, rounded up to whole bags. Coverage per bag: R-13 ~106 sq ft, R-15 ~68, R-19 ~87, R-21 ~68 — rounded manufacturer bag-chart numbers, so check your brand's chart before ordering; batt width and count per bag vary.
Blown-in attic. Attic area ÷ coverage per bag: blown-in fiberglass runs about 40 sq ft per bag at R-38 and 31 sq ft at R-49, rounded up to whole bags. The blower rental is usually free with a ~10-bag purchase, so plan the buy in one trip.
Air-sealing first. Caulk and foam the top plates, wire holes, and flue penetrations before a single bag goes in — it is the highest-ROI hour on the job, and insulation will not stop the air leaks it merely hides.
What's not here. Rim joists, basement walls, spray foam, and rigid exterior foam — those are sized from your plans and climate zone, not a coverage chart. R-value requirements come from local code; the insulation phase guide has the climate-zone map.
Assumptions
Coverage, deductions, and prices
ADJUST ABOVE
- Batt coverage, R-13~106 sq ft/bag
- Batt coverage, R-15~68 sq ft/bag
- Batt coverage, R-19~87 sq ft/bag
- Batt coverage, R-21~68 sq ft/bag
- Blown-in coverage, R-38~40 sq ft/bag
- Blown-in coverage, R-49~31 sq ft/bag
- Wall openings deduction15%
- Batt cut allowance5%
- Batt bag prices$55–$95 by R-value
- Blown-in bag price$38–$52
- PricesNational range, Aug 2026
Questions
Owner-builders ask
FAQ
How many square feet does a bag of insulation cover?
Typical bag-chart coverage: R-13 batts run about 106 sq ft per bag, high-density R-15 about 68, R-19 about 87, and high-density R-21 about 68. Blown-in fiberglass covers roughly 40 sq ft per bag at R-38 and 31 sq ft at R-49. Coverage varies by brand because batt width, length, and count per bag all differ between manufacturers — the chart printed on the bag is the authority; these are rounded typical numbers for estimating.
What R-value do I need in my walls?
2×4 walls fit R-13 or R-15 batts; 2×6 walls fit R-19 or R-21. Which one your house needs comes from your climate zone and local energy code, not from a calculator — colder zones require more wall R-value, and some jurisdictions add continuous exterior foam on top. The insulation phase guide on this site has the climate-zone map; when in doubt, ask your building department what they enforce.
How many bags of blown-in insulation for 1,200 sq ft at R-38?
At roughly 40 sq ft of coverage per bag at R-38, a 1,200 sq ft attic takes 1,200 ÷ 40 = 30 bags. At $38–$52 per bag that is about $1,140–$1,560 in material, and at 30 bags the blower rental is usually free — most big boxes waive it with a purchase of around 10 bags or more.
Can I compress batts to fit a smaller cavity?
No. Fiberglass insulates with the air it traps; squash it and the R-value collapses — an R-19 batt compressed into a 2×4 cavity performs like R-13. Buy the batt made for your cavity depth, and cut around wires and boxes instead of stuffing the batt behind them.
Batts or blown-in for the attic?
For an open, accessible attic, blown-in usually wins: it fills irregular joist bays and odd gaps without cutting, goes in faster, and the blower rental is typically free with a ~10-bag purchase. Batts earn their keep at kneewalls, sloped ceilings, and tight-access areas where you cannot aim a hose, and on small jobs where renting and returning a machine is not worth the trip.