This insulation calculator estimates how much insulation a space needs — batts or rolls counted from framing bays, or blown-in bags from manufacturer coverage — so the supply run matches the attic, not the other way around.
R-Value First, Material Second
Insulation is bought to a target R-value (resistance to heat flow; higher = warmer): code-typical attic targets run R-30 (mild) to R-49/R-60 (cold climates), walls R-13 to R-21, and the bag/batt label converts R to coverage at settled thickness. Material choice is lifestyle: batts for clean rectangular bays, blown for attics and retrofit walls, rigid foam where inches are scarce (~R-5 to R-6.5 per inch vs ~R-3.5 for fiberglass). Whatever you install, the heating load it relieves is our BTU Calculator‘s input. Part of our Construction Calculators collection.
Worked Examples (Verified)
- Blown attic, 1,000 sq ft at 45 sq ft/bag coverage: ⌈1000 ÷ 45⌉ = 23 bags — buy the 23, because a half-covered corner at the eaves is where ice dams are born
- Batt walls, 1,200 sq ft gross, 16″ OC: framing takes 1.5/16 = 9.4% → 1,087 sq ft of bays → at 80 sq ft/package, 14 packages
- The R cheat-sheet: fiberglass batt R-13 (3.5″ wall), R-19 (5.5″), R-30 (9.5″); blown fiberglass ≈ R-2.2–2.7/inch, cellulose ≈ R-3.2–3.8/inch — depth gauges, not vibes, decide when to stop blowing
Installation Truths That Matter More Than Brand
- Air sealing beats R: a leaky attic floor (top plates, chases, can lights) can waste more heat than 10 points of R add back — seal first, then insulate.
- Gaps and compression are silent thieves: a 5% void area can cut effective R by double digits.
- Ventilation (baffles at eaves, soffit-to-ridge path) protects the roof deck and the insulation’s dryness; more R with a choked vent path trades one problem for another.
- Vapor control is climate-specific — follow local code rather than internet folklore.
Related Tools
- BTU Calculator — the heating load your insulation reduces
- Drywall Calculator — the surface going over wall insulation
- Construction Calculators — the full collection
Frequently Asked Questions
How many bags of blown insulation do I need?
Area ÷ the bag’s coverage at your target R-value, rounded up. 1,000 sq ft at an R-38 chart coverage of 45 sq ft/bag = 23 bags. Always read coverage off your specific product’s chart — brands and R-targets differ by 2×.
How much R-value do I need?
By climate zone and assembly: attics typically R-30 (Zones 1–2) up to R-60 (Zones 7–8) in current US energy codes; walls R-13–R-21; floors R-19–R-30. Your local code amendment is the authority; the trend is only upward.
Do I deduct for studs and joists?
For batts, yes — batts only occupy the bays (about 9–10% of a 16″ OC wall is wood). For blown attic insulation, no: it covers the joists too. This calculator applies the right convention per type.
Can I layer new insulation over old?
Usually yes (unfaced over faced; never sandwich a vapor barrier mid-assembly). Total R is the sum of layers. Fix air leaks and any moisture problem first — burying a wet or moldy layer inherits it.
Is fiberglass or cellulose better blown-in?
Cellulose packs more R per inch (~3.5 vs ~2.5), resists air movement better and is recycled content; fiberglass doesn’t settle appreciably and resists moisture history better. Both fail the same way: installed too thin. Depth discipline beats brand loyalty.
Is this insulation calculator free?
Yes — free, instant, no sign-up.