Insulation Calculator
Enter the area and the R-value you want to reach — the calculator estimates bags of blown-in or batts.
Runs entirely in your browser — your numbers never leave this page.
How it works
R-value measures resistance to heat flow — higher is better. The DOE recommends R-38 to R-60 for attics (by climate zone) and R-13/R-21 for walls:
- Blown-in (attic): cellulose or fiberglass blown to depth — ~R-3.2 per inch. R-38 needs roughly 12 inches. Bags cover less area as R-value rises.
- Batts: pre-cut blankets for stud/joist bays — R-13 for 2×4 walls, R-21 for 2×6, R-30/R-38 for attic floors.
- Air-seal first: caulk top plates, seal can lights and penetrations. Insulation without air sealing is like a winter coat left unzipped.
- Don't compress batts — squeezing R-30 into a 2×4 cavity drops it to ~R-13. Depth is the rating.
- Settling: blown insulation settles ~10–20% over time — install 10%+ extra or it underperforms within a few years.
Frequently asked questions
How much insulation do I need?
Area × target R-value ÷ coverage. 1,000 sq ft attic to R-38 blown needs ~55 bags with settling allowance. Batts: ~11 bags at R-38 coverage.
What R-value should I target?
Attics: R-38 minimum (R-49+ in cold zones). Walls: R-13 (2×4) or R-21 (2×6). Check the DOE zone map — most US attics are under-insulated.
Blown-in vs batts — which is better?
Blown-in wins for attics: it fills gaps and covers joists with no seams. Batts suit open wall cavities during construction or remodels.
How deep should attic insulation be?
Divide target R by ~3.2 per inch for blown. R-38 ≈ 12 inches, R-49 ≈ 15 inches. Measure in several spots — it is never even.
Can I add new insulation over old?
Yes — the most cost-effective upgrade there is. Just do not put a vapor barrier (kraft facing) between layers; moisture gets trapped.
Why air-seal before insulating?
Air leaks carry far more heat than conduction through insulation. Sealing top plates, wire penetrations, and can lights can matter more than adding R-value.
Does insulation settle?
Blown-in settles 10–20% over a few years. Install extra now (this calculator adds 10%) or lose R-value later.
Is more than R-49 worth it?
Diminishing returns kick in hard past R-49 in most climates — the money is better spent on air sealing, duct sealing, or windows.
Can I compress batts to fit?
Do not — compression destroys the rating. An R-30 batt squeezed into 3.5 inches performs around R-13. Buy the right thickness.
How do I know my current R-value?
Measure depth and multiply: blown ~3.2/inch, fiberglass batt ~3.1/inch, cellulose ~3.7/inch. Most 1990s attics have 6 inches ≈ R-19 — half of recommended.