Insulation Calculator
Attic insulation is sold by the bag, and each bag covers a rated square footage that shrinks as R-value climbs. This insulation calculator divides your attic area by the coverage rate for your insulation type and target R-value, rounds up to whole bags, and estimates the installed depth. Air-seal first — insulation slows heat flow, but only air sealing stops the leaks.
Wall-to-wall attic floor you plan to insulate.
Higher R = more resistance to heat flow.
Bags needed
28
- Target R-value
- 38R
- Coverage per bagRule of thumb — confirm on the bag label.
- 36ft²/bag
- Estimated installed depthRule-of-thumb depth for the target R-value.
- 142.5in
How it was calculated
- Coverage: 36 ft² per bag at blown 38R (rule of thumb)
- Bags: 1000 ft² ÷ 36 = 27.78 → round up to 28 bags
- Depth: ≈ 11.9 inches for R-38
How it works
Bags = ceiling(attic area ÷ coverage per bag). Coverage is a rule of thumb that falls as R-value rises: blown fiberglass covers about 48/36/28 ft² per bag at R-30/R-38/R-49, blown cellulose about 40/30/24, and fiberglass batts about 48 regardless of R-value.
Always round up to whole bags: a half-covered attic performs far below its rated R-value, and one extra bag is cheap insurance against a second trip to the store.
Installed depth is estimated from R-value: blown insulation runs about R ÷ 3.2 inches deep (so R-38 ≈ 11.9 in), while batts come in fixed thicknesses (R-30 ≈ 9.5 in, R-38 ≈ 12 in, R-49 ≈ 14.5 in) — both rules of thumb.
Before buying, check the coverage chart printed on the bag: the manufacturer’s label governs over any calculator.
Worked examples
Attic to R-38 with blown fiberglass
A 1,000 ft² attic: 1,000 ÷ 36 = 27.78, so round up to 28 bags of blown fiberglass. Installed depth lands around 38 ÷ 3.2 ≈ 11.9 inches.
Small attic, cellulose to R-30
A 500 ft² attic: 500 ÷ 40 = 12.5, so 13 bags of blown cellulose at roughly 9.4 inches deep. Budget an evening with a caulk gun first to seal top-plate gaps and plumbing chases — air sealing comes before insulation.
How to measure
- Measure the attic floor wall-to-wall; multiply length × width for rectangular attics.
- Check what is already there: measure existing depth and note the material before adding more.
- For batts, note joist spacing (16 or 24 inches on center) so the batt width matches.
- Buy one spare bag — running short mid-blow means a second trip.
- Verify coverage on the bag label at the store; this calculator is a planning estimate.
Assumptions & limitations
Planning estimate
- Bag coverage rates are a rule of thumb; the manufacturer’s printed coverage chart on the bag governs.
- Blown depth ≈ R ÷ 3.2 inches and batt thicknesses are rules of thumb — settled depth varies by product.
- Assumes a clear attic floor; obstructions, decking, and storage platforms reduce real coverage.
- Air sealing, ventilation baffles, and damming around fixtures are not included but are required for a proper job.
- planning estimate only — Manual J load calculation by a professional is required for equipment selection; this tool does not replace it.
Frequently asked questions
R-value measures resistance to heat flow — higher means better insulation. An R-38 attic resists heat about 27% better than an R-30 one. Building codes set minimum R-values by climate zone; check your local code before choosing a target.
Blown-in fills gaps, voids, and irregular framing better, which is why it dominates attic retrofits. Batts are faster for open, regular joist bays but leave gaps around wiring and pipes that hurt real-world performance. Both hit the rated R-value only when installed to full depth without compression.
Yes, if the old insulation is dry, uncompressed, and mold-free. Lay new material perpendicular to joists (for batts) or blow over the top, and never put a second vapor barrier over the old one. Remove any wet or contaminated insulation first.
Insulation only performs at its rated R-value at full, even depth. Stretching 27.8 bags’ worth of material over the whole attic leaves thin spots that drag down the entire job. The calculator always rounds up so every square foot reaches the target.
About 15.3 inches of blown insulation (49 ÷ 3.2) or roughly 14.5 inches of batts, by the rules of thumb used here. Measure depth in several spots after installation — blown material settles, so installers often blow slightly deep to compensate.