Roofing
Insulation Calculator
Bags of blown-in insulation for an attic, by target R-value — with the manufacturer coverage chart.
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About this calculator
Blown-in insulation is bought by the bag, and each bag covers less area as the target R-value rises — the same fibres spread deeper cover fewer square feet. This calculator sizes the bag count from your attic area and target R-value using manufacturer coverage estimates for loose-fill fiberglass.
What this number means
Attic insulation is bought by the bag and installed by depth: the R-value you want decides how deep the loose-fill must sit, and the depth decides how much floor area each bag covers. A bag that covers 40 sq ft at R-30 covers only 25 sq ft at R-49, because the same fibres are spread thinner at the higher target — the bag count climbs faster than the R-value does.
The coverage figures on this page are manufacturer estimates for loose-fill fiberglass, the most common blown-in product. Brands differ by a few square feet per bag, and cellulose and mineral wool have their own charts — always cross-check the bag label for your exact product before ordering.
The formula
- Area = Length × Width
- Bags = Area ÷ coverage per bag at the target R-value, rounded up
- Installed depth ≈ R-value ÷ 2.7 (loose-fill fiberglass, approximate)
- Coverage chart (manufacturer estimates, loose-fill fiberglass): R-30 → 40 sq ft/bag, R-38 → 32 sq ft/bag, R-49 → 25 sq ft/bag
Worked example
A 25 × 40 ft attic (1,000 sq ft) topped up to R-38.
- Bags: 1,000 ÷ 32 = 31.25 → 32 bags
- Installed depth: 38 ÷ 2.7 ≈ 14.1 in of loose-fill
- If 6 in of old insulation is already there (about R-16), you only need the R-22 top-up — roughly 19 bags at the R-38 coverage rate is the wrong shortcut; measure the existing depth and subtract its R-value first
Assumptions and limitations
- Coverage figures are manufacturer estimates for loose-fill fiberglass and vary by brand; the bag label for your product governs.
- The calculation assumes a bare attic (no existing insulation). With existing insulation, subtract its R-value from the target and size the top-up from the remaining R-value, not the full target.
- Installed depth is approximate (R ÷ 2.7 per inch for loose-fill fiberglass). Settling over the first year can reduce depth by 5–10% — blowing slightly deep compensates.
- Batts follow different maths (sold by area at a fixed R-value per thickness) and are not covered here; this calculator is for blown-in loose-fill.
Frequently asked questions
At R-38 with 32 sq ft per bag: 32 bags. At R-30 (40 sq ft per bag) it is 25 bags, and at R-49 (25 sq ft per bag) it is 40 bags. The higher the target, the faster the bag count climbs.
The US DOE recommends R-38 to R-60 for attics in most climate zones — R-38 in warmer zones, R-49 to R-60 in cold ones. If your attic already has insulation, you usually need only a top-up to reach the target, not the full amount.
About 14 in of loose-fill fiberglass (38 ÷ 2.7 per inch). Cellulose runs a little deeper per R-value and batts are fixed by product thickness — a standard R-38 batt is about 12 in thick.
Yes — that is the whole point of the chart. The same bag spread to a greater depth covers less floor area, so coverage per bag falls as the target R-value rises: 40 sq ft at R-30, 32 at R-38, 25 at R-49 for loose-fill fiberglass.
Yes, that is the normal top-up method — but size it from the gap, not the full target. If 6 in of old fiberglass (about R-16) is already there and you want R-38, you need about R-22 more, not another full R-38.
Blown-in loose-fill covers irregular joist bays, wiring and pipes with no gaps, which is why it dominates attic top-ups. Batts suit open, regular framing and DIY installs. Gaps and compression both cut real-world R-value, so coverage matters more than the label number.