Insulation Calculator
Free attic insulation calculator. Enter attic size, current and target R-value to get the depth and number of bags of blown-in fiberglass, cellulose or batts.
Roughly R-3 per inch of old insulation; 0 if none
- Bags
- 25
- Depth to add
- 15.2 in
- R-value to add
- R-38
- ends at R-49
- Area
- 1,000 sq ft
Blown-in details
| Material weight | 697 lb |
| R-value per inch | R-2.5 |
The bag's coverage chart is the final word
Price
$1,125
Insulation only. Many stores lend the blowing machine free with a bag purchase.
Default prices are typical U.S. retail figures, reviewed November 2026. Enter your supplier's prices for an exact total.
Insulation is chosen by R-value, the resistance to heat flow, and bought by the bag. How many bags you need depends on your attic area, how much insulation is already there and the R-value you want to reach.
This calculator works out the R-value to add, the depth that takes for blown-in fiberglass, blown-in cellulose or batts, and the number of bags. Bag weight, density and coverage differ by brand, so they are editable; the chart on your bag is the final word.
How to use the insulation calculator
- Measure the attic floor: length × width, or the ceiling area of the rooms below.
- Measure the depth of any existing insulation and multiply by about 3 for its R-value.
- Pick a target R-value for your climate (see the table below).
- Subtract the existing R-value from the target, then divide by the R-value per inch for depth.
- Convert depth to bags using the bag's coverage chart or weight.
Depth (in) = (target R − existing R) ÷ R per inch. Bags (blown-in) = area × depth ÷ 12 × settled density ÷ bag weight.
Recommended attic R-values
ENERGY STAR recommendations for an uninsulated attic, by IECC climate zone, with the depth that takes.
| Climate zone | Target | Blown fiberglass | Blown cellulose |
|---|---|---|---|
| 1 (South Florida, Hawaii) | R-30 to R-49 | 12–20 in | 9–14 in |
| 2–3 (Gulf Coast, South) | R-49 to R-60 | 20–24 in | 14–17 in |
| 4–8 (Mid-Atlantic and north) | R-60 | 24 in | 17 in |
Tips before you buy
- Air seal the attic floor before insulating; it often saves as much energy as the insulation itself.
- Keep insulation at least 3 inches away from recessed lights unless they are IC-rated.
- Install baffles at the eaves so insulation doesn't block soffit vents.
- Mark the target depth on rulers stapled to joists so you can see when you've blown enough.
Frequently asked questions
How many bags of blown-in insulation do I need?
To bring 1,000 square feet of empty attic to R-38, plan on roughly 25 bags of blown fiberglass or 55 bags of cellulose. Bag sizes differ, so check the coverage chart on the bag you buy.
How thick is R-38 or R-49 insulation?
Blown fiberglass gives about R-2.5 per inch, so R-38 is about 15 inches and R-49 about 20 inches. Cellulose gives about R-3.5 per inch: roughly 11 inches for R-38 and 14 inches for R-49.
Can I put new insulation over old insulation?
Yes, if the old insulation is dry and free of mold or pests. Blown-in or unfaced batts can go right on top. Don't add a second vapor barrier.
Is blown-in or batt insulation better for an attic?
Blown-in fills gaps around joists, wiring and framing better and is faster for large areas. Batts are easier for a small or open attic you can stand in.