How it works
The formula
walls = wall length × height − 20 per door − 15 per window
batts = area × 1.05, in sq ft, rounded up
blown-in bags = area ÷ sq ft per bag at your R-value, rounded up
Assumptions
- The area is the gross area: the full attic floor or wall, with no deduction for joists or studs. That is the simple, common way to estimate, and the 5% allowance on batts covers cutting.
- Batts fill the cavities between joists or studs. They are priced per square foot here, but they are sold in bags that print the square feet each covers. Check the bag's coverage and buy enough bags to reach the total.
- Blown-in cellulose coverage per bag changes with the R-value. We use the maximum from GreenFiber's SANCTUARY attic chart for a 25 lb bag: 25.9 sq ft per bag at R-30 and 19 sq ft at R-38. Other brands and bag sizes differ, so the chart on your bag is the one to trust; never spread a bag further than its chart says, or you won't reach the R-value.
- A door takes off 20 sq ft and a window 15 sq ft, the same as the paint calculator. Leave them at 0 to insulate the full wall.
- Metric: type meters. Sizes convert to feet for the math, and products are still sold by US square foot and by the bag.
Worked example
A 50 × 30 ft attic floor, R-38 blown-in cellulose
- Area: 50 × 30 = 1,500 sq ft
- Coverage at R-38: at most 19 sq ft per 25 lb bag
- Bags: 1,500 ÷ 19 = 78.94736842105263, so buy 79 bags
- At $15.13 a bag (the middle of our typical range): $1,194.88
Which R-value
Attic R-values by climate zone
The U.S. Department of Energy (energy.gov, Insulation guide) suggests these R-values for an uninsulated attic. Across the country that is R30 to R60. Look up your zone on the map in that guide.
| Climate zone | Suggested attic R-value |
|---|---|
| Zone 1 (southern tip of Florida, Hawaii) | R30 to R49 |
| Zones 2 and 3 (the South) | R38 to R60 |
| Zones 4 to 8 (the middle of the country and north) | R49 to R60 |
This calculator offers R-30 and R-38, the two common levels. If your zone calls for more, add a second layer or ask your store for higher R-value bags or batts.
How deep to blow cellulose for each R-value
Blow to the installed depth; loose cellulose settles to the settled depth over the first months, which is where the R-value is reached. You need both the depth and the bag count: a thin layer of the right weight, or a fluffy one with too few bags, falls short.
| R-value | Blow to | Settles to | Sq ft per bag | Bags per 1,000 sq ft |
|---|---|---|---|---|
| R-13 | 3.9 in | 3.6 in | 65.8 | 15.2 |
| R-19 | 5.7 in | 5.3 in | 44.2 | 22.6 |
| R-22 | 6.6 in | 6.1 in | 38 | 26.3 |
| R-30 | 8.9 in | 8.3 in | 25.9 | 38.6 |
| R-38 | 11.2 in | 10.4 in | 19 | 52.6 |
| R-44 | 12.9 in | 12 in | 15.7 | 63.5 |
| R-49 | 14.3 in | 13.3 in | 14.1 | 70.8 |
| R-60 | 17.3 in | 16.1 in | 11.6 | 86 |
From GreenFiber's SANCTUARY coverage chart (25 lb bags, loose-fill attic). Other brands differ a little: blow to the chart printed on your bags. Energy.gov suggests R-49 to R-60 for attics in most of the country (zones 4 to 8).
Common attic sizes
| Attic floor | R-30 blown (bags) | R-38 blown (bags) | Batts (sq ft) |
|---|---|---|---|
| 1,000 sq ft | 39 | 53 | 1,050 |
| 1,500 sq ft | 58 | 79 | 1,575 |
| 2,000 sq ft | 78 | 106 | 2,100 |
Blown-in uses at most 25.9 sq ft per 25 lb bag at R-30 and 19 at R-38 (GreenFiber's chart; check your bag). Batts include 5% waste and are the same square feet at R-30 or R-38; only the price differs.
Walls of a 12 × 12 room
With 8 ft walls, 1 door and 1 window the wall area is 349 sq ft, so buy about 367 sq ft of batts. At typical prices that is $58.72 to $293.60 for R-13 or $146.80 to $367 for R-19.
Questions people ask
How much insulation do I need for my attic?
Enough to cover the whole attic floor to the R-value you choose. Multiply length by width: a 50 × 30 ft attic is 1,500 sq ft. That takes 79 bags of blown-in cellulose at R-38, or 1,575 sq ft of batts including 5% waste.
What R-value do I need?
It depends on your climate zone. Energy.gov suggests R30 to R60 for an uninsulated attic, from R30 to R49 in the warmest zone up to R49 to R60 in the north. Walls are usually R-13 in a 2×4 wall and R-19 in a 2×6 wall. Local building codes can ask for more, so check yours.
How many bags of blown-in insulation do I need?
Divide the area by the square feet one bag covers at your R-value. At most 19 sq ft per 25 lb bag for R-38, 1,000 sq ft takes 53 bags, 1,500 sq ft takes 79 and 2,000 sq ft takes 106. Coverage differs by brand, so use the chart on the bag.
How many inches of blown-in insulation for R-38?
About 11.2 in of cellulose as you blow it, settling to 10.4 in. R-30 is 8.9 in, R-49 14.3 in and R-60 17.3 in (GreenFiber's chart; check yours). So 8 in of settled cellulose is a little under R-30.
Batts or blown-in: which is better?
Batts are easy to fit between joists and need no machine, but gaps and cuts around wiring and pipes matter. Blown-in cellulose fills around obstacles and uneven spots, and many stores lend a blowing machine with a purchase of bags. For a plain, open attic floor either works; for walls that are already closed, batts are not an option. Prices differ, so type yours above and compare.
Can I add insulation over old insulation?
Usually yes, if the old insulation is dry and not moldy or damaged by pests. Lay new batts across the joists, not along them, and use unfaced batts so you do not trap moisture between layers. Blown-in can go right over old loose fill or batts. Keep insulation clear of recessed lights unless they are rated for contact, and leave soffit vents open. If you are unsure, ask a local insulation contractor.