How Much Insulation Do I Need? Attic, Shed and Wall Charts
It depends on where it goes. Attics need the most: R-49 to R-60 for most of the US, which is about 14 to 17 inches of blown cellulose. Walls are limited by the studs: R-13 or R-15 batts for 2 × 4 walls and R-19 or R-21 for 2 × 6. To get the amount, measure the area, pick the R-value, and divide by the coverage on the bag. The insulation calculator does the depth and bag count for any area.
Recommended R-value by location
| Where | R-value | Usual product |
|---|---|---|
| Attic floor, warm climate (zone 1) | R-30 to R-49 | Blown-in or batts |
| Attic floor, most of the US (zones 2 to 8) | R-49 to R-60 | Blown-in, or 2 layers of batts |
| 2 × 4 wall | R-13 to R-15 | Batts, 3 1/2 in |
| 2 × 6 wall | R-19 to R-21 | Batts, 5 1/2 to 6 1/4 in |
| Floor over a crawl space or garage | R-25 to R-30 | Batts |
| Shed walls (2 × 4) | R-13 | Batts or rolls |
Attic levels are ENERGY STAR's recommendations for an uninsulated attic. Your local building code may ask for a specific value on new work.
How much insulation for a shed
Most sheds have 2 × 4 walls and rafters, so R-13 batts are the usual choice. This chart assumes 8 ft walls, takes 20 sq ft off for a door and a window, and adds 10% for cuts. Batts are 15 × 93 inches (9.7 sq ft each) for studs 16 inches apart.
| Shed size | Wall area | Wall batts | Roof area (6/12 pitch) | Roof batts |
|---|---|---|---|---|
| 8 × 10 ft | 268 sq ft | 31 | 89 sq ft | 11 |
| 10 × 12 ft | 332 sq ft | 38 | 134 sq ft | 16 |
| 10 × 16 ft | 396 sq ft | 45 | 179 sq ft | 21 |
| 12 × 16 ft | 428 sq ft | 49 | 215 sq ft | 25 |
| 12 × 20 ft | 492 sq ft | 56 | 268 sq ft | 31 |
| 12 × 24 ft | 556 sq ft | 64 | 322 sq ft | 37 |
If your shed has a flat ceiling instead of insulating the roof, use the floor area for the ceiling. Studs 24 inches apart take 23 inch wide batts, so you'll need about a third fewer pieces.
How much blown-in insulation for an attic
Blown-in insulation settles, and its R-value depends on the final depth. These are typical depths; the bag chart on your product gives the exact depth and how many square feet each bag covers at each R-value.
| Target | Blown cellulose | Blown fiberglass |
|---|---|---|
| R-30 | 8.3 in | 12.0 in |
| R-38 | 10.6 in | 15.2 in |
| R-49 | 13.6 in | 19.6 in |
| R-60 | 16.7 in | 24.0 in |
bags = attic floor area (sq ft) ÷ sq ft per bag at your R-value
For a 1,000 sq ft attic, if the chart says a bag covers 30 sq ft at R-49, you need 1,000 ÷ 30 = 34 bags. Many home stores lend the blowing machine free with a big enough order.
Topping up old insulation
Most older attics already have some insulation. Measure its depth in a few spots and count about R-3 per inch for old fiberglass. Then add the difference. This table shows how much to add to reach R-49:
| Insulation already there | R-value to add | Blown cellulose to add | Blown fiberglass to add |
|---|---|---|---|
| None | R-49 | 13.6 in | 19.6 in |
| 3 in (about R-9) | R-40 | 11.1 in | 16.0 in |
| 4 in (about R-12) | R-37 | 10.3 in | 14.8 in |
| 6 in (about R-18) | R-31 | 8.6 in | 12.4 in |
| 8 in (about R-24) | R-25 | 6.9 in | 10.0 in |
| 10 in (about R-30) | R-19 | 5.3 in | 7.6 in |
Only cover old insulation that is dry and free of mold or pests. If you use batts on top, choose unfaced batts and lay them across the joists.
Worked example: 10 × 12 shed
- Walls: 2 × (10 + 12) × 8 ft = 352 sq ft, minus 20 sq ft for the door and window = 332 sq ft
- Add 10%: 332 × 1.1 = 365 sq ft
- Batts: 365 ÷ 9.7 = 38 R-13 batts
- Roof: 10 × 12 × 1.118 = 134 sq ft, so 16 more batts if you insulate between the rafters
Before you insulate, seal gaps around the door frame, windows and where the walls meet the floor. Cover the batts with plywood, OSB or drywall so they don't get damaged or become a home for mice.
Frequently asked questions
How much insulation do I need for a 10x12 shed?
With 8 ft walls, a 10 × 12 shed has about 332 sq ft of wall after a door and window, which takes about 38 R-13 batts (15 × 93 in) with 10% extra. A 6/12 roof adds about 134 sq ft, or 16 more batts if the rafters are 2 × 4.
How much insulation do I need in my attic?
Most of the US should aim for R-49 to R-60 in the attic, which is about 14 to 17 inches of blown cellulose or 20 to 24 inches of blown fiberglass. South Florida and Hawaii can stop at R-30 to R-49.
How many inches of insulation should be in my attic?
If you can see the tops of the ceiling joists, you almost certainly need more. A typical 2 × 6 joist is 5 1/2 inches deep, and most attics need well over 12 inches of insulation.
What R-value do I need for 2x4 and 2x6 walls?
R-13 or R-15 batts fill a 2 × 4 wall. R-19 or R-21 batts fill a 2 × 6 wall. Squashing a thicker batt into a thinner wall lowers its R-value.
How many batts of insulation do I need?
Divide the wall or ceiling area in square feet by 9.7 (one 15 × 93 inch batt for studs 16 inches apart) and add about 10% for cuts. A bag holds anywhere from 4 to 20 batts depending on the R-value, so check the square feet on the label.