How to use the insulation calculator
- Measure the attic floor (length × width) or the wall (length × height). Batts are sold by the area they cover, so don't subtract the framing.
- Choose batts or blown-in insulation, and the R-value you want to reach.
- If the attic already has insulation, enter its R-value. Measure its depth and multiply by about 3 for fiberglass or 3.5 for cellulose.
- For a bag count, enter the coverage per bag from the package chart. Blown-in bags list a different coverage for each R-value.
Insulation formulas
R-value to add = target R − existing R
depth (in) = R to add ÷ R per inch
bags = area (sq ft) ÷ coverage per bag
Standard batts come in fixed thicknesses for each R-value (R-13 is 3.5 in, R-19 is 6.25 in). For other cases the calculator uses a typical R per inch: fiberglass batts about 3.2, blown cellulose about 3.6 (settled), blown fiberglass about 2.5. Products vary, so the depth on the bag chart is the one to follow.
Recommended R-values for attics
ENERGY STAR recommends these levels for an uninsulated attic, by US climate zone:
| Climate zone | Example areas | Attic R-value |
|---|---|---|
| 1 | South Florida, Hawaii | R-30 to R-49 |
| 2–3 | Gulf Coast, Texas, Southern California, Georgia | R-49 to R-60 |
| 4–8 | Mid-Atlantic, Midwest, Pacific Northwest, Northeast, Alaska | R-60 |
Walls are limited by the stud depth: R-13 or R-15 fits a 2×4 wall, R-19 or R-21 fits a 2×6 wall.
Approximate depth for each R-value
| R-value | Fiberglass batts | Blown cellulose | Blown fiberglass |
|---|---|---|---|
| R-13 | 3.5 in | 3.6 in | 5.2 in |
| R-19 | 6.25 in | 5.3 in | 7.6 in |
| R-30 | 9.5–10 in | 8.3 in | 12.0 in |
| R-38 | 12 in | 10.6 in | 15.2 in |
| R-49 | 14–16 in (often 2 layers) | 13.6 in | 19.6 in |
| R-60 | about 19 in (2 layers) | 16.7 in | 24.0 in |
Tips
- Seal air leaks around light fixtures, pipes and the attic hatch before adding insulation. Air leaks undo a lot of the benefit.
- Keep insulation away from recessed lights that aren't IC-rated, chimneys and flues, and keep soffit vents clear with baffles.
- When adding batts over old insulation, use unfaced batts laid across the joists.
- Many home stores lend the blowing machine free when you buy enough bags of blown-in insulation.
Frequently asked questions
How thick is R-38 insulation?
About 12 inches of fiberglass batts, about 10.5 inches of blown cellulose after it settles, or about 15 inches of blown fiberglass.
How many bags of blown-in insulation do I need?
Divide the attic area by the coverage per bag shown on the bag chart for your target R-value. For example, 1,000 sq ft at 40 sq ft per bag takes 25 bags.
Can I put new insulation on top of old insulation?
Yes, if the old insulation is dry and free of mold or pests. Use unfaced batts or blown-in insulation, and subtract the existing R-value from your target.
How much insulation do I need for 1,000 square feet?
For an attic at R-38 that is about 12 inches of fiberglass batts or about 10.5 inches of blown cellulose over the whole 1,000 sq ft. At around 40 sq ft per bag that is 25 bags, but use the coverage chart on your bag.
Is batt or blown-in insulation better for an attic?
Blown-in usually does a better job on attic floors because it fills gaps around joists and wiring. Batts are easier in walls and open joist bays.