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.
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.