How the insulation calculator works
Insulation is rated by R-value, and each material delivers a fairly consistent R-value per inch. To reach a target, subtract whatever R-value you already have and divide the rest by the material's R-value per inch. That gives the depth to add.
R-value to add = target R − existing R
Depth to add (in) = R-value to add ÷ R-value per inch
Bags or rolls = area ÷ coverage per bag at your target
Example: a 40 × 30 ft attic with about 3.5 inches of old fiberglass (roughly R-11) and a target of R-38 needs R-27 more. With blown fiberglass at about R-2.5 per inch, that's about 10.8 inches of new insulation over 1,200 sq ft.
R-value per inch by material
Typical values. Actual R-value depends on the product and installation, so use the manufacturer's numbers when you have them.
| Material | Approx. R per inch | Inches for R-38 |
|---|---|---|
| Blown-in fiberglass | 2.2–2.7 | about 15 |
| Blown-in cellulose | 3.2–3.8 | about 11 |
| Fiberglass batts | 2.9–3.8 | about 12 |
| Mineral wool batts | 3.0–3.3 | about 11.5 |
| Open-cell spray foam | 3.5–3.7 | about 10.5 |
| Closed-cell spray foam | 6.0–7.0 | about 6 |
Recommended R-values
For attics, US recommendations range from about R-30 in the warmest climates to R-49 or R-60 in the coldest. Walls are limited by stud depth: 2 × 4 walls typically hold R-13 to R-15, and 2 × 6 walls R-19 to R-21. Floors over unheated spaces commonly get R-19 to R-30. Check your local energy code or the ENERGY STAR climate zone map for your exact area.
Before you insulate
- Seal air leaks around attic hatches, light fixtures, plumbing and wiring holes first. Air sealing often saves as much as the insulation.
- Keep insulation clear of recessed lights not rated for contact, chimneys and soffit vents. Use baffles to keep eave vents open.
- Wear a mask, goggles, gloves and long sleeves when handling fiberglass or blown insulation.
Frequently asked questions
How much attic insulation do I need?
It depends on your climate. In most of the US, current recommendations for attics fall between about R-30 in the warmest areas and R-60 in the coldest. Check the ENERGY STAR or Department of Energy climate zone map for your area.
How thick is R-38 insulation?
It depends on the material. Blown fiberglass at about R-2.5 per inch needs roughly 15 inches, fiberglass batts at about R-3.2 per inch need about 12 inches, and blown cellulose at about R-3.5 per inch needs about 11 inches.
Can I add new insulation on top of old?
Usually, yes, as long as the old insulation is dry, free of mold and not compressed. Unfaced batts or loose fill can go over existing material. Don't put a second vapor barrier, such as faced batts, on top of existing insulation.
What does R-value mean?
R-value measures how well a material resists heat flow. Higher numbers insulate better. R-values add up, so R-19 of existing insulation plus R-19 added gives about R-38.
How many bags of blown-in insulation do I need?
Each brand prints a coverage chart on the bag showing square feet per bag at each R-value. Enter that coverage figure for your target R-value, and the calculator divides your area by it.