Enter the area to insulate, the R-value you want and the material. The calculator divides the target R-value by the material’s R-value per inch to give the thickness you need, plus the area and board feet for ordering.
Insulation formula
Thickness (in) = target R-value ÷ R-value per inch Board feet = area (sq ft) × thickness (in) Bags or bundles = area ÷ coverage per package at that R-value
R-value per inch by material
| Material | R-value per inch | Inches for R-13 | Inches for R-38 |
|---|---|---|---|
| Fiberglass batt | R-3.5 | 3.7 | 10.9 |
| EPS rigid foam | R-4.0 | 3.3 | 9.5 |
| XPS rigid foam | R-5.0 | 2.6 | 7.6 |
| Open-cell spray foam | R-4.2 | 3.1 | 9 |
| Closed-cell spray foam | R-6.5 | 2 | 5.8 |
These per-inch figures come from Oklahoma State University Extension, which gives polyisocyanurate board as about R-6.2 to 7.2 per inch. Real products vary, so the R-value printed on the package or data sheet always wins.
Worked example: R-38 in a 40 × 30 ft attic
- Area: 40 × 30 = 1,200 sq ft.
- Fiberglass at R-3.5 per inch: 38 ÷ 3.5 = 10.9 in.
- Closed-cell spray foam would need 38 ÷ 6.5 = 5.8 in.
Tips
- Compressing a batt into a shallower cavity lowers its R-value; buy batts sized for the cavity depth.
- Blown-in products list coverage per bag at each R-value; enter that figure to get a bag count.
- Air sealing before insulating makes a large difference to comfort and energy use.
Questions
How thick is R-30 insulation?
About 8.6 in of fiberglass batt, or 4.6 in of closed-cell spray foam.
What R-value do I need?
It depends on your climate zone and local energy code; check with your building department.
Can I add insulation on top of old insulation?
Usually yes in an attic, if the old insulation is dry. Add the existing R-value to the new layer.
Sources
- BAE-1410: The Importance of Residential and Commercial Building Insulation, Oklahoma State University Extension.
Used for: R-value per inch for fiberglass batts, EPS, XPS, polyiso and open- and closed-cell spray foam.