What R-value tells you
R-value measures how well a layer resists heat flow. In the US it is expressed in ft²·°F·h/BTU, and every product lists it on the label. Layers add up: old insulation plus a new layer on top gives the sum of both R-values. This calculator finds your total R-value, the thickness needed to reach a target, and how much heat loss you avoid.
The formulas
- R-value of the new layer = R per inch × thickness in inches.
- Total R-value = existing R + new R.
- Thickness needed = (target − existing R) ÷ R per inch.
- Heat flow = area × temperature difference ÷ total R, in BTU per hour. The U-factor is 1 ÷ R.
Worked example: topping up an attic
An attic floor of 1,000 sq ft has about 3 in of old fiberglass, worth roughly R-11. You add 12 in of blown cellulose at R-3.5 per inch.
- New layer: 3.5 × 12 = R-42.
- Total: 11 + 42 = R-53, above the R-49 target.
- Thickness needed for R-49: (49 − 11) ÷ 3.5 = 10.9 in.
- Heat flow with a 40 °F difference: 1,000 × 40 ÷ 53 = 755 BTU/h, against 3,636 BTU/h before: a cut of 79%.
R-value by thickness added
| Cellulose added | Total R | Heat flow | Cut |
|---|---|---|---|
| 3.5 in | R-23.3 | 1,720 BTU/h | 53% |
| 6 in | R-32 | 1,250 BTU/h | 66% |
| 10 in | R-46 | 870 BTU/h | 76% |
| 12 in | R-53 | 755 BTU/h | 79% |
| 16 in | R-67 | 597 BTU/h | 84% |
The first inches give the largest gain. Each extra inch helps less, which is why recommended levels stop around R-49 to R-60.
Typical R-values per inch
Blown fiberglass is about R-2.5 per inch, fiberglass batts R-3.2, blown cellulose R-3.5, mineral wool R-4, EPS foam board R-3.8, XPS R-5, polyiso R-6 and closed-cell spray foam about R-6.5. Use the figure on your product, since it varies by brand and density.
Recommended levels
ENERGY STAR guidance for attics ranges from R-30 in the warmest parts of the country to R-60 in the coldest, with R-49 to R-60 for much of the US. 2×4 walls usually hold R-13 to R-15 batts and 2×6 walls R-19 to R-21. Floors over unheated crawlspaces call for R-25 to R-30. Your local energy code may set higher minimums.
Before adding insulation
Seal air leaks first: gaps around chimneys, light fixtures, plumbing stacks and the attic hatch let warm air bypass the insulation. Keep soffit vents clear with baffles, and never cover recessed lights that are not rated for insulation contact. Federal tax credits and utility rebates may cover part of the cost of qualifying insulation and air sealing.