How to use the insulation calculator
- Enter the R-value you have now. Measure the depth of existing insulation and multiply by its R-value per inch, or use 0 for an uninsulated space.
- Enter the target R-value (see the ENERGY STAR table below).
- Enter the R-value per inch of the product you will install, from the bag or batt label.
- Optionally enter the area to see the volume of insulation.
Check the R-value already in place before you buy, because you only need to add the difference; Decoratoradvice takes your current level into account.
Insulation formula
R-value to add = target R − current R
Thickness = R-value to add ÷ R-value per inch
Volume = area × thickness
Worked example
An attic with R-11 now, a target of R-49, and loose-fill rated R-3.2 per inch:
R to add: 49 − 11 = 38
Thickness: 38 ÷ 3.2 = 11.9 in
For 1,000 sq ft: 1,000 × 11.875 ÷ 12 = 990 cu ft
Recommended attic R-values
ENERGY STAR's recommendations, based on the 2021 International Energy Conservation Code:
| Climate zone | Attic, uninsulated | Attic with 3–4 in existing | Floor |
|---|---|---|---|
| 1 | R30 | R25 | R13 |
| 2 | R49 | R38 | R13 |
| 3 | R49 | R38 | R19 |
| 4A and 4B | R60 | R49 | R19 |
| 4C, 5 and 6 | R60 | R49 | R30 |
| 7 and 8 | R60 | R49 | R38 |
The "existing 3–4 in" column is the R-value to add on top of what is there.
Walls
Wall cavities limit thickness. The U.S. Department of Energy notes that 2 × 4 walls can hold R-13 or R-15 batts and 2 × 6 walls R-19 or R-21 products. Adding more R-value to walls usually means insulated sheathing outside the studs. The stud calculator covers framing.
Tips
- Seal air leaks into the attic before adding insulation.
- Keep insulation clear of soffit vents with baffles so the attic still breathes; the attic ventilation calculator works out the vent area.
- Loose-fill bags list coverage by R-value; use that chart to count bags once you know the depth.
- See how long the upgrade takes to pay back with the energy upgrade payback calculator.
Common mistakes
- Using a generic R-per-inch figure. Products differ; use the one on your label.
- Compressing batts. Squashing insulation into a smaller space lowers its R-value.
- Covering vents or recessed lights not rated for contact with insulation.
Frequently asked questions
How many inches of insulation do I need for R-49?
It depends on the product. At R-3.2 per inch, about 15.3 in from zero; check your product's label.
What R-value should my attic have?
ENERGY STAR recommends R30 to R60 for uninsulated attics depending on climate zone; see the table above.
What is R-value per inch?
How much thermal resistance each inch of the product provides. Manufacturers must provide R-value information under the FTC's R-value Rule.
How do I convert metric RSI to R-value?
Multiply RSI by 5.678. RSI 8.6 is about R-49.
Does insulation help my air conditioner?
Less heat gain means a smaller cooling load. Size room units with the BTU calculator and ducts with the duct size calculator.
Key takeaways
- Thickness = (target R − current R) ÷ R per inch.
- Use the R-value per inch from your product's label.
- ENERGY STAR recommends attic levels from R30 to R60 by climate zone.
- Seal air leaks first and keep soffit vents clear.