Our Insulation R-Value Calculator works out how thick your insulation needs to be to hit a target R-value, based on the material type — and how much total volume you’ll need to cover your area.
Insulation R-Value Calculator
Insulation R-Value Formula
Thickness (in) = Target R-Value ÷ R-Value per Inch
For layered insulation (e.g. batt over existing attic insulation), total R-value is simply the sum of each layer’s R-value: R-total = R₁ + R₂ + R₃ …
Recommended Attic R-Value by Climate Zone (US)
| Climate Zone | Recommended Attic R-Value |
|---|---|
| Zone 1-2 (Hot — FL, South TX) | R-30 to R-49 |
| Zone 3-4 (Mixed — GA, VA, TN) | R-38 to R-49 |
| Zone 5-6 (Cold — NY, IL, OH) | R-49 to R-60 |
| Zone 7-8 (Very Cold — MN, AK) | R-60 |
R-Value per Inch by Material
- Fiberglass batt: R-3.0 to R-3.4 per inch.
- Blown cellulose: R-3.6 to R-3.8 per inch.
- Spray foam (open-cell): R-3.5 to R-3.8 per inch.
- Spray foam (closed-cell): R-6.0 to R-7.0 per inch — highest R-value per inch of common materials.
- Rigid foam board (XPS): R-4.5 to R-5.0 per inch.
FAQs
What R-value do I need for my attic?
Most US climate zones recommend R-38 to R-49 for attics, with colder northern zones needing up to R-60. Check your local energy code or use the table above as a guide.
Can I just add more insulation on top of old insulation?
Yes — adding a new layer on top of existing attic insulation is common and effective. Total R-value is the sum of both layers, as long as the old insulation isn’t compressed, wet or moldy.
Which insulation gives the highest R-value per inch?
Closed-cell spray foam offers the highest R-value per inch (R-6 to R-7), making it ideal for tight spaces where you can’t add much thickness.
Does compressing fiberglass batt reduce its R-value?
Yes — compressing batt insulation into a smaller space than it’s rated for significantly reduces its effective R-value, since the trapped air pockets that provide insulation get squeezed out.