Attic Insulation R-Value Calculator
Calculate the total R-value of your attic insulation based on insulation type and thickness. R-value measures thermal resistance — the higher the R-value, the better the insulation performance.
Formula
Total R-Value = (R-Value per Inch × Thickness × Layers) + Existing R-Value
- R-Value per Inch — Thermal resistance per inch of the selected insulation material (units: °F·ft²·h/BTU per inch)
- Thickness — Depth of new insulation being installed (inches)
- Layers — Number of separate insulation layers installed
- Existing R-Value — R-value of any insulation already present in the attic
- R-values are additive — stacking layers increases total thermal resistance proportionally
- Additional insulation needed: Deficit R ÷ R-per-inch = extra inches required
Assumptions & References
- R-values per inch are based on ASHRAE Handbook of Fundamentals and DOE/Oak Ridge National Laboratory data
- Climate zone recommendations follow DOE/ENERGY STAR guidelines for unfinished attics (2023)
- Fiberglass Batts: R-3.14/inch; Blown Fiberglass: R-2.2/inch; Blown Cellulose: R-3.7/inch; Mineral Wool: R-3.2/inch
- Open-Cell Spray Foam: R-6.5/inch; Closed-Cell Spray Foam: R-6.9/inch
- Rigid EPS Foam: R-4.3/inch; Rigid XPS Foam: R-5.0/inch; Polyisocyanurate: R-6.2/inch
- Energy savings estimate is approximate — actual savings depend on climate, HVAC efficiency, air sealing, and home construction
- R-values assume standard installation conditions; compression, moisture, or improper installation can reduce effective R-value
- This calculator does not account for thermal bridging through joists or rafters
- Reference: DOE Insulation Fact Sheet (2023), IECC Climate Zone Map, ENERGY STAR Recommended Levels of Insulation