Insulation R-Values
Thermal Resistance
Learn R-ValuesR-value measures thermal resistance—how well a material resists heat flow. Higher R-values mean better insulation. Unlike thermal conductivity (which measures how easily heat flows), R-value measures how well heat is blocked.
Insulation R-Values per Inch
| Material | R-value/inch |
|---|---|
| Fiberglass batts | R-2.9 to R-3.8 |
| Cellulose (blown) | R-3.2 to R-3.8 |
| Spray foam (closed) | R-6.0 to R-7.0 |
| Spray foam (open) | R-3.5 to R-4.0 |
| Rigid foam (XPS) | R-5.0 |
| Mineral wool | R-3.0 to R-3.3 |
Conclusion
R-values guide insulation choices—attics need R-38 to R-60 in cold climates, walls R-13 to R-21. Total R-value is additive, so multiple layers add up. Higher-performing spray foams achieve the same R-value with less thickness than traditional fiberglass.