Insulation R-Value per Inch Reference
| Insulation Type | R-Value per Inch | Common Use |
|---|---|---|
| Fiberglass Batt | R-3.2 | Walls, attics |
| Cellulose (blown) | R-3.5 | Attics, walls |
| Spray Foam - Open Cell | R-3.7 | Walls, rooflines |
| Spray Foam - Closed Cell | R-6.5 | Walls, crawlspaces, moisture barrier |
| Rigid Foam XPS | R-5.0 | Foundation, exterior sheathing |
| Rigid Foam Polyiso | R-6.0 | Roof decking, exterior walls |
| Mineral Wool | R-3.3 | Walls, fire-rated assemblies |
About the R-Value Calculator
Polyiso Cold-Weather Note: Polyisocyanurate (polyiso) is rated at R-6.0/inch at 75°F, but performance drops to approximately R-5.0/inch in cold conditions (below 40°F). Use R-5.0/inch for cold-climate exterior applications.
Code Minimums by Climate Zone (IECC): Zone 1-2: Walls R-13, Attic R-30 | Zone 3: Walls R-20, Attic R-38 | Zone 4: Walls R-20, Attic R-49 | Zone 5-8: Walls R-20+5ci, Attic R-49-60. Check local energy code for exact requirements.
R-value measures an insulation material's resistance to heat flow. The higher the R-value, the better the insulation performs. This calculator allows you to convert between insulation thickness and R-value for seven common insulation types, making it easy to compare materials and determine what thickness you need to achieve your target R-value.
Different insulation materials achieve different R-values per inch of thickness. Closed-cell spray foam offers the highest R-value per inch at R-6.5, making it ideal for spaces where thickness is limited. Fiberglass batts and mineral wool are more economical but require greater thickness to achieve the same R-value. The choice of insulation depends on your budget, available space, moisture conditions, and fire safety requirements.
Building codes specify minimum R-values based on climate zone and building component. Walls in most U.S. climate zones require R-13 to R-21, while attics typically need R-38 to R-60. Always check your local building codes for specific requirements, and consider that real-world performance may differ from rated values due to installation quality, thermal bridging through framing, and compression of insulation materials.