Insulation R-Value Calculator
Building codes specify minimum R-values by climate zone and assembly type. This calculator determines the insulation thickness needed to meet code based on your material choice and the IECC (International Energy Conservation Code) requirements.
Project Parameters
Based on 2021 IECC residential prescriptive requirements. Local codes may differ. Continuous insulation requirements (ci) are in addition to cavity insulation for some assemblies.
IECC R-Value Requirements by Climate Zone
| Zone | Attic | Wall | Floor | Basement |
|---|---|---|---|---|
| 1 | R-30 | R-13 | R-13 | R-0 |
| 2 | R-38 | R-13 | R-13 | R-0 |
| 3 | R-38 | R-20 | R-19 | R-5/13 |
| 4 | R-49 | R-20 | R-19 | R-10/13 |
| 5 | R-49 | R-20+5ci | R-30 | R-15/19 |
| 6 | R-49 | R-20+5ci | R-30 | R-15/19 |
| 7 | R-49 | R-20+5ci | R-38 | R-15/19 |
| 8 | R-49 | R-20+5ci | R-38 | R-15/19 |
Understanding R-Value
R-value measures thermal resistance — how well a material resists heat flow. Higher R-values mean better insulation. The total R-value of an assembly is the sum of all layers:
Rtotal = Rmaterial1 + Rmaterial2 + Rair films
Thickness needed = Required R-value ÷ R-value per inch of material
Frequently Asked Questions
Can I just add insulation on top of existing insulation?
Yes, for attics. You can add unfaced batts or blown insulation over existing insulation to reach the target R-value. Do not add a second vapor barrier — only the bottom layer (closest to living space) should have facing.
What is continuous insulation (ci)?
Continuous insulation is rigid foam installed over the exterior of framing, covering studs and reducing thermal bridging. In climate zones 5–8, walls require cavity insulation plus continuous exterior insulation to meet code.
References & Methodology
- U.S. Department of Energy (DOE), "Insulation: R-Value Recommendations by ZIP Code," energy.gov. — Climate-zone-based R-value recommendations for attic, wall, and floor insulation.
- International Energy Conservation Code (IECC), Table R402.1.2, "Insulation and Fenestration Requirements by Component," 2021. — Minimum R-values for residential building envelopes by climate zone.
- Oak Ridge National Laboratory (ORNL), "Insulation Fact Sheet," ornl.gov. — R-value per inch for common insulation materials (fiberglass, cellulose, spray foam, mineral wool).
References
- ASHRAE Climate Zone Map — U.S. Department of Energy Building America Program
- 28 CFR Part 35 — Nondiscrimination on the Basis of Disability in State and Local Government Services
- Advisory Council on Historic Preservation — Section 106 of the National Historic Preservation Act
- ADA Standards for Accessible Design — U.S. Department of Justice
- 2010 ADA Standards for Accessible Design — U.S. Department of Justice
- 21 CFR Part 177 — Indirect Food Additives: Polymers, U.S. FDA / Electronic Code of Federal Regulatio
- International Residential Code (IRC)
- Center for Universal Design, NC State University — 7 Principles of Universal Design