The International Energy Conservation Code Council (IECC) specifies both the R-value and minimum distance for the insulation from the top of the slab downward based on a locality’s Heating Degree Days (HDDs). Consult your local weather bureau for your area’s actual Heating Degree Days. Then use the table below to find the IECC’s recommended depth and R-value based on your Heating Degree Days.
| Heating Degree Days | Feet Installed Vertically | R-Value |
|---|---|---|
| 0 to 2,499 | none required | none required |
| 2,499 to 4,500 | 2 feet | R-4 |
| 4,500 to 6,000 | 4 feet | R-5 |
| 6,000 to 7,200 | 4 feet | R-6 |
| 7,200 to 8,700 | 4 feet | R-7 |
| 8,700 to 10,000 | 4 feet | R-8 |
| 10,000 to 12,400 | 4 feet | R-9 |
| 12,400 to 14,000 | 4 feet | R-10 |
Select and install insulation using these recommendations. Be sure to use only insulation approved for below-grade use.
Source: EERE, U.S. Department of Energy