According to the DOE, 8 inches of high-density batt insulation will provide around an R-30. In cooler climates, or in cases where you have sufficient space for more insulation material, you may opt ...

use batt insulation with higher R-value per inch in combination with rigid foam insulation. Rigid foam sheeting has an R-value ranging from R-5 to R-10 per inch of thickness. This means you can ...