Attic insulation levels are measured by R-Value, which indicates resistance to heat flow. Most attics should be insulated to R-38, or about 10 to 14 inches, depending on the insulation type.
The 3.5 inch thick R-15 insulation offers 36% more R-value compared to the standard R-11 fiberglass insulation. It features a kraft facing that must not be exposed as it can burn. Ensure the facing is in substantial contact with the finishing material and protect it from open flames or other heat sources.
Batt insulation consists of pre-cut, flat pieces that resemble small blankets. This type of insulation is one of the most common forms and is available in various materials, including fiberglass, mineral wool, and sheep's wool.