A medium-density batt offers R-13 for the same thickness. High-density fiberglass batts for a 2 by 4-inch (51 by 102 millimeter ) stud-framed wall has an R-15 value, compared to R-11 for 'low density' types.
The denser products are intended for insulating areas with limited cavity space, such as cathedral ceilings. Manufacturers now produce medium- and high-density fiberglass batt insulation products that have slightly higher R-values than the standard batts. It's commonly used in many different forms of insulation: blanket (batts and rolls), loose-fill, and is also available as rigid boards and duct insulation. Fiberglass consists of extremely fine glass fibers and is one of the most ubiquitous insulation materials.
