What Is the R-Value for 4 Inch Batt Insulation?

Fiberglass blankets are the preferred insulation for walls in new houses. They also are used in attics and other areas. They are easy to install and effective, relatively inexpensive and fire-resistant, although the batts will melt in high heat. Fiberglass insulation comes in two blanket styles, rolls and batts. The difference is that batts are pre-cut to specific construction dimensions, such as wall heights. The insulation is the same.

Faced and Unfaced

Fiberglass batts come in two styles, faced and unfaced. Faced has a moisture barrier on one side, which typically has flaps on both sides of the batt for easy installation. A batt is placed into a wall cavity, and the flaps are stapled to the studs on both sides. Wall batts are nominally 16 inches wide and nominally 4 inches thick to fit the wall cavity; the actual thickness conforms to the 3 1/2-inch thickness and 15-inch inside dimension of stud framing.

Heat Resistance

Insulation effectiveness is measured in R-value, its capacity to resist heat. Standard fiberglass has an R value of 3.0 to 3.2 per inch. That would give a nominal 4-inch wall batt an overall R value between 10 and 12. Actual R values, however, will vary by manufacturer. It will even vary within manufacturers by type of product. One manufacturer's product line has batts with R values varying from 11 to 15.

High Density

High density (HD) fiberglass batts have a higher R value, from 3.6 to 4 per inch. A typical high-density fiberglass batt will have an R-value of 15 for a 3 1/2 inch thickness. The actual R-value of high-density batts will vary to as little as about 12.6, again depending on the manufacturer and the specific brand. Many manufacturers produce more than one type of batt.


Facing can affect R-value. Facing is typically kraft paper, and batts are labeled on the face by R-value. Unfaced insulation has no outer covering to block heat or moisture; manufacturers typically provide a code to indicate the R value, but these codes vary by maker. Some faced batts are covered with a foil barrier, which not only blocks moisture but also adds to the heat resistance.

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