How Much Insulation Is in 2x4 Wall?

3 1/2-Inch Walls

• Most houses are framed with 2- by 4-inch vertical studs placed with centers 16 inches apart. Because 2 by 4s are actually 1 1/2 by 3 1/2 inches, that creates an interior cavity 3 1/2 inches deep by approximately 16 inches wide. The most common insulation for these interior spaces is fiberglass batts, but loose fill cellulose is used sometimes. Both styles fill the wall cavity completely.

Fiberglass Batts

• Insulation is rated by heat resistance or R value. Fiberglass has an R value of about 3.2 per inch, depending on the manufacturer, so a 3 1/2-inch thick fiberglass batt would have a total R value of between 11 and 12. High density fiberglass value is 3.6 to 4 or 12 to 14 for a 3 1/2-inch batt. Most fiberglass insulation batts are rated R 13.

Loose Fill Celllulose

• Loose fill cellulose has an R value of about 3.5 per inch, between 12 and 13 for a 3 1/2-inch wall, but is not used as often because it is harder to install. It is blown into wall cavities with a high-pressure air hose, which requires special equipment. Batts are faced with kraft paper with lips on either side which are stapled to the wall studs.

Vapor Barrier

• Any wall cavity insulation requires a moisture or vapor barrier on the inside. Batts have this with kraft paper. Loose fill cellulose must be covered with a waterproof membrane. Both types must be covered by interior drywall, which adds .45 R value at 1/2-inch thick. Seams of fiberglass batts are sealed with a special tape to eliminate heat loss through small gaps at the stud edges.

Spray Foam

• Another type of wall insulation, used mainly in adding insulation to older houses, is sprayed polyurethane foam. This requires special equipment and professional installation. Holes about 2 inches in diameter are drilled into the siding or sheathing at every wall cavity and foam is sprayed into the interior. It expands to fill the cavity completely. It generally has an R value of about 3.5, so a 3 1/2-inch cavity would have an R value between 12 and 13.

Rigid Foam Addition

• Rigid foam board outside wall cavities can add significantly to the insulation. The basic types are two forms of polystyrene, with R values of about 4 to 5 per inch, so adding an inch outside would raise the total R value for the 2 by 4 wall to about R 17 to 18.