How Much Cellulose Insulation For a Wall?

Requirements Vary

• The amount of insulation required will vary depending on the geographic location and climate. For most of the country, the recommended R-value for wall insulation is R13 to R15, which is roughly the middle of the R-value scale. Cold northern areas may require R21. A map on the North American Insulation Manufacturers Association website shows variations by region (see Resources).

Fill Standard Cavities

• Cellulose insulation is made from old newspapers that are shredded, treated with a fire-resistant chemical and formed into granules. Its R-value ranges from 3.2 to 3.8 per inch, varying with the specific manufacturer, but typically is about 3.7. Standard 2-by-4-inch framed wall cavities are 3 1/2 inches deep, which means cellulose insulation must completely fill the wall to meet the minimum standard.

Thicker Walls

• Walls in some extremely cold regions are framed with 2-by-6-inch studs, which creates a cavity 5 1/2 inches deep. R-value requirements in these areas range up to R21, which means this wall also needs to be filled completely with cellulose.

Obstructions

• Obstructions in wall cavities can create problems, although one advantage of cellulose is that it can filter into small nooks and crannies where more rigid material cannot fit. Walls, however, sometimes have internal braces and other obstructions. In new construction, these can be identified before insulation, which typically is blown into the wall after exterior sheathing and interior wallboard are installed.

Existing Walls

• Using cellulose to add insulation to an existing wall requires opening the wall. Holes are drilled either through the exterior sheathing or through the interior drywall. Holes are placed at the top of each cavity between studs and cellulose is blown in until the cavity is filled to the top. The holes are then repaired.