Photo by: Charles Bickford
R-2 to R-4.2 per in.
COST: About 60¢ to $1.20 per sq. ft., depending on installation and density
APPLICATION: Above-grade walls, ceilings, and vented roof assemblies
Blown-in fiberglass consists of chopped glass fibers (and in some cases, an adhesive). The R-value of blown fiberglass increases with density and ranges from R-2.2 to R-4.2 per in. High densities also help to lower air-infiltration rates.
Like cellulose, fiberglass can be blown into a closed cavity through drilled holes, installed behind air-permeable netting stapled to studs, or blown into open framing cavities, if the product includes adhesive. Among brands of blown-in fiberglass that include an adhesive, perhaps the best known is Johns Manville’s Spider. Spider fibers are fine and short, and special spray equipment adds a polyester-based liquid adhesive during installation. Dry Spider fibers (without adhesive) also can be installed behind netting or dense-packed into wall cavities.
Typically installed on an attic floor, loose-fill fiberglass has a lower R-value per inch (R-2.2 to R-2.7) than any other common insulation product, so it’s best applied in attics that have enough room at the eaves to accommodate a very deep layer—as much as 16 in. to 26 in. The cost to install R-19 to R-21 blown-in fiberglass in a 2x6 wall is around 90¢ per sq. ft. To reach R-38 in an attic, you’ll pay around 60¢ to $1.20
per sq. ft.