Cellulose Insulation Calculation

R Value

• Insulation blocks heat transfer. It prevents warm air, which rises, from escaping in the winter. It blocks penetration of hot air through attics into cool spaces in the summer. It is rated by heat resistance or R value, usually figured by the inch for the depth of the material. Cellulose R value ranges from 3.6 to 4.0 per inch, varying with manufacturer.

Climate Variations

• Insulation requirements vary with climate. The United States has seven climate zones. Zone 1 is the very southern tip of Florida. Zone 7 is extreme northern North Dakota, Minnesota and Michigan and parts of the high Rocky Mountains in Colorado and Wyoming. Zones 2 through 4 cover most of the east, south, Midwest, Gulf Coast and up the west coast from California to Washington. Zone 5 covers the upper Midwest and west. In general, colder regions require more insulation.

General Minimum

• Zones 2 through 5 require minimum attic insulation of R 25. Cellulose with an R value of 4 would require slightly more than 6 inches of insulation to meet this requirement. Some energy guides suggest installing cellulose to about 2 inches above the joists in an attic, which usually equates to about 6 inches. Zone 6 minimum is R 30, Zone 7 R 35. Adding more insulation increases the R value but most of the benefit is in the first 6 inches.

Adding Cellulose

• Cellulose usually is installed as basic attic insulation, but it will settle over time and lose some of its insulating ability. It can be covered with new blown-in cellulose or with fiberglass batts to restore maximum R value. Cellulose also can be added over fiberglass batts in an attic to increase R value; adding 5 or 6 inches of cellulose over fiberglass can boost R values to between 38 and 60.