Roof Insulation Requirements

Double Roof Insulation

• Insulation is rated by thermal resistance, or R value. Most house roof areas today have double insulation: a sheet of foamboard insulation on the deck of the roof with some type of insulation between the joists in the attic. Several types of rigid foamboard are used for roof insulation, but all typically are 1 to 2 inches thick and installed with a vapor or moisture face on the side against the roof decking.

Climate Zones

• Roof insulation varies by climate zones. There are six in the United States. Zone 1 is the very tip of south Florida. Most of the country is in zones 2 through 5, from the eastern seaboard, along the Gulf coast and up the west coast from California to Washington. Zone 6 covers the northern and Rocky Mountain areas. Small sections of the Rockies in Colorado and Wyoming, extreme northern United States from North Dakota to upper Michigan and the northern tip of Maine are in Zone 7, which covers most of Canada and Alaska.

Minimum Insulation

• The recommended minimum insulation for Zones 2 through 5 is R-25. Most houses are insulated for between R-25 and R-30. That value is hard to achieve with exterior roof deck insulation alone, so most houses are rated on a combination of rigid foamboard exterior roof insulation and internal attic insulation. Rigid foam has R values of from four to eight per inch, loose fill about three per inch, so a combination of R-4 foam and 7 inches of loose fill would satisfy the R-25 minimum.

Attic Insulation

• The most common interior roof/attic insulation is cellulose, made from old newspapers which are shredded and treated with a fire-resistant chemical. This has an R value of 3.2 to 3.8 per inch and usually is laid 10 to 12 inches deep for R-25 to 30. This will vary if foamboard also is installed. Rock wool, the next most common attic insulation, is made of wool fibers treated with chemicals and formed into granules. It has an R value from three to 3.3.