Q:  How can I get a higher R-value in a 2x4 or 2x6 cathedral ceiling cavity space?

A:  There are two recommended ways to achieve a higher R-value in a cathedral ceiling where it cannot be achieved by adding a fiber glass insulation batt to the stud/joist cavity.

1. Foam insulation can be added in the roof system. Johns Manville produces one product applicable to this situation -- Nailboard. This is not a feasible means of retrofit if the roof is not going to be replaced.

2. A combination of fiber glass insulation and rigid foam insulation. First, place an unfaced fiber glass insulation batt in the cavity. Choose the fiber glass insulation product that will give the highest R-value for the designed cavity space. Since fiber glass is designed to fit between standard joist or stud spacing, it will completely fill the space ensuring no voids, which could lead to other problems. It is too difficult to cut the rigid foam product to the exact spacing of the studs or joists. Second, install a foil faced rigid insulation to the inside of the fiber glass filled stud cavity. The thickness of the rigid insulation will be dependent upon the R-value of insulation in the stud cavity and the overall R-value desired. The two R-values can be added together to achieve the final R-value. The foil faced rigid insulation will provide a vapor barrier to the warm in winter side of the structure while only compromising a small amount of space. Cover the rigid insulation with a 15 minute fire barrier such as 1/2" Gypsum board. The Gypsum board must be fastened through the foam and into the framing.