Cathedral ceilings must provide space between the roof deck and ceiling for adequate insulation and ventilation. For most areas of the U.S., the 1995 MEC recommends R-25 to R-38 insulation in a cathedral ceiling. This can be achieved through the use of truss joists, scissor truss framing, or sufficiently large rafters. For example, cathedral ceilings built with 2×12 rafters have space for standard 10-inch, R-30 batts and ventilation.
Foil-faced batt insulation is often used in cathedral ceilings because it has a 0.5 perm rating and provides the permeability often required for use in ceilings without attic spaces. A vent baffle should be installed between the insulation and roof decking to ensure that the ventilation channel is maintained.
If roof framing provides insufficient space for required insulation, higher insulation values can be obtained by either attaching furring strips to the underside of the rafters (that permit additional insulation to be installed), using high-density batts (high-density R-30 batts are about the same thickness as R-25 batts and fit into 2×10 framing), or adding rigid foam insulation under the rafters. Rigid foam insulation offers a resistance to thermal bridging through wood rafters. Rigid foam insulation must be covered with a fire-rated material when used on the interior of a building. Half-inch drywall usually complies, but check with local building codes for confirmation.