Roofing Systems & Insulation
Properly insulating your ceilings will allow ceiling temperatures to remain closer to room temperatures, providing an even temperature distribution throughout the house. Cathedral ceilings must provide space between the roof deck and home’s ceiling for adequate insulation and ventilation. This can be achieved through the use of truss joists, scissor truss framing, or sufficiently large rafters. For example, cathedral ceilings built with 2x12 rafters have space for standard 10-inch batts (R-30) and ventilation. Unvented (hot roof design) cathedral ceilings are also an option. The hot roof design allows more insulation to be installed in the roof cavity as the need for a vent space is eliminated. It is important that the roof cavity be totally air sealed from the conditioned space below to prevent moisture intrusion and roof degradation.
Foil-faced batt insulation is often used in cathedral ceilings because it provides the permeability rating often required for use in ceilings without attics. A vent baffle should be installed between the insulation and the roof decking to maintain the ventilation channel.
Consider using high-density R-30 batts, which are as thick as R-25 batts, but fit into 2x10 framing. You can also add rigid foam insulation under the rafters, which adds R-value and eliminates thermal bridging through wood rafters. However, rigid foam insulation must be covered with a fire-rated material when used on the interior of a building. Half-inch drywall is usually sufficient, but check with local building officials before installing.