Q. My house has cathedral ceilings with 2-by-12-inch rafters to which the builder nailed 2-inch-high sleepers, bringing the effective depth for insulation to 135 / 8 inches. I had wanted to use 12 inches of insulation, leaving enough of a gap above the insulation for airflow. But then I realized the 12-inch insulation would expand to fill the whole space, so I went with batting 9 inches thick. Since then, squirrels have gotten between some of the rafters and “squirreled” up some of the insulation. I wish to add additional insulation, perhaps fill the entire cavity by blowing some in. I heard that leaving an air space is no longer important. Is this correct?
A. It’s true that many building codes now permit fully insulated cathedral ceilings without an air gap, but you can’t just stuff in more insulation and meet code. There’s too much risk that moisture-laden air will sneak through gaps in the ceiling, move through the insulation and condense on the underside of the roof sheathing, eventually causing it to rot.
To prevent moist air from meeting a surface cold enough to allow condensation, you need to either keep the air channels or coat the underside of the roof sheathing with spray-on foam insulation that blocks airflow. To install that foam, you would need to remove the drywall or other ceiling material and take down the existing insulation. You could then fill the cavities between rafters entirely with foam. Or, to save money, you could use a thin layer of foam and then install fiberglass or cellulose below that. Either way, the total insulation value needs to be at least R-38 for your area. And you need a ceiling finish that blocks airflow as much as possible. Painted drywall works well. If you want a board ceiling, install drywall first and then cover it with the wood.
If you don’t want to take down the ceiling, you can add rigid foam board insulation above the roof sheathing. But that means taking off the roofing and then installing a new roof. If you need a new roof anyway, it’s a great option. Otherwise, it’s probably not cost-effective.