Insulating a Cathedral Ceiling
Vented or unvented roof assemblies can work, but failure carries a heavy price.
Synopsis: Cathedral ceilings provide a majestic crown for many a great room, but insulating them can be tricky. A poorly detailed cathedral ceiling can lead to energy losses and rotted roofing. In this article, senior editor Martin Holladay describes eight ways to get a cathedral-ceiling roof to R-45 or better using both vented and unvented processes. Holladay says that vented cathedral ceilings can be difficult to insulate well, so it’s important to place the vent baffles and insulation properly. He also says that unvented cathedral ceilings have a mixed track record, but can work when the details are managed correctly. This article includes cross-section diagrams showing four ways to prepare a ceiling from above (if the roofing is going to be replaced) and four ways to access the ceiling from inside. A sidebar outlines various roof-venting myths.
Because older cathedral ceilings are usually insulated with thin fiberglass batts, they are often thermal disasters. These ceilings usually leak air, leak heat, create monumental ice dams, and permit condensation and rot. Roofers sometimes try to solve these problems by improving ventilation openings at the soffits and ridge, but these “improvements” often make every symptom worse.
Fortunately, there are better ways to build cathedral ceilings. Whether they are vented or unvented, such ceilings can perform well, as long as they are properly detailed.
Although many builders still follow the time-tested technique of installing vent channels directly under roof sheathing, a vented cathedral ceiling makes sense only if the geometry of your roof is simple. You need a straight shot from the soffits to the ridge. That’s easy on a gable roof without any hips, valleys, dormers, or skylights, but if the geometry of your roof is complicated, it’s impossible to ensure adequate airflow through all of the rafter bays. Until recently, building…