Most attics are vented, although some attics are unvented and conditioned. Cathedral ceilings can be vented or unvented—and either approach can work, as long as the details are right. Because of the wide range of possible venting scenarios, designers and builders are sometimes unsure of how to vent their roofs.
In a recent episode of the “BS + Beer” show, participants posted lots of venting questions on the chat forum. In this article, I’ll do my best to answer some of the questions.
Readers who are looking for a comprehensive article on attic ventilation should read “All About Attic Venting.” Those who are looking for a comprehensive article on venting cathedral ceilings should read “How to Build an Insulated Cathedral Ceiling.”
Q. I want to build an unvented roof assembly that includes continuous rigid foam above the roof sheathing. Do I need to create vent channels between the rigid foam and the roofing?
A. No. In most cases, you’ll install a layer of OSB or plywood roof sheathing above the rigid foam, fastening the sheathing to the rafters below with long screws. Above this roof sheathing, you’ll install roofing underlayment and roofing. Building codes don’t require this type of assembly to have vent channels. For detailed instruction on this approach, see “How to Install Rigid Foam On Top of Roof Sheathing.”
That said, note that some asphalt shingle manufacturers don’t allow their products to be installed on an unvented assembly. If this concerns you, choose a different brand of shingles or a different type of roofing.
While vent channels aren’t required above a continuous layer of rigid foam, they may make sense in areas of the country with lots of snow. Vent channels under roofing reduce the chance of ice dams—so if you’re building near…