So much information has been devoted to the subject of roof venting that it’s easy to become confused and to lose focus. So I’ll start by saying something that might sound controversial, but really isn’t: A vented attic, where insulation is placed on an air-sealed attic floor, is one of the most underappreciated building assemblies that we have in the history of building science. It’s hard to screw up this approach. A vented attic works in hot climates, mixed climates, and cold climates. It works in the Arctic and in the Amazon. It works absolutely everywhere—when executed properly.
Unfortunately, we manage to screw it up again and again, and a poorly constructed attic or roof assembly can lead to excessive energy losses, ice dams, mold, rot, and lots of unnecessary homeowner angst.
Here, I’ll explain how to construct a vented attic properly. I’ll also explain when it makes sense to move the thermal, moisture, and air-control layers to the roof plane, and how to detail vented and unvented roofs correctly.Theory behind venting
The intent of roof venting varies depending on climate, but it is the same if you’re venting the entire attic or if you’re venting only the roof deck.
In a cold climate, the primary purpose of ventilation is to maintain a cold roof temperature to avoid ice dams created by melting snow and to vent any moisture that moves from the conditioned living space to the attic. (See “Energy Smart Details
” in FHB
#218 for more on ice dams.)
In a hot climate, the primary purpose of ventilation is to expel solar-heated hot air from the attic or roof to reduce the building’s cooling load and to relieve the strain on air-conditioning systems. In mixed climates, ventilation serves either role, depending on the season.