Joe Lstiburek says: The only reason to vent a roof is to prevent ice-damming
The dean of building science, Dr. Joe Lstiburek, is out with a new* article where he says: “The only time you should vent a roof is to control ice-damming. And when I do a vented roof to control ice-damming I install the vented roof over the top of an unvented under-roof.”
The whole article is here:
This is a pretty radical departure from what seems to be the conventional wisdom. His introduction:
“Vented roofs blow off more than unvented roofs. Vented roofs burn more than unvented roofs. Vented roofs are energy inefficient compared to unvented roofs. Vented roofs cause moisture problems south of the Mason-Dixon Line and east of Interstate 35 in Texas. Venting a roof in a hot-humid and mixed humid climate is a very, very bad idea.”
I have to quibble with that last part, Washington, DC, is south of Mason-Dixon and has tens of thousands of houses build in the decades after the Civil War. Virtually all of them have vented roofs and they seem to last OK.
Something else he says that I disagree with: “In cold climates roof venting controlled attic moisture accumulation only in poorly insulated attics… In the winter the air outside is cold. Yup. Cold air is not capable of holding much moisture. So bringing cold air into your attic does not remove moisture unless there is heat loss from the building. The heat loss warms up the incoming cold air giving it the ability to pick up moisture and carry it to the outside.”
Even in the winter, when the sun hits a roof, the attic warms up. That warming is enough to drive air exchange. In my experience, even in well-insulated houses the wood in the attic is desert-dry.
*(OK, new to me, it came out in March).
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