There are several reasons why builders have so many roof-venting questions. For one thing, building codes concerning roof venting are confusing. For another, builders are often uncertain about when unvented roofs are acceptable. If they are carefully detailed, either vented or unvented roofs perform well—but whether vented or unvented, poorly detailed roofs can fail quickly, especially in a cold climate.
Why do we vent roofs?
Builders include a ventilation channel under roof sheathing in hopes that moving ventilation air will remove moisture that has escaped from the house through air leaks in the ceiling. In some climates, we include a ventilation channel under roof sheathing in hopes of reducing the frequency of ice dams.
According to building scientists, roof ventilation is less effective at drying roof sheathing or reducing ice dams than commonly thought. Moreover, some building codes imply that indoor air travels to roof sheathing via vapor diffusion. (These codes are a legacy of an earlier era—the benighted era when vapor barriers were mistakenly seen as the best solution to moisture problems.) In fact, vapor diffusion has very little to do with the types of moisture transport that affects roof sheathing. In most homes with damp roof sheathing, the moisture reaches the roof sheathing via air leaks from the interior, not via vapor diffusion, so the presence of a vapor retarder or vapor barrier near the insulation is far less important than the presence of an air barrier.
If you are concerned about keeping roof sheathing dry or reducing ice dams, make sure that your house has an airtight ceiling, and verify your air-sealing techniques by performing a blower-door test. For dry roof sheathing, an airtight ceiling is more important than the effectiveness of your vent channel.
For more information on roof-venting effectiveness, see “All About Attic…