Building codes require attic ventilation, nominally 1:300.  Inappropriate Attic  Ventilation may increase the amount of moisture retained in the attic cavity.  Ventillation is not as simple as meeting the 1:300 nominal ventilation requirement of Building codes in Greater Vancouver, B.C.  Some of the Factors that need to be considered are the existing Vapor Barriers if present in the building, the attic layout, and the Roofing System or Underlay being used.  Attic Ventilation is driven by wind and thermal cycle.  A thorough ventilation system will have air intake, at minimum 50% of the required ventilation, in the soffit or lower area of the attic and will have openings for air flow exits in the higher area of the roof. I.E. Ridge and Gable’s and at least 50% of the requirement. Even distribution of the ventilation is as well important in ensuring that all areas of the attic have air flow to remove moisture.

It has been speculated that on the West Coast, over ventilation in a dry attic (homes with vapor barrier’s on the ceiling) may actually increase the amount of moisture in the attic in some circumstances.  In contrast, older homes with no vapor barrier have an increased amount of moisture in the attic and if a vapor barrier underlay has been used in the roofing system this moisture may be driven up to the underside of that vapor barrier and collect on the roof sheeting.  It is more integral that older homes without vapor barriers have appropriate ventilation to protect the building from moisture on the West Coast.

In Greater Vancouver we have many critters trying to make a home in our attics.  Sometimes the screening suplied with the vents installed on a roof are insuficient to keep the determined animals out.  Placing a 1/4″ galvanized steel mesh over the opening will go a long way in ensureing that you have a critter free attic.

Reece Jorgensen