Powered ventilation

As an alternative to a passive ventilation system, forced or powered ventilation may be appropriate. Air displacement in the amount of 1 cubic foot per minute per square foot of attic floor area generally is considered to be equivalent to a 1:150 ventilation ratio.

Similar to passive attic ventilation, when using powered ventilation, it is important to provide for adequate amounts of intake air into the attic space to account for and offset the amount of air that will be exhausted by the powered ventilation.

Figure: Powered ventilation of attic space. A powered vent near ridge with intake vents at the soffits or eaves are shown.

Powered vents should not be used in combination with other vents placed high on a roof because forced air flow may bypass the intake vents, largely canceling the benefits of balanced ventilation. Also, in some instances, the air suction created by powered exhaust vents may result in water infiltration through other exhaust vent openings.