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.