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which diminishes its effectiveness and in sufficient
quantities may damage interior ceilings.
Proper attic ventilation can also help prevent the
formation of “ice dams” in the winter by equalizing the
indoor and outdoor temperatures. Ice dams occur
when snow hits a part of a home’s roof warmed by
heat loss from the interior. The snow then melts and
freezes at the eave area of the roof, where it’s colder.
Here the mounds of ice and snow often drip and form
icicles.
Powered and static attic ventilators
Effective attic ventilation requires air to exit the attic
at or near the peak of the roof and replacement air to be
drawn in under the eaves of the house. This can be
achieved using a combination of static vents, or the
ventilation can be given a boost using powered attic
ventilators (PAVs).
PAVs pull air out of the attic and are usually
equipped with automatic thermostats, which activate
the unit as the temperature rises above the setpoint.
They are usually placed near the roof peak on the back
slope of the roof, where they are less visible from the
street, or high on a gable end wall. A humidistat may
also be used to start the unit automatically when the
humidity rises above the selected setting in the winter.
Static vents, both intake and exhaust, are not electrically
powered and depend on natural airflow for ventilation.
Intake vents are necessary whether using a
static exhaust system or a powered attic ventilator. Static
vents include ridge vents, roof vents, gable vents
and turbines.
• Ridge vents are installed at the peak of the roof, often
along its entire length. Roof vents cover holes cut in
the roof near the peak to let air escape and to prevent
rain, snow and insects from entering the attic.
• Gable end vents, generally triangular, rectangular
or round in shape, have either adjustable louvers to fit a
wide range of roof pitches, or fixed louvers for a specific
pitch. They are mounted at the highest point of the
gable.
• The intake vents, necessary to balance the systems,
are installed under the eaves in the soffits.
To be effective, both intake and exhaust vents must
be present to promote air movement. Static ventilators
are also available for installation in sidewalls to reduce
peeling of siding paint and moisture damage.
Guidelines for determining which type of static
vents best fit a particular application are available from
HVI member companies. ■
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