JLC Extra Q&A
Construction Tips & Techniques
Wouldn't a single-sided ridge
vent be more effective than a standard ridge vent for a home
built on a windy site? It seems that with openings on both
sides of the ridge, a standard vent would simply short-circuit
when the wind blows rather than draw air from the eaves
Paul Fisette, director of
Building Materials and Wood Technology at the University of
Massachusetts Amherst, responds: In order for a ridge vent to
exhaust, you need a pathway and a reliable driving force. Both
a single-sided and a double-sided ridge vent provide a pathway,
so the important question is this: What is the main driving
force that pushes attic air up and out of the ridge vent?
While wind direction can induce roof venting — and it's
been shown that even soffit-only venting can draw air out of
roofs — most often the answer is the buoyancy of the
attic air. If there is a fair amount of heat loss from the
house into the attic, then the buoyancy of the warm air rising
causes it to escape at the highest point, the ridge.
However, a more energy-efficient house experiences less heat
loss, so in that case buoyancy becomes less of a driving force.
Based on some tests I've run, I think it's important to install
ridge vents that have an external baffle, like ShingleVent II
(Air Vent Inc., 800/247-8368,
www.airvent.com). As wind
passes over the roof ridge, the airstream jumps over the vent's
baffle, causing suction as the air lifts upward — the
same way an airplane wing works (see illustration). Called the
Bernoulli effect, this driving exhaust force works regardless
of wind direction. Without an external baffle, either a single-
or a double-sided roof vent can allow outside air to come in
and short-circuit the venting process.