A.Mike Keogh, a specialist in
energy conservation and ventilation from Campbellford, Ont.,
responds: Upgrading existing ventilation when reroofing is
a smart move -- an opportunity that is missed all too
frequently. But should you add a ridge vent to a roof with
gable vents? No way -- that's a prescription for disaster.
Gable vents will alter the air flow around the ridge vent and,
especially when wind is parallel to the ridge (at right angles
to the gable), can actually reverse air flow through the ridge
vent, pulling rain or snow into the attic.
Unfortunately, however, even when they're matched with
continuous eaves venting, gable vents are not very effective
because most of the air flow is along the floor of the attic.
This leaves much of the attic volume unvented (see
illustration), with pockets of dead air that can store summer
heat and radiate it into the living space below.
On a house with eaves and gable vents
but no ridge vents, wind perpendicular to the ridge tends to
create air flow along the floor of the attic but leave hot dead
air in the roof peak (left). When the wind is perpendicular to
the gable, the cooler outside air enters the gable vent and
drops to the floor of the attic before rising to exit at the
other end, again leaving zones of dead air
The most efficient option is ridge venting combined with
continuous soffit vents. If in this case, you choose to add
ridge vents, you must either remove the gable vents or seal
them up from inside the attic. Sealing and leaving them may
look better from outside, to avoid creating a blank gable.
There are also some attractive, strictly decorative gable vents