- Q.Here in the West, we often
build with exposed rafter tails. For attic roof vents,
we typically drill three 1 1/2-inch holes through every
other soffit block, and install ridge vents. Is this
enough ventilation?
A.Clayton DeKorne
responds: Most codes rely on the old FHA
Minimum Property Standards, which call for enough
net free ventilation to equal 1/300 of the
attic floor area. If the ceiling does not have a
vapor barrier, you need enough ventilation to equal
1/150 of the attic floor. Ridge and soffit vents
should be balanced (equal openings along soffit and
ridge).
Net free area means the unobstructed openings of
a vent. If you cover the holes in the soffit blocks
with 8x8 screen (64 openings per inch), the vent
area is reduced by 25%. This means your three 1
1/2-inch holes in each rafter bay equal about 4
square inches of net free vent area.
Let’s take an example. For a 24x30-foot
attic, you need about 346 square inches of net free
vent area to satisfy the code requirement. A
low-profile ridge vent will supply 360 to 510
square inches (based on manufacturer estimates of
12 to 17 inches per linear foot of net free vent
area). But your soffit vents will only give you
about 88 square inches — hardly enough to
balance the ridge vent. You’d be better
off drilling three holes in every bay. This would
equal about 176 square inches of vent area
— a solid half of the required vent area,
and a suitable balance to the ridge vent.
This answers the question from the code point of
view. But codes are simply minimums: What about
good building practice? Current research by Bill
Rose at the University of Illinois indicates that
ventilation probably plays a less important role in
controlling moisture in roof cavities than the air
tightness of the ceiling and the pressure
difference across the ceiling. This
doesn’t mean you can get around the code.
But it does suggest that in addition to providing
roof ventilation, you should also do everything you
can to control indoor humidity levels and install
an airtight ceiling. This should include sealing
around vent stacks and chimney chases that pass
through attics, avoiding can lights and other
penetrations in the ceiling, and, of course,
exhausting the dryer outside, not into the
attic.
Clayton DeKorne is a senior editor of
the Journal of Light
Construction.