- 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.