Q. Would an attic that has a properly sized and installed soffit and ridge venting system benefit from the installation of a power vent?
A. Paul Fisette, director of Building Materials and Wood Technology at the University of Massachusetts Amherst and a JLC contributing editor, responds: There are two reasons to vent an attic: to remove moisture and to control attic temperature.
If the attic has moisture problems despite functioning soffit and ridge vents, find the bypasses that are allowing moisture-laden air to reach the space and seal them with expanding foam; this is a more effective — and less expensive — solution than adding power venting. If the attic is too hot, consider roof color. Research suggests that it — not attic venting — has the biggest impact on reducing roof temperature.
However, if you decide to add a power vent anyway, remember that air follows a driving force along the path of least resistance. If there's a ridge opening located within a few feet of the power vent, air will simply be pulled from the ridge into the exhaust vent, achieving nothing. To avoid that, seal the ridge vent and make sure the lower soffit vents are distributed uniformly throughout the attic.
By design, a power vent depressurizes the attic and requires makeup air. So be sure to seal air-leakage pathways in the attic floor/house ceiling interface to prevent the vent from sucking conditioned indoor air into the attic and out the exhaust port. And carefully follow the manufacturer's recommendations regarding fan sizing; an oversized fan can create enough negative indoor pressure to make dangerous back-drafting of gas-fired appliances a real concern.
