Problems with Attic Ventilation Fans

Smart Inspector Science

by Tom Feiza, Mr. Fix-It, Inc.

We all know attics must be vented to remove heat and moisture. Intense heat buildup in an attic makes a home uncomfortable and forces the HVAC system to work harder, wasting energy. In the winter, proper ventilation reduces moisture levels; this, too, enhances comfort in the living spaces. Controlling attic heat and humidity can even help spare the roof shingles from damage.

At times we encounter powered ventilation fans that draw air out of an attic. Are they a good solution? What might go wrong?

Rule of thumb

For many years we understood that attics must be evenly ventilated. About half of the static vent openings were evenly spaced near the ridge of the roof and the other half evenly spaced near the soffits or overhangs. These created a balanced flow of air throughout the attic.

Different types serve different needs

In those days, upper vents – can vents, static vents and box vents – were placed high, near the ridge of the roof. Lately these have been replaced with a ridge vent: an opening in the ridge of the roof covered with a mesh or plastic frame and shingles (illustration V005).

In static vent systems, wind or rising heat in the attic pushes air from lower vents to upper vents, removing heat and moisture. This worked well with simple roof designs. Older homes often had gable end vents, too, which worked in combination with soffit vents.

A powered fan can help, but…

As homes were built with more complicated roof designs and tight envelopes that traps moisture, attic ventilation became an issue. The need to “cool” the attic to supplement air conditioning also became a concern. Solution: the powered attic ventilator, a fan that sucks air from the attic (illustration V004). The fan responds to levels of heat and humidity.

This is a great solution, but most of these fans were installed wrong. (illustration V024) When a powered attic ventilator is installed, all the upper vents in a roof must be closed. Yes, all the upper vents must be closed! The fan pictured here (photo: Attic Fan with Ridge Vent) sucks air only from the adjacent ridge vent opening, which means the attic still is not ventilated.

Whenever a home has roof vents or gable end vents, they also must be closed to allow an attic fan to vent an attic properly. A lower vent at the overhang will enable complete ventilation.

The takeaway

During your inspection, note powered attic ventilators and check the installation; you may find an issue to be recorded. In many cases, the attic fan is incorrectly combined with upper vents. If the ventilation system is performing well, with no moisture issues, the best recommendation may be to turn the fan off – but that’s not your call.

Tom Feiza has been a professional home inspector since 1992 and has a degree in engineering. Through HowToOperateYourHome.com (htoyh.com), he provides high-quality marketing materials that help professional home inspectors educate their customers. Copyright © by Tom Feiza, Mr. Fix-It, Inc. Reproduced with permission.