Attic Fans & Ventilation

History Lesson

The original purpose for putting ventilation in attics was to exhaust moisture from the crawl spaces under houses.  That’s right – most older buildings were framed in such a way that there were lots of direct connections between the crawl space and the attic.  Creating holes in the attic had the effect of drawing moist air up and could actually help dry out the basement.

We’ve since discovered lots of other reasons for venting attics, but interestingly, most building codes are still the same as they were 50 years ago when the purpose was to dry out the area under the house.

Overview

Today we know that venting the attic can cut air conditioning bills in the summer, reduce moisture problems in the winter and extend shingle life.  These potential benefits have many people thinking that it’s a good idea to install some sort of forced ventilation system in the attic.  However, this measure is not as clear cut as it may appear on the surface.

Pros

• Can reduce temperatures in upstairs rooms by 3-4 degrees in the summer.
• Works best in attics with little or no insulation.

Cons

• Energy saved by exhausting hot air is about the same amount of energy as it takes to run the fan.
• When the fan comes on (usually temperature-controlled switch), it has a tendency to pull conditioned air from the house into the attic through air leaks – around pull-down staircases, around and through recessed light fixtures, and around holes drilled for plumbing, wiring, and ductwork.
• While the attic fan is pulling air out of the house, it is also pulling make-up air into the house from outside or from the crawl space.
• If the fan is powerful enough, it may create negative pressure in the house.  This can cause a gas water heater or other combustion appliance to back draft and release poisonous gasses into your home.
• Can cause fires.

Idaho Power’s Recommendation

The bottom line is that code-required attic ventilation levels are generally adequate.  However, if you want to be sure there is sufficient passive ventilation, a Home Performance Specialist can perform an evaluation.  If you are experiencing hot air radiating into your home from the attic floor, here are some actions to consider:

• Increase attic insulation to at least R-30.  We have found that insulating the attic up to R-50 continues to increase comfort and efficiency benefits in both summer and winter
• Idaho Power’s Home Improvement Program will pay fifteen cents per square foot for adding attic insulation up to R-50.
• Seal air leaks around pull-down staircases, recessed light fixtures and other penetrations.
• Install a truss-mounted radiant barrier.
• When upgrading a roof, consider white reflective roof surfaces that limit solar gain.