[Patrick]

Originally Posted by DocAir

Don’t Waste Money on Powered Attic Fans
Barry Westbrook, P.E.

Most of us were taught from an early age that we can reduce our summer time cooling costs and extend the life of our asphalt shingles by sucking the hot air out of the attic. This is a wasteful, damaging, and dangerous myth that needs to be debunked.
Ironically, powered attic fans will almost always increase the cost of cooling the home. In many cases, they will also elevate radon gas levels in the living spaces of the home and create moisture management problems during hot weather. Finally, powered attic fans will not extend the life of the shingles. Why is this true?
Building scientists at Oak Ridge research facilities have conclusively proven that the amount of ventilation in attics does not significantly affect the cooling requirements of the building nor does it affect the life of the asphalt shingles. These findings are in direct contradiction of the long-held beliefs about the need for attic ventilation.
It turns out that actively ventilating the attic has almost no effect on the surface temperature of the asphalt shingle. Also, most of the heat transmission from the roof is radiant energy coming directly from the roof decking to the ceiling of the home. Although the air in the attic also gets hot, the air is not heating the ceiling. The ceiling is heating the air. Ironically, in most homes, powered attic fans literally suck the conditioned air out of the living space and increase the cost of cooling.
Considering these facts, is there a need for attic ventilation at all? For most attics, the answer is yes. During the winter months, the roof surface becomes cold. Without some ventilation, moist air from the living spaces will leak into the attic and form condensation on the roof decking whenever the temperature drops below the dew point of the air. Condensation on the roof decking can promote mold growth and wood rot. However, only passive venting is needed to prevent wintertime condensation in the attic. Powered attic fans are almost always installed due to concerns about hot weather.
Some attics need no ventilation at all. Many new homes and some existing homes are retrofitted with what is termed as a "closed attic assembly". In a closed attic, spray foam insulation is installed directly to the underside of the roof decking creating an “igloo” effect. This insulation thermally isolates the roof assembly from the attic. This prevents the radiant heating of the house in the summer time and prevents moisture condensation on the roof decking in the winter.

Closed Attic Assembly with Spray Foam Insulation

In summary, although there is a need for ventilation in most attics, the installation of powered attic fans are counterproductive. Homeowners looking to reduce summertime energy costs should invest in additional insulation or install radiant barriers instead.


Questions and comments can be submitted to the following address:
Barry Westbrook
℅ DocAir
4014 Flagstone Ct
Franklin, Tn 37069
615-373-2498

What if the ducts and/or air handler are located in the attic space?