Should powered roof fans be used in the attic?

[James W.]

We have a one-story house with an attic ridge vent that does not do the job. As a result, there is moisture collecting in the tray around the furnace (located in the attic) that builds up and drains out through the eaves' flashing leaving brown rust streaks on the siding.

Should powered roof fans be used in the attic?

[home improvement guy]

The purpose of attic venting is to cool the attic space by removing hot air through a ridge vent, can vents, turbine or gable vents in conjunction with soffit vents. By removing the hot attic air, cooler, dryer air is drawn into the attic through soffit vents or by opposing gable vents. All of the vents listed are passive, meaning no energy is used in the natural ventilation of the attic. When a powered fan is the source of ventilation, not only are you using additional energy, but also there is the chance that cooler conditioned air is being mechanically drawn from the home's interior.

When conditioned air is removed from the home's interior, the air conditioner has to work harder, which in turn uses even more energy. Power fans also have a limited useful life expectancy whereas passive vents work year-round. If the fan's blades cause the motor to lock up, the motor is still drawing energy and can cause overheating. It is possible for an overheated motor to ignite an attic fire. In some cases a powered fan is necessary, such as homes with hip roof designs, but I usually recommend a ridge vent system balanced with an equal amount of soffit venting. In some designs, the area at the peak of the roof is too short to accept the necessary amount of ridge vent.

The general rule for venting is to have at least one square foot of free ventilation for every 300 square feet of attic floor space. Half of the venting needs to be at the peak or ridge of the roof and the other half at the eave or soffit. If you only have vents at the ridge or peak of the roof, the ventilation required is one square foot for every 150 square feet of attic. To calculate how much venting is needed you first need to consider that a 1-foot-long piece of ridge vent offers only 10 to 20 square inches of free ventilation depending on the product used. A square foot of metal or vinyl soffit material has only 9 square inches of free venting. Rather than converting square feet to square inches and back again, go togaf.com and type in "venting calculator" in the search box in the upper right corner of the home page. To prevent condensation from forming on the attic or crawl space ductwork, wrap the exposed cold metal with insulation and an approved vapor barrier.