Keep your cool with an attic fan

Published 7:26 pm Wednesday, July 28, 2010

Intense solar radiation in the summer can send temperatures in an attic soaring to 150 degrees and higher. While you may not spend time in your attic, its heat is transferred to the living space below. What’s the answer? Ventilate.

Traditional forms of passive ventilation like screened vents uniformly placed at the eaves around the house, louvered vents at the gable ends, a continuous vent along the ridge of the roof or a wind-driven roof turbine are not enough for some homes, especially those in arid climates. We suggest you take a look at electric attic ventilators or attic fans.

Attic fans are usually installed in the roof, or at a gable (if one exists), at one end of the attic. These thermostatically controlled fans remove hot air from the attic while drawing in fresh air through eave vents.

The result is a significant drop in attic temperature. A cooler attic radiates less heat. This means that your air-conditioning system that doesn’t have to work as hard. Bottom line — you’ll see a drop in your utility bill.

While the attic fan does use some electricity, the amount is a fraction of what is saved by easing the burden on the air conditioner.

According to the Home Ventilating Institute, a consumer-oriented trade association, an attic fan should provide a minimum of 10 air changes per hour to be really effective. Attic fans are rated by the cubic foot of air moved per minute.

To determine the proper CFM rating for a fan, which will provide that number of air changes for your attic, multiply the total square footage of your attic by 0.7. For example, a home with a 1,500-square-foot attic would require a fan with a minimum rating of 1,050 CFM (1,500 X 0.7 = 1,050). Add 15 percent to the CFM calculation for dark roofs or if your roof has a steep pitch (greater than 6 inches of rise for every foot of run).

Once you’ve determined the fan size, you’ll have to decide whether to install it in the roof or along a gable end. If your home has a gable with an existing vent, this is probably the simplest way and, due to design, will require a less expensive fan. Since the gable vent exists, there are no holes to cut.

If your home doesn’t have a gable or if you’d prefer installing the fan in the roof, the project is a bit more complicated.

Put the fan up high on the roof close to the ridge and somewhere near the center of the home. The opening should be located in a space between two rafters where there are no obstructions, such as ducting, plumbing pipes, framing or electrical.

Measure the size of the intake throat on the fan and, using a piece of string and a pencil as a compass, transfer the opening size onto the surface of the roof. Use a reciprocating saw to cut out the hole, roof and all.

Then install one 2-by-4 block between the rafters at each end of the opening for stability. Before placing the fan over the opening, coat the underside of the fan flashing with an asphalt roofing cement. This makes for a waterproof connection.

Place the attic fan over the opening and carefully insert the upper and side flashing flanges up between two courses of shingles. You may need to use the flat blade of a hacksaw to cut a nail or two in order to accommodate the flashing.

The bottom or low end of the flashing should lay on top of the roof, be attached with 3/4-inch barbed roofing nails and be sealed with a dab of liquid aluminum. You may find it necessary to trim some of the roof cover that surrounds the collar of the fan housing in order to achieve a more watertight fit.

Wherever the fan is installed, you’ll need to get 110-volt power to it. This can be done by accessing existing power in the attic that will not overload a circuit or by running a new separate circuit from the power panel to the attic, provided there is space in the panel.

Since working with electricity can be a shocking experience, you might consult an electrician.

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