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Solar Attic Fans

I have a few attic vents, but I can still feel the sun’s heat radiating from the ceiling during the afternoon. It is uncomfortably warm. What do you think of installing more standard vents or new solar vents?—John A.

People are often surprised when they realize how much heat blasts down into their homes from a hot attic in the afternoon sun. A dark roof can easily reach 150 degrees in the hot sun. This heat radiates down to the room ceilings below and on to your body. This also forces your air conditioner to run longer, increasing your utility bills.

Attic floor insulation is excellent for blocking low-temperature conductive heat flow through the ceiling during both winter and summer, but it is relatively ineffective at blocking radiant heat from a super-hot, 150-degree roof. The radiant heat flow can make the insulation itself get hot and then it holds the heat even longer.

The best method for blocking this heat from a hot roof is a combination of adequate attic ventilation and low-emissivity, reflective foil stapled under the roof rafters. The ventilation helps reduce the roof temperature and the foil shields the attic floor and ceiling from the radiant heat transfer.

I installed attic foil and additional vents in my own home when I moved in and the temperature in the second-floor bedrooms immediately dropped by eight degrees. Before that, the air coming out the few old vents was so hot, I could not hold my hand over it.

Attic ventilation is also needed during the winter to remove moist air that leaks up from the living areas. If the attic is not well-vented, this moisture in the air can condense and drip down on the insulation. Wet insulation is not only ineffective, but it can actually damage the attic lumber.

There are many types of roof and attic ventilation methods, but solar-powered attic vent fans are effective and operate for free from the sun’s energy. They are an ideal fit with summer attic cooling because the solar fan runs faster as the sun gets more intense and hotter. At night, they stop running.

Solar-powered vent fans range in size from mini 4-inch-diameter models, to the more typical 12- to 16-inch sizes. The maximum airflow rates are about 1,000 cubic feet per minute for the larger ones. The mini-models have a built-in rechargeable battery so they continue to run when the sun goes down, removing any residual heat in the attic.

The two basic designs of solar fans are self-contained (integral) and remote. The self-contained models have the solar cell panel built neatly into the top of the fan housing. The housing can be either round or rectangular. These models are ideal for a roof with an unshaded, southern exposure because no external wiring is required.

For non-southern roof exposures, or for shade problems, use a remote design with a separate solar cell panel and a fan. With up to a 20-foot, safe, low-voltage wire, you can usually find a location for the panel so it faces the sun for maximum electricity output. A remote design also allows you to install an on/off switch if you wish.

Since there is no high-voltage electrical wiring needed with solar-powered fans, installing one or two solar fans is a simple do-it-yourself project. All of the designs have flashing attached to the housing that slips under the shingles, to eliminate leaks when it rains. Carefully remove a few shingles and saw a round hole through the roof. Nail the fan in place through its flashing and replace the shingles with some roofing cement.

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