Should You Invest in a Solar-Powered Attic Fan?

A solar-powered attic fan costs nothing to operate, but that doesn't necessarily make it a good investment. It depends on the condition of the attic.

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A few years ago, I was helping a friend install baffles around her soffit vents. In the middle of the day, neither of us could stay in the attic longer than a few minutes. We didn’t measure the temperature, but we think it easily exceeded 120 degrees. It’s possible for the temperature in an attic to reach as high as 150 degrees — hot enough to fry an egg.

That much heat in the attic is bound to make the house warmer and drive up cooling costs. So if there was a way to cool down the attic without using electricity, that would be a good investment, right? That’s the promise of solar attic fans. But whether they’ll work for you or not depends on your climate and the conditions of your house and attic.

How Does a Solar-Powered Attic Fan Work?

You can install an attic fan in a roof or gable. When it’s running, it blows hot air out of the roof or gable vents and sucks cool air in through the soffit vents.

The installation procedure is similar to any type of roof or soffit vent: Cut a hole, nail the fan in place so it covers the hole, then top the edges with flashing and sealant.

A solar attic fan draws its power from a solar panel, mounted on the fan itself or the roof and wired in. The solar panel comes with a fan. Because the sun provides the power, operating costs are zero. Naturally, a solar fan works best when the sun is out and you need the cooling breeze the most.

We have a solar attic fan, and it really does help keep the attic temperature bearable on really hot days. It doesn’t make the attic anything that could be characterized as cool, however.

What Are the Benefits of Solar Attic Fans?

Its main benefit is a cooler attic in the summer. This brings related benefits, such as:

• A more comfortable environment in the house;
• Reduced energy costs for cooling;
• Protection for the roof, which can be damaged by excessive heat.

An important disclaimer: All these benefits are possible only if the attic is sealed off from the rest of the house. If there are gaps, the circulation can pull cool air from the house to the attic and blow it out the roof vents, which can actually increase cooling costs.

The additional air circulation has another benefit: It promotes the evaporation of attic moisture in cooler weather. That controls mold that grows in the insulation and on the framing.

Most attics don’t need this, because natural convection between the soffit and roof vents usually provides adequate circulation. But poorly ventilated attics already experiencing moisture problems can benefit from the active ventilation a fan provides.

What Are the Drawbacks of Solar Attic Fans?

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The main attraction is also its main drawback: It runs on the power of the sun. That means the fan won’t work at night or on cloudy days.

If you need a fan to control moisture in the attic, you’re better off with an electric (wired) fan that will run all the time. Wired fans are more powerful than solar ones and are better suited for attics with ventilation problems.

Another problem with solar fans — especially roof-mounted ones — is the potential for roof leaks. A properly installed roof fan shouldn’t leak. But anyone with a skylight knows anytime you make a hole in the roof, there’s a good chance water will find its way through eventually, no matter how well you seal it.

Are Solar Attic Fans Effective?

Yes, to a certain extent. It will definitely cool the attic. Whether it will have any effect on the temperature in the rest of the house depends on three conditions:

• As mentioned above, the attic must be sealed from the house. Even small gaps in corners and around light fixtures are pathways for conditioned air to be pulled into the attic.
• Air needs to pass freely into the attic through the soffit vents. In many older homes, insulation partially blocks these vents. This can be easily remedied by installing inexpensive polystyrene or cardboard baffles.
• The attic must have enough roof vents to allow circulated air to exhaust freely.

Another factor that limits the cooling effect: Heat radiating into the attic through the roof decking. Air circulation has no effect on radiative heat. Anything close to the decking will also get hot, fan or no fan.

How Much Do Solar Attic Fans Cost?

Generally $300 to $500, with installation adding another $150 to $250 depending on your roof. Installation is DIY-able, so you could potentially install a single fan for as little as $300, but most attics need two or three. So the final cost could be $600 to $1,500 or more.

If you own a new house with a tight, well-ventilated attic, this is an unnecessary expense, because natural convection provides all the cooling you need.

In an older house, you’ll probably need to seal the attic before an attic fan will make your house any cooler, and the cooling may not even be noticeable. In the end, you probably won’t save enough in energy costs to offset the expense of sealing the attic and installing the fans.

If you find yourself going into the attic often on hot days, a solar fan can help you be more comfortable. In that case, it’s a good buy. Otherwise, it’s probably not the best investment.

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