Why ventilate your attic?

By Hannah McKenzie
Why ventilate your attic?

Cooling the attic is not recommended by anyone I know as an effective strategy for reducing your bills.

Q: My sister Louise cut an article out of the newspaper about solar powered ventilation fans for attics. Do these things really work? I could install it myself, since no wiring is required, and the article said it would save money by cooling off the attic.

Ask Louise what she's doing hanging out in the attic. And if she's not spending time up there, why does she want to make the squirrels and bats more comfortable? Powered attic ventilators are generally not a good idea, whether they're powered by nuclear electricity, burning water buffalo dung, landfill-generated methane gas or directly by the sun.

Powered attic ventilators are promoted as doing three things:

Reducing summer air conditioning bills

  • Removing moisture from the attic
  • Extending shingle life
  • Let's look at each of these.

Reducing summer air conditioning bills

Theoretically, these fans reduce attic temperatures by pulling outside air into the attic. Unfortunately, somebody forgot to tell that to the air. In most of the houses we've tested, the attic fans were drawing some of the air from the house, rather than from the outside.

In other words, they are cooling the attic by drawing air-conditioned air out of your house and into the attic. Cooling the attic is not recommended by anyone I know as an effective strategy for reducing your bills. Effective strategies include sealing the air leaks between the house and the attic and making sure there is enough attic insulation and that it is installed properly.

Another problem is that a large fan in the attic that is pulling air out of the house can create a negative pressure in the house. This negative pressure can suck the flue gasses out of a water heater or other combustion appliance. In one house we tested, we measured substantial levels of carbon monoxide (CO) in the daughter's bedroom in the basement. The CO was coming from the water heater next to the bedroom, which was backdrafting. The daughter had been suffering from flu-like symptoms for some time. The backdrafting was caused by the powered attic vent fan.

Removing moisture from the attic

Vent fans are also promoted to remove moisture from the attic. In our part of the country, the humidity is typically high in the summer, when we're advised to run the fans to "cool" the attic. To the extent that the fan is pulling outside air into the attic, that air will tend to have a high relative humidity, so it's unlikely that it will reduce the moisture level in the attic.

If there is an air conditioning unit with a leaky cabinet or leaky ductwork in that attic, there's a chance that the moisture in the air will condense on the cold spots and cause moisture damage. Rather than solve moisture problems, the powered attic vent fan may, in fact, cause it.

Extending shingle life

Finally, powered attic vent fans are advertised as a strategy to extend shingle life by reducing attic temperatures. Shingles are heated by radiant heat from the sun. It's possible that ventilating the attic can reduce the temperature of the air in the attic, which could reduce the temperature of the roof decking, which could reduce the temperature of the back of the shingles. I've seen no research, however, that supports the idea that powered attic fans actually increase shingle life.

If you are in a climate where you can be comfortable in your house without air conditioning, an attic fan won't have the drawback of pulling costly air conditioned air out of the house. Also, if there is no air conditioning, there is little likelihood of having cold condensing surfaces in the attic, so moisture problems are unlikely. In these circumstances, a powered attic fan may be helpful. But be sure to check that the water heater is not back-drafting.

If you're in a warm, moist climate where you need air conditioning, I would tell Louise that you'd much rather get a solar-powered outdoor lighting system or a solar hot water system. But a solar-powered attic fan? It's like smoking cigarettes made with Vitamin C.

About the Author

Hannah McKenzie is a residential building science consultant at Advanced Energy in Raleigh who specializes in working with nonprofit developers like Habitat for Humanity to make new affordable housing energy efficient.

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