Disable attic fan or leave alone?
I recently bought a house that has an attic fan on a thermostat that seems to run whenever the outside temperature gets above about 70. The house was constructed in 1958, and is 2×6 wood frame on 16″ centers. The attic has about 2-3″ of blown-in cellulose, is pitched (I don’t know the pitch but it’s about average for a typical suburban house), and has a gable vent on each end (L-shaped house), soffit vents, and ridge vents.
This is in Albuquerque, NM, where it’s very dry and very hot in summer (90-110 degrees is a normal high from May – September).
I’ve tried to research attic fans, but the information is conflicting. As in, apparently they’re either good or terrible? On the one hand, I’m thinking the previous owner had a reason for installing it (it definitely was not a feature of the original home, nor probably was the ridge vent). On the other hand, I don’t want to waste electricity running it if it makes little or no difference. In the summer it runs from about 8am to 10pm so I’m sure it adds up. Cooling is one roof-mounted evaporative unit, with the ducts in the attic. Conditioned space is about 1450 square feet.
If it was easy to disable, I would try with and without, but it’s a long, long crawl and I’d really like to only unplug it once, ideally when I’m up there adding more cellulose (but I don’t have the time or money for that project right now).
Does anyone have any advice about what I should do here? Thanks!
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Replies
Gen,
You wrote, "I don't want to waste electricity running it if it makes little or no difference." Your instincts are correct. This device is wasting electricity without any benefit. In fact, this device may be pulling cool air out of your house, making your house warmer than it would otherwise be, and may also be lowering your indoor air quality by causing your water heater to backdraft.
Disable it.
Here is a link to an article that will explain the reasoning behind my advice: Fans in the Attic: Do They Help or Do They Hurt?
If there's only 2-3" of cellulose in the attic it's WELL worthwhile to add more, after air-sealing the ceiling plane and air sealing the ducts. Don't add the cellulose without air sealing first.
If the ducts are not insulated it may be worth doing either retrofitting insulation onto the ducts, or if that's difficult/impossible, installing radiant barrier to the underside of the rafters, which will limit direct gain on the ducts and lower the peak air temps of the attic.
Cooling attics via ventilation is usually the lousiest comfort return on investment, and often a negative return on energy use. Powered attic ventilation often uses more power than the air conditioning power it offsets, and in many cases even increases air conditioning power use.
Are the walls 2 x 6 16" o.c. construction, or is that what the spacing of the attic joists & rafters? Is there a plan to re-roof any time soon? (If yes, that may change what makes the most sense to do.)