HVAC

Published on July 28th, 2015 | by Henri

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Attic fans are seldom helpful

Q.  My wife and I are fans of your blog. And speaking of fans…

We have read several of your opinions recently about attic fans, and I understand your reluctance to install one that might pull heated or cooled air from the living spaces below when you didn’t want it to. But, on summer days when it has cooled down outside, and inside the house, it is still oppressively steamy, wouldn’t an attic fan (either fixed in the gable end, or in the attic floor), combined with an open attic door and open house windows, pull in the fresh, cool outside air to make it bearable inside?

This condition of cool evening air seems to be common in Vermont in summer, and it seems to me that an attic fan might be cheaper than installing air conditioners in key rooms. What do you think?

A.  Yes, it could work nicely. It’s quite different from having an attic fan in a gable or on the roof for the sole purpose of cooling the attic in summer and removing moisture in winter; that system is what draws heated or cooled air from the living quarters because there is seldom enough net free ventilation area (NFVA) in the attic itself to satisfy the CFM requirement (cubic feet per minute) of the fan.

But a fan whose sole purpose is to cool the house at night (with open windows and no AC) during the summer would not have the same deleterious effects. It is best to have it installed in a gable wall, as a fan is more effective drawing air out than pushing it through screened areas.

Be sure to make provisions for effective insulation of the ceiling louvers in other seasons — otherwise you will incur a tremendous heat loss through them. That means that, unless you can work out an effective insulation system from below, you will have to have easy access to the attic, such as a disappearing stairway, which will also need to be effectively insulated. Having to lug a ladder upstairs to climb in the attic through a usually small, difficult-to-access, and hopefully insulated, hatch (referred to as a scuttle hole) often found in a closet is a serious deterrent.

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