Anyone who has wanted to clear up a foggy bathroom mirror after taking a shower knows the value of a ventilation fan. What you might not know, however, is that today’s models use humidity sensors and motion sensors to clear the air better. Ventilation fans now move more air at a higher speed (measured in cubic feet per minute, or cfm) and are quieter than ever before. They also are marketed as using less energy than older models required.
Lately, ventilation fans also are marketed toward use in more rooms than just your bathroom. The attic is the latest space for which products are being promoted, but it’s a venture that’s borne out of marketing. Broan made a ventilation fan that’s geared specifically toward use in the attic when it introduced a solar-powered model in February 2012. (At press time, we learned that Broan plans to replace that ventilation fan with a new version sometime in 2013, but it wouldn’t disclose further details. Stay tuned.)
Broan says attic ventilation delivers untapped energy savings. In the summer, an attic can reach 150 degrees Fahrenheit, and heat can find its way into the home and increase the load on the air conditioner, which adds to the energy bill. Broan says a ventilation fan will relieve the attic of that heat and also will protect the roof and housing structure from degrading that’s due to moisture buildup.
However, two independent experts with whom we spoke disagree on both counts. First, a ventilation fan that’s in the attic starts to work only when the thermostat detects a rise in temperature during the daytime, so it does little to contribute to heating the moist air that’s left at night. Second, these experts tell us that, because a ventilation fan that’s in the attic turns on during the daylight hours, it has to run faster and run all day long, which means that it consumes more electricity. In fact, its mere operation is counterproductive, according to the experts.
“Attic ventilation fans move huge amounts of air and create a negative pressure ‘bubble’ right over the conditioned space of the home,” says Brett Dillon, who owns IBS Advisors, which is an energy-efficiency and building-sustainability consulting company. “This sucks conditioned air into the attic, typically when outside conditions are really hot, through all the holes in the ceiling and wall top plates. Although this cools the attic down, it also causes outside air to get sucked into the house, which then makes the air conditioner run even more.”
Broan didn’t respond to questions about negative air pressure that an attic ventilation fan creates.
STRONG, SILENT TYPE. If you haven’t shopped for a ventilation fan in the past 3 years, you’ll notice that some models deliver far more power than older models did. Today, the maximum speed of a bathroom ventilation fan goes as high as 380 cfm, compared with the former maximum of 100 cfm.
Beyond the Bathroom: Spot Ventilation Moves to Other Spaces
However, you should know that size matters only in terms of how it affects your wallet. The two experts whom we interviewed say you shouldn’t pay a lot for a ventilation fan that delivers more power, because a model that moves air at, say, only 70–100 cfm is adequate to clear the moisture from a typical bathroom of 100–200 square feet.
A formula that’s recommended by Home Ventilating Institute (HVI) to calculate the speed of a ventilation fan results in speed specifications that are greater than what our experts and others believe to be necessary. HVI’s formula reads like this: If a bathroom is, say, 10 feet wide and 10 feet long and has an 8-foot ceiling, you multiply 10 x 10 x 8, which will equal 800 cubic feet. If you divide 800 cubic feet by 60 minutes (1 hour of use), this will equal 13.3. HVI would then have you multiply that number (13.3) by eight air changes per hour. You will get an estimate of 106.7 cfm to ventilate your bathroom properly.