Drawbacks Of Powered Attic Ventilators


Powered attic ventilators, already suspected of using more energy than they save, can also create excess moisture, structural problems, discomfort, and combustion safety problems for home occupants, according to a recent study. John Tooley of Natural Florida Retrofit, and Bruce Davis of Alternative Energy Corporation's Applied Building Science Center in North Carolina, presented "The Unplanned Impacts on Houses by Powered Attic Ventilators" at the 1995 meeting of the Energy Efficient Building Association.

The paper describes research conducted on eight homes over a period of three months. As a result of this research, Davis said that he wouldn't recommend the use of powered attic ventilators. He emphasized, "If someone chooses to use a powered attic ventilation strategy, they need to do additional performance tests and take responsibility to be sure that it won't cause other problems." The potential for hazardous conditions is particularly high in homes with combustion gas appliances, because the ventilators can create negative pressures that cause backdrafting.

Tooley and Davis took measurements from July through September 1993 at eight homes in Research Triangle Park, North Carolina. Each attic contained passive ventilation in addition to a powered ventilator. All the houses had some depressurization when the ventilator operated, with pressures ranging from -0.5 to -2.5 Pascals (Pa). The tests showed that the powered attic ventilators, on average, drew 231 cubic feet per minute (CFM) of conditioned air out of each house and caused, by themselves, 0.72 air changes per hour (ACH). This flow from the house to the attic ranged from a low of 104 CFM to a high of 646 CFM, and ACH ranged from 0.38 to 1.2.

The houses had air volumes ranging from 13,000 to 32,000 ft3. The high figures above were for the largest house in the study, which had two ventilators and noticeable air leakage paths between the house and the attic. Tooley and Davis concluded that all eight of the sample homes wasted energy due to the high leakage of conditioned air into the attic.

Two houses had combustion safety problems resulting from the ventilators operating in conjunction with other mechanical equipment. At one, the water heater, located inside the conditioned space, stayed in a complete backdraft mode when the ventilators were operated with other equipment. During the seven minutes of the testing cycle, the area containing the water heater reached 40 ppm (parts per million) of carbon monoxide, and at five minutes the water heater flue contained 700 ppm of carbon monoxide (see "Combustion Safety Checks," HE Mar/Apr '95, p. 19). The study also reports anecdotal cases of combustion safety problems, including a Colorado family of three who died from carbon monoxide poisoning when an attic ventilator caused the furnace to backdraft.

Another two houses had moisture problems resulting from ventilator-induced negative pressures. At one house, a chain of events started with the repair of a leaky return duct for the air conditioner. Condensation occurred on the tighter (and colder) duct, which was located in a framing cavity, and moisture began to appear on the ceiling Sheetrock of the first floor. The suggested remedy was to increase the powered attic ventilation and turn on bath fans to remove moisture from the structure. This actually exacerbated the problem by increasing the pressure difference, moving more humid outside air through the building cavity, which created more condensation on the sheet metal duct. The moisture saturated the Sheetrock, which eventually fell to the floor. Discomfort problems were found in two study houses, one of which also had safety problems, while the other had moisture problems.

Tooley and Davis concluded that if a powered attic ventilator is to be used, the installer should provide a good air barrier between the ceiling and the attic, and ensure that adequate net free vent area is provided in the attic. The amount of air the fan moves must also be considered so that it can be properly sized (a bigger fan is definitely not better in this case), and pressure measurements should always be used to confirm safe operation.

Powered attic ventilators are commonly used in some regions of the United States, such as the Southeast and the Midwest, and they have a positive reputation. They are actively promoted by builders, roofers, HVAC contractors, ventilator manufacturers, do-it-yourself retailers, and even some utilities, although few utilities are sponsoring installation programs as they once did. Davis said that ventilators are sometimes used as a quick fix to meet attic vent codes in homes with an unusual roof structure or design.