According to a paper by John J. Tooley Jr., Natural Florida Retrofit and others, powered attic ventilation is counterproductive and, in some cases, dangerous. Is this paper wrong, or are proponents of powered attic ventilation unaware of the problems?
Barry Elkin • via email
The purpose of powered attic ventilation is to remove heat from an attic so cooling loads are reduced. The ventilators exhaust hot attic air while drawing in lower-temperature ambient air. This reduces heat conduction through the ceiling to living quarters below. A typical control strategy is to use a simple thermostat that turns on the attic fan once a certain temperature is reached, and turns off the fan once the attic is sufficiently cooled.
The authors of the paper you mention discovered many instances of attic ventilators causing depressurization of the attic. Although they do not address the root cause, it would seem that either the ventilators are oversized or the intakes are undersized. Whatever the cause, a depressurized attic could cause air to flow from the living space to the attic. This could, according to the authors, cause energy loss, indoor air quality issues, and decreased thermal comfort. They do briefly address some possible remedies to the main problem—proper sizing of fans and intakes. And airflow can be minimized by proper sealing, though the vast majority of houses are quite leaky. However, they do question the basic effectiveness of powered attic ventilation—does the strategy really save energy?
Unfortunately, the energy savings rarely exceed the energy costs of running a powered attic ventilator. This is because most attic ventilators use a fair bit of energy— a typical 250-watt fan would use 180 kWh per month if run continuously (60 kWh per month if used 8 hours per day). To put this electricity use in perspective, the average U.S. home uses about 950 kWh per month.
Heat flow from the attic to the living space comes in several forms. Besides conduction (direct heat transfer from material to material), there is also considerable radiation heat transfer. Solar radiation is absorbed by the roof, which in turn radiates heat in all directions, including toward the ceiling insulation. Ceilings should be well-insulated to significantly reduce heat flow to the living space, even with very high attic temperatures.
In my opinion, using electricity to ventilate an attic for reducing cooling loads is rarely justified. Instead, there are techniques that have better results. Air-seal the attic floor/interior room ceiling, and maximize the insulation there. This is usually much more effective in reducing heat transfer than attic ventilation. A well-designed passive attic ventilation system, with adequate ventilation openings, can be implemented. Half of the ventilation openings should be as low as possible on the roof, and the remainder as high as possible. The hotter, less-dense air rises through the high openings, while replacement air comes in through the lower openings.
Solar-powered attic fans can help, as long as there is adequate and low intake vents. Powered attic ventilators should only be used if the other options have been implemented and extreme temperatures persist in the attic, possibly causing premature failure of building materials.
Note that people often confuse attic fans and whole-house fans. A whole-house fan cools a house by pulling air from the house; cool air is allowed to enter through open doors and windows. Most whole-house fans are mounted in the ceiling and push air into the attic. This air then exits the attic through attic vents.
Neil Smith • AirScape Fans