The recent hail storm and replacement of everything that is roof-related has many of us having to deal with attic fans and wondering what it is exactly that these fans do. Often being situated on the roof, attic fans are often in need of repairing or replacing following hail storms. There are really two categories of attic fans; the whole house fan and the more common powered attic ventilator.
Whole house fans
Whole house fans are specifically designed to cool your home at night using the outside air. Whole house fans are best suited for dry climates, such as ours, where the temperature drops down at night. The fan is mounted in the floor of the attic, sitting above a central hallway in the home. To operate, windows in the house are opened and the fan is turned on. Outside air is then drawn in through the windows and up through the attic where the hot inside air is vented out of the home.
Whole house fans are very energy efficient; drawing only 10-15% of the energy drawn by a central air conditioning system. They are also efficient from a time standpoint. Generally speaking, they can cool down a house in less than an hour. Once the house has cooled, the fan can be turned off and the windows closed. The windows remain closed until the next evening to keep the cool air from escaping.
Whole house fans take all the hot air from the house and move it up into the attic and out through the attic vents. Because of this, they require significant ventilation in the attic. Often their ventilation requirements exceed the minimum required by building codes. Although people had complained that whole-house fans are noisy, much quieter models are available today. In the winter months, the fan opening needs to be sealed. Whole house fans are now also available with built-in motorized doors as a part of the unit. These doors open and close without having to crawl into the attic to seal the unit.
Ventilation fans, or powered attic ventilators, are designed to cool the attic, pushing out the hot attic air and rushing in a flow of cool outside air. Powered attic ventilators are housed in the slope of the roof or in the gable wall of the attic. The intention of the ventilation fan is to save energy and the roof. Energy would be saved by reducing the burden of the air conditioner in the home. The roof would be saved by decreasing the amount of heat moving from the attic through the roof. The lifetime of a roof with asphalt shingles is significantly reduced by excess heat moving from the attic below through the shingled roof above.
Disadvantages of ventilation fans
There have been a number of problems identified with powered attic ventilators that often go unmentioned by installers and manufactures of these fans. Even though the hope with these fans is that cool outside air is pulled into the attic through the attic vents, it is common that the fan instead pulls interior, air-conditioned air up from the home into the attic through small cracks in the ceiling. As it turns out, few ceilings are completely crack free. This negative pressure in your house in turn pulls in hot exterior air to replace the exhausted cooled air. Your air conditioner is then running longer as it is working to cool more warm air.
Another significant problem with powered attic ventilators is that this negative pressure situation within the home can also lead to backdrafting. If gas appliances, such as gas water heaters, are operated within the home this backdrafting can introduce toxic carbon monoxide into your home.
Even with a perfectly tight ceiling, powered attic ventilators use more energy than they save. In response to this criticism, the industry has now produced solar powered attic ventilators that are “off the grid”. Although these devices still don’t address backdrafting, they do not require electricity to run. However, if you look at the cost of the purchase and installation of these devices alone, in most cases you won’t break even until 20 years plus down the road.
Many experts caution against installing powered attic ventilators. In fact, some go as far as to suggest that if you already have them you should disable them. If it’s energy efficiency you’re looking for, the whole house fan is the best bang for your buck.