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How to defeat the purpose of the exhaust fan, common in homes built from the mid-80's on
Saturday, February 24, 2007

Atlanta Home Inspections: Roofs. Here's a common sight in homes built from the mid-80's on.

Rusting mushroom vent - Dan Curl

Note the rusting mushroom cover for the attic exhaust fan:

Detail of rusting mushroom vent - Dan Curl

This fan operates on a thermostat: when the temperature exceeds the set temp the fan activates until the temp is lowered. Great idea, most homes have them. Except:

Note the two passive vents on either side of the fan housing. These vents (known as "turtleback vents") defeat the purpose of the exhaust fan.

When the fan is activated, it pulls most of it's air from the adjacent turtleback vents, NOT from the lower sections of the attic

As we all know air, like water, follows the path of least resistance. Air flows from high to low pressure.

In the HVAC trade this is known as a short circuit. A triumph of product over method I call it just plain stupid.

Fans need regular lubrication - a fact most homeowners are unaware of. So, the fan probably doesn't work. 

Even if they knew - the fan is virtually inaccessible to the average homeowner: You either have to climb a steep pitched roof or place a tall ladder on the floor of the attic (if there is a floor) to reach the fan.

Likewise with temperature controls: the control is located near the (inaccessible) fan

Thermostatic exhaust fans are of questionalbe energy value: the cost to run one usually exceeds what little benefit they offer.

Contrary to common wisdom attics are hot because of RADIANT energy (broadcast in the infared spectrum), not CONVECTIVE energy which the fan is designed to control.

If you want to reduce summertime heat gain through the roof your best bet is to install a radiant barrier (reflective foil) on the underside of the rafters. One sees this occainsionally in older homes. Think of those survival foil blankets.

Your best bet is to get a reflective roof covering (metal).

The best use of attic exhaust fans is to move a little air in the attic to make me, the home inspector, a little more comfortable AND to provide income for the guys who replace them.


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