San Antonio Sustainable Living member and owner of Solar Texas(soltx.com), Tony Stephenson, explains the effect of the sun on a simple ventilated attic, as contrasted to a ventilated attic with an attic fan.
#1 - A home with simple vents and no attic fan.
Effects of the sun's heat on a home in the morning: Attic is about the same temperature as the ambient temperature. The sun hits the roof and the attic starts to warm.
#2 - A home with simple vents - no attic fan. Effects of the sun's heat on a home in the afternoon:
Towards the afternoon, the attic is considerably hotter than ambient. Air has expanded with the heat in the attic. There is so much heat, the vents and soffits just allow the attic air room to expand...everywhere. The hot air pushes out from the vents and soffits and gable vents.
Ideally, it would be great to have fresh, outside air coming in under the eaves through the soffits or gable vents, across the ceiling joists and rafters and out the vents on top of the roof. This is what a good solar powered attic ventilation fan is designed to do.
#3 - A home with simple vents - no attic fan.
Effects of the sun's heat on a home in the evening:
In the evening, insulation holds in the heat from the day like a sleeping bag left out in the sun. Cool, humid night time air returns to the attic as the attic cools and the air brings moisture with it. Process repeats every day.
When installed, a solar powered attic fan comes on when the sun comes up and goes off when the sun sets and will keep the attic close to ambient temperatures all day. In doing so, the extreme temperature variations are minimized and moisture and mold issues are virtually eliminated. The AC ductwork is usually in the attic and keeping the attic close to ambient allows the AC system to work a little easier.
My best results have been with attic fans near roof peaks with ample airflow from the attic extremities. Best result was with a home in Shavano Park where the homeowner's utility bill went from $385 in May 2008 to $247 in July (not a scientific evaluation). Fans were installed in mid-June.
A good attic fan should draw at least 1150 cubic feet per minute (cfm) of airflow. Anymore than 1575 cfm will start to bring conditioned air up from the home below through light fittings and other ceiling penetrations.
#4 - A home with solar attic ventilation fan in the afternoon.
With a good solar attic ventilation fan, the heat from the sun doesn't get a chance to build in the attic. The fan comes on when the sun is shining on the roof and brings a flow of fresh air through the attic. The fan comes on in the morning, even before the heat in the attic starts to build.
Originally posted to the SASL Facebook group.