Is the Attic Supposed to Have a Fan?

Roof Construction

• A properly constructed roof has soffit and ridge venting along with insulation on the attic floor. Fresh air enters through soffits into the attic space and moves air out through the ridge vent. Insulation on the attic floor protects the ceiling below from gaining the attic heat. This roof design operates independent of the rest of the ventilation systems in the house. It should not require any powered fans to move air.

Ventilation in Hot Climates

• Building codes indicate that attic ventilation is necessary to prevent condensation from developing on the roof sheathing. This ventilation extends the life of roofing products and cools attic temperatures. Hot air builds up in the attic from sun exposure and shingle color. Removing the hot air with fans was thought to reduce energy required for air conditioning within the home. If the attic is properly designed, those fans are unnecessary, and may even cause an increase in the amount of cooling required. Attic fans draw cooled air from the house into the attic through chimneys and electrical fixtures, causing more humid outside air to enter the house and air conditioners to run longer.

Ventilation in Cold Climates

• In cold climates, improper attic ventilation can cause ice damming -- an ice build-up on the lower edge of the roof. If an attic fan is running, warm air from inside the house can be pulled into the attic space, melting the snow on the roof and causing ice to form. On a properly vented roof, snow cover should remain consistent. If there are patches where the snow has melted and others where it has not, the attic ventilation likely is improper for the house.

Other Considerations

• If a roof is designed and constructed to building standards, there is no need for an attic fan. Introducing a fan into an attic where it is unnecessary can increase heating, cooling and dehumidifying costs. A fan also can change the pressure within the house, drawing up carbon monoxide and radon into the living space. A professional engineer or reputable roofing contractor should be knowledgeable about the latest advances in roofing ventilation, and can advise you on the requirements for a particular house.