In our central California climate, an attic fan is equally important in summer and winter — but for different reasons. In summer, heat accumulation in a poorly ventilated attic can send temperatures up to 150 degrees. All that heat baking the wooden structure of the attic and sub-roof can be damaging. Even worse, it migrates down into living spaces and makes your A/C work harder, raising utility costs. During winter, humid air infiltrating from rooms below condenses in the cold attic. This saturates attic insulation, destroying its heat-resistant properties. The wet environment also promotes mold and warps wooden structural components.