In most residences, the attic is a broiling oven in summer and a wet, cold void in winter. An “out of sight, out of mind” attitude toward the attic is common with many homeowners. But what happens in the attic, doesn’t stay in the attic. It shows up as increased energy costs and expensive structural repairs. The culprit is lack of ventilation. Whether it's baking hot air during summer or damp winter conditions, an unventilated attic is costing you money. So how do you ventilate an attic without consuming more energy than you’re saving?
Solar energy striking your roof conducts through the shingles and subroofing, and radiates into the attic, heating the air in the enclosed space as much as 50 degrees above outdoor temperatures. This accumulation of stored heat energy lasts all day, and often persists into the night, as the wooden structure of the house gradually re-radiates energy back to the outdoors. Life with a furnace above your head can be uncomfortable. Attic heat conducting down through the ceiling raises the temperature in living spaces below, offsetting the effect of your air conditioner, and causing it to run longer to meet thermostat settings. This hikes electricity costs as well as making affected rooms less consistently cool and comfortable for occupants. In summer, the influence of attic heat can account for as much as 20 percent of your home cooling expenses. Years of exposure to attic heat can warp wood structural materials, particularly the subroofing, causing shingles to buckle, and spawning roof leaks and the ensuing expensive repairs.
During the winter, humid air rising from living spaces below collects in an unventilated attic and condenses on surfaces when temperatures fall at night. This continued cycle of condensation gradually soaks attic insulation, drastically reducing its insulating properties, and promoting household heat loss through the ceiling into the attic. Your furnace runs overtime to make up for the loss of heat because of perpetually damp insulation, increasing electric or natural gas costs. A wet, unventilated attic is a welcoming environment for mold growth, which permanently ruins insulation, necessitating expensive replacement.
Most residential attics are constructed with a passive ventilation design. Outside air enters the attic through the soffit vents down near the eaves. The air is warmed, and rises to the peak of the roof, where it exits the attic through roof vents. This natural “chimney effect” maintains a continuous circulation of air through the attic. However, passive attic ventilation methods only accomplish slightly less than one complete air exchange per hour. This usually is insufficient to keep up with heat accumulation during the summer. Most experts recommend 10 to 12 air exchanges per hour to maintain attic temperatures in the proper range. Only a powered ventilation system can generate the necessary circulation to meet that standard.
Power to the Attic
Hardwired electric attic ventilation fans generally are mounted at the roof peak or on the side of the house at the gables. Attic fans powered by an AC source usually are actuated by a thermostat installed in the attic to begin ventilating when attic temperatures reach a certain level. These fans may feature humidistat activation during winter to ventilate the attic when humidity exceeds preset limits. The air-moving capabilities of an attic ventilation fan are expressed in cubic feet per minute (CFM), and correspond to the square footage of the attic. To calculate the required CFM for adequate air exchange, multiply the total square footage of the attic times 0.7. For example, a 1,000-square-foot attic would require a fan or fans that deliver a minimum total CFM of 700.
The drawback to powered attic ventilation fans is the electricity required to power them. The electricity consumed by an attic ventilation fan may largely negate the energy savings that result from ventilating the attic in the first place. Solar attic vent fans are mounted on the outside of the roof, penetrating the attic space at the roof peak, and powered entirely by the sun. An attic ventilation fan is a particularly appropriate application for solar energy. When the sun’s energy is at its peak heating the attic, the solar energy to run the fan and exhaust that heat is at the maximum level. Whether in summer or winter, solar attic ventilation fans begin running automatically at the first light of day, and continue to run until sunset. Where winter condensation in the attic is not an issue, solar fans can be controlled by a temperature switch that activates them only when attic temperatures are high. Generally available in power ratings from 20 to 60 watts, the higher the wattage, the greater the fan’s CFM. While solar attic ventilation fans have higher upfront cost than hardwired attic fans, the payback period can be as short as two to three years.
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