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Even in areas with moderate climates year-round, attic air temperatures can soar on sunny days. That may not seem like a problem if the space is used only for storage. But the floor of an unfinished attic is the ceiling of a finished living space below, and even if the common floor/ceiling is insulated, oven-like temperatures of 125 degrees Fahrenheit and higher can radiate to the living space and increase the load on air conditioning.
However, reducing attic temperatures to increase cooling efficiency is only one reason for ventilating an attic. You also need attic airflow to carry away moisture that rises from the living space. Even if the attic floor is insulated and protected with a vapor retarder (commonly 6-mil polyethylene sheeting), some of the moisture produced in cooking, washing, and other household operations inevitably seeps into cracks and seams. Again, that may not seem like a problem if the space isn't lived in. But moisture collecting in the attic can condense on wood framing to foster rot, and drip puddles onto insulation and ceiling drywall.
To prevent these problems, the best approach is to treat unfinished attics as outdoor space. Wrap the living areas below with layers of insulation and vapor barriers, but flood the attic with fresh air. There are several ways to provide thorough ventilation. Most of the systems are easiest to install on new construction, where they can be conveniently woven into weatherproof layers of shingles and siding. But the three main options of soffit vents, as well as roof vents, gable vents, and ridge vents also can be installed on existing homes.
While gable vents do not involve dismantling the roof, they do require cutting through your home's siding and the underlying plywood sheathing.
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