Attic Ventilation

Depending on house geometry, an unvented attic can have 10% to 60% more square feet of surface area needing insulation than a vented attic. This larger area is due to the fact that the total area of a sloped roof plus gable and dormer walls is greater than the area of the attic floor. This directly affects not only cost, but also the efficiency of the home. All else being equal, an envelope assembly with 10% to 60% more area will have 10% to 60% more heat losses and gains, resulting in more load on the HVAC system.

Vented attics can reduce heat gains to the conditioned space by ventilating and exhausting heat from the attic. The insulation on an attic floor is essentially "shaded" from the sun by the roof above it. Attics, though hot in summer, are not as hot as the roof above them.