How much ventilation does an attic need, and why does an attic need ventilation?
Short answer. A vented attic needs at least 1 square foot of net free vent area for every 150 square feet of attic floor, or 1 square foot per 300 square feet when 40 to 50 percent of the vents are exhaust located high in the attic and the rest are low intake. It needs that ventilation to carry out the moisture that rises from the house and the heat that builds under the roof.
Why an attic needs ventilation
- Moisture: everyday living puts water vapor into the air, and some of it leaks into the attic. In winter it can condense on the cold roof deck, causing mildew, rusted nails, and weakened decking.
- Heat: Georgia summers bake roofs. Ventilation moves some of that heat out, which reduces stress on shingles and on ducts in the attic.
- Code and warranty: building codes set the minimum, and shingle manufacturers expect it for full warranty coverage.
Worked example: a 1,400 square foot attic
- At 1/150: 1,400 divided by 150 is about 9.3 square feet, or about 1,344 square inches of net free area.
- At 1/300: about 4.7 square feet, or 672 square inches, split into about 336 intake and 336 exhaust.
Which ratio applies to you
Most homes with soffit intake and a ridge vent or high box vents qualify for the 1/300 ratio. Homes with gable vents only, or with exhaust well down the slope, should use 1/150. In Georgia's climate zones, the vapor retarder condition the code applies in very cold regions does not come into play.
More is not automatically better
Balance matters more than extra area. Many manufacturers recommend intake equal to or greater than exhaust, and one type of exhaust per attic. Sealing air leaks in the ceiling reduces the moisture the ventilation has to handle in the first place. See roof ventilation basics and heat, humidity, and roof life.