How do you properly ventilate an attic?
Principle 1: size it
Net free vent area should be at least 1/150 of the attic floor area, or 1/300 when 40 to 50 percent of it is exhaust within 3 feet of the ridge. For a 1,500 square foot attic on the 1/300 path, that is about 720 square inches in total.
Principle 2: balance it
Split the area roughly evenly between intake and exhaust, with intake equal to or greater than exhaust. Exhaust that outruns intake pulls air from the house.
Principle 3: one exhaust type
Ridge vents, box vents, gable vents, turbines, and fans should not be mixed on one attic. Two exhaust types short circuit, with air flowing between them instead of in from the soffits.
Principle 4: keep the path open
- Install baffles at each soffit bay so insulation cannot block intake.
- Maintain at least a 1 inch air space between insulation and the roof deck at the eaves.
- Make sure vented soffit panels are not covering solid wood.
- Vent separate attic spaces, like those over additions, on their own.
Principle 5: no exhaust into the attic
Bath fans, kitchen hoods, and dryers must vent outdoors through a proper cap. Moist exhaust dumped into the attic overwhelms any ventilation system and causes mildew on the decking.
Plus: seal the ceiling
Air sealing top plates, light fixtures, plumbing chases, and the attic hatch reduces the moisture ventilation has to remove and keeps conditioned air in the house. See roof ventilation basics and heat, humidity, and roof life.