What is proper attic ventilation, and how can you tell if you have it?
Short answer. Proper attic ventilation means enough net free vent area for the attic, roughly 1 square foot per 300 square feet of attic floor when balanced, split about evenly between intake low at the eaves and exhaust high near the ridge, with one type of exhaust and nothing blocking the airflow. You can check most of it yourself with a flashlight and a look at the roof.
A homeowner checklist
- Intake you can see: soffit or edge vents along the eaves, not painted over, and from inside the attic, daylight or a baffle channel at the eave of each bay.
- Exhaust near the top: a ridge vent or box vents within a few feet of the peak.
- One exhaust type: not a ridge vent plus box vents, or a power fan plus a ridge vent. Gable vents closed off if there is a ridge vent.
- Baffles at the eaves: insulation stops short of the roof deck, leaving at least a 1 inch air channel.
- Exhaust fans to the outdoors: bath fans, kitchen hoods and dryers ducted outside, not into the attic.
- Clean, dry deck: no frost, rusty nail tips, dark staining or mildew on the underside of the decking.
- Enough area: the rated vent area adds up to what the attic size requires.
What it feels like
Even a well vented Georgia attic is hot on a summer afternoon, because the sun heats the roof faster than air can carry the heat away. A good system keeps it from getting hotter than it needs to and, just as important, keeps it dry. Moisture is the better test: a dry deck with clean nail tips in February is a very good sign.
Why it matters
Shingle manufacturers require adequate ventilation, and excess heat and moisture shorten roof life, rot decking, and make attic air handlers work harder. Sorting it out during a roof replacement costs far less than doing it later. For the background see roof ventilation basics and heat, humidity and roof life.