How can you tell if your attic is vented properly, not just vented?
Short answer. A properly vented attic has intake low at the eaves and exhaust high near the ridge, at least as much intake as exhaust, only one type of exhaust vent, clear baffled paths from soffit to deck, and no bath or kitchen fans dumping into it. An attic can have plenty of vents and still fail several of those checks.
Check 1: balance
Intake should equal or exceed exhaust. Plenty of roof vents with blocked or tiny soffit vents means the attic pulls air from the house.
Check 2: one exhaust type
A ridge vent plus box vents, a turbine, a fan or open gable louvers lets air short circuit between the exhaust vents. Pick one and close the rest.
Check 3: placement
- Exhaust within 3 feet of the ridge, measured vertically.
- Intake at the lowest part of the roof, in the soffit or at the edge.
- Box vents grouped high, not scattered midway down the slope.
Check 4: open paths
- Baffles in the bays over soffit vents.
- At least 1 inch of air space between insulation and the deck at the eaves.
- Holes cut in any plywood behind vinyl soffit.
Check 5: no indoor exhaust in the attic
Bath fans, kitchen hoods and dryers must vent outdoors through their own caps, never into the attic, soffit or ridge vent.
Check 6: screens and openings
Openings should be small enough to keep out pests and large enough not to clog: the code's range is 1/16 to 1/4 inch.
Check 7: results
Clean, dry decking, bright nail tips and insulation that is not damp are the proof. See roof ventilation basics and ridge vent vs box vents.