What are the signs of poor ventilation in an attic, and how do you fix it?
Short answer. Signs of poor attic ventilation include a stifling attic and hot upstairs rooms in summer, frost or dripping under the roof deck in winter, dark stained or delaminating plywood, rusty nails, musty smells and shingles that blister or age early. The fix usually starts with clearing or adding intake at the eaves, then matching one type of exhaust near the ridge.
Signs you can check yourself
- In the attic: black or gray staining on the deck, especially near the top; rust on nail tips; damp or matted insulation; a sour, musty smell.
- In the house: second floor rooms that never cool, an air conditioner that runs constantly in July, peeling ceiling paint near exterior walls.
- On the roof: blistered shingles, curling on some slopes but not others, or a wavy deck line.
- At the eaves: soffit vents painted over, clogged with dust and webs, or vented vinyl soffit installed over solid wood with no openings cut behind it.
Common causes
The most frequent cause is missing intake. Insulation blown into the eaves smothers the soffit vents, so the exhaust has nothing to pull from. Next is mixed exhaust, such as a ridge vent combined with gable vents or a powered fan, which short circuits the airflow. Third is moisture coming from the house, such as bath fans that dump into the attic.
Fixes in order of priority
- Route every bath and kitchen fan to the outdoors through its own cap.
- Install baffles at the eaves and pull insulation back so air can enter.
- Open up or add soffit or edge intake.
- Choose a single exhaust type near the ridge, sized to the intake.
- Air seal the attic floor around lights, the hatch and plumbing penetrations.
Steps 3 and 4 are most economical during a reroof. Learn more in attic heat and shingles and heat, humidity and roof life.