How do you fix attic ventilation problems?
Short answer. Fixing attic ventilation means matching the symptom to its cause: open or add intake when the soffits are blocked, remove extra exhaust types when vents short circuit, reroute bath and dryer exhaust that dumps into the attic, and add high exhaust only when the intake is already adequate. Air sealing the ceiling is often the most important fix of all.
Problem and fix
- Insulation blocking the soffits (no daylight at the eaves): pull it back and install rafter baffles at every bay with a soffit vent.
- Solid soffits (no intake at all): cut in soffit vents, swap to vented panels and open the wood behind them, or add edge intake vents at a reroof.
- Mixed exhaust (ridge vent plus box vents, gable vents, or a fan): keep one type and close or remove the others.
- Bath fans or a dryer venting into the attic (mildew near the fan, wet insulation): extend the duct to a proper roof or wall cap with a damper.
- Too little exhaust (hot, stagnant upper attic despite good intake): add a ridge vent or more box vents high on the slope.
- Powered fan with weak intake (higher cooling bills, drafts at fixtures): add intake or remove the fan.
- Air leaks from the house (dirty streaks in insulation, condensation on nails): seal top plates, light fixtures, chases, and the hatch.
- Dead zones around dormers and additions: add baffles and vent channels, or vent those sections separately.
Order of attack
- Stop moisture sources and roof leaks.
- Air seal the ceiling.
- Restore intake.
- Simplify and size the exhaust.
- Verify with a look at the decking and a few weeks of attic temperature and humidity readings.
If the roof is near replacement, do the exhaust and intake upgrades then, when they cost the least. See roof ventilation basics and attic heat and shingles.