Attic Ventilation Troubleshooting: Moisture, Ice Dams, and Intake vs Exhaust

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Moisture and Mold on Rafters

Black staining on the north roof deck, fuzzy growth on rafter sides, rusty nail tips poking through the sheathing: winter moisture, not a leak from last Tuesday’s rain (those leave a drip path). Indoor air hit a cold surface. Ventilation helps only if the ceiling is not a chimney of can lights and an unsealed hatch. If the attic smells like a basement in January, check intake first — stuffed soffits are more common than a missing ridge vent.

A hygrometer hung from a rafter tells you whether the space is actually drying after a cold night. Relative humidity stuck high while it is 20°F outside is a ventilation-plus-air-leak problem, not a reason to add a second gable louver next to a ridge vent.

Ice Dams in Winter

Ice at the eaves means the attic was warm enough to melt snow on the deck while the overhang stayed below freezing. Ventilation is one control: keep the attic cold by washing outdoor air along the underside of the sheathing. The other controls are air sealing the ceiling and putting enough insulation above it so heat from the rooms never reaches the deck. A ridge vent on an attic with zero soffit intake and a leaky hatch will not stop dams. Neither will a turbine spinning over a stuffed eave. The membrane that keeps ponded melt off the ceiling is a different measurement: ice & water shield coverage calculator.

Premature Shingle Aging From Heat Buildup

Granule loss on the field, curled tabs, and a deck that feels like a griddle in May are heat and UV, amplified by a stagnant attic. Exhaust high only works if intake is open. Adding a powered fan to a blocked-soffit attic pulls conditioned air from the house and can void shingle warranties. Size NFA with the calculator, then clear the baffles (R806.3 wants an air space at the eave) before you add more holes in the cap.

Diagnose Intake Shortfall vs Exhaust Shortfall

Walk it in this order:

1. Look at the soffits from the yard. Perforated vinyl that got painted shut, foam stuffed into the eave, or insulation covering the intake from inside is an intake shortfall. Ridge vent will whistle and do nothing useful.
2. Look at the ridge and gables from the ground, then from the hatch. A short run of ridge on a long building, or gable vents on a hip roof with no ridge, is an exhaust shortfall. Hip roofs often need more thought than a simple gable.
3. Do not mix exhaust types. Ridge plus gable, or ridge plus turbines, is the classic short-circuit. Air takes the nearest pair of openings. The far bays stay wet and hot. Pick one exhaust method; size intake to match.
4. Count NFA, do not count holes. Three gable vents that look huge can still lose to 40 ft of ridge at 18 in²/ft once you read the stamps. Run attic floor area through the calculator, then divide each side by the NFA of the product that is actually installed.
5. Check the ceiling plane. A perfect 1:150 attic with an unsealed whole-house fan lid still sweats. Air sealing is not optional diagnostics.

Shingle quantity problems (short a square, skipped pitch) are a different page: roofing estimating troubleshooting.

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