Attic ventilation that is all exhaust and no intake will:

Attic ventilation is a balanced system: intake low at the soffits, exhaust high at the ridge, with air moving between them by buoyancy and wind. Remove the intake and the exhaust openings do not stop working — they simply find their air somewhere else, and the nearest available source is the conditioned house below, through recessed lights, top plates, attic hatches and duct penetrations. The attic then depressurises relative to the living space and pulls heated, humid indoor air up into a cold assembly, where the moisture condenses on the underside of the sheathing. So an unbalanced system does not merely underperform; it converts the attic from a space that dries the roof into one that wets it, and it drives up heating cost at the same time. Balance is the point, not total vent area.

Exhaust openings must draw air from somewhere, and with no soffit intake the nearest source is the house below through ceiling penetrations. That pulls warm humid indoor air into the attic, where it condenses on cold sheathing.

Attic ventilation relies on balanced intake at the eaves and exhaust at or near the ridge. Blocked or absent soffit intake causes the attic to draw replacement air from the conditioned space through ceiling air leaks, carrying moisture into the assembly.

Exhaust Needs Intake, or the house becomes the intake. Air leaving the ridge is replaced from somewhere — make it the soffits, not the living room.

Blocked soffits are more common than absent ones — insulation stuffed into the eaves without baffles produces the same result. Check for baffles while you are in the attic.

• •Judging ventilation by total vent area rather than balance
• •Overlooking insulation blocking soffit intake
• •Treating an exhaust-only attic as merely less efficient rather than harmful