How attic ventilation works.
A vented attic works a lot like a chimney. Cool outside air enters low, through intake vents in the soffits or at the eaves, and warm, moist air leaves high, through a ridge vent or box vents near the peak. That steady flow keeps the underside of the roof deck close to the outdoor temperature in winter and carries away the moisture that rises out of the house all year long.
When the balance is off, a Minnesota winter finds it fast. Too little intake, often because insulation was blown over the soffit vents, starves the whole system. Mixing different exhaust types on one roof can short-circuit the airflow and pull air from the wrong place. Bath fans and dryer vents that end in the attic instead of outdoors pump in moisture that freezes on nail tips and sheathing. The results are ice dams at the eaves, frost that melts into a "leak" on the first warm day, and shingles that blister or curl years ahead of schedule.
Ventilation does not replace attic insulation and air sealing. The two work together: insulation keeps the heat in the house, and ventilation clears out whatever gets past it. When we look at an attic, we tell you honestly which fix your house needs first.
