A ridge vent with
nothing feeding it
is a hole in your roof

Intake and exhaust, in balance

The rule of thumb the codes are built on is one square foot of net free ventilating area for every 300 square feet of attic floor when the intake and exhaust are split roughly evenly, high and low. The number that matters is net free area, not the size of the hole — a vent's screen and louvers cut its actual opening down considerably, and it is printed on the product.

Split it about half and half. If anything, err towards more intake than exhaust, never the other way round.

What goes wrong, nearly every time

Somebody installs a long ridge vent, because ridge vent is easy to sell and easy to fit. Nobody checks the soffit. Perforated soffit panels are fitted, they look right from the driveway, and behind them the original plywood soffit board is solid with no holes cut in it at all — or there are holes, and the attic insulation has since been blown right over them.

Now the ridge vent is still pulling. With no air available at the eave, it draws it from the path of least resistance instead: the gaps round your recessed lights, the attic hatch, the top plates of your walls. You are exhausting conditioned air out of the house through the roof, all summer, and paying to cool the attic.

The fix is not a bigger ridge vent. It is cutting real intake into the soffit and fitting baffles at each rafter bay so the insulation cannot slump back over it.

Do not mix exhaust types on one plane

A ridge vent and a set of box vents on the same roof plane, or a ridge vent and a powered fan, short-circuit each other. The high vent pulls its air from the nearest opening, which is now the other vent a few feet below it, rather than from the eave. The bottom two-thirds of the attic stops moving air at all, and in wind-driven rain the lower vent can be pushed into taking water. When we cut a ridge vent, the old box vents get removed and the deck patched, not left in place as a belt and braces.

Why the shingle manufacturer cares

Read the exclusions on a shingle warranty and inadequate ventilation is on the list. A shingle is warranted on the assumption the deck beneath it can lose heat and moisture. Cook it from below all summer and the mat ages faster than the printed number, and the first thing a manufacturer's inspector counts if a claim is ever made is the intake vents. Ventilation is the cheapest way to protect a warranty you have already paid for.

The bathroom fan

Half the ventilation calls we go to are really this. A bathroom or dryer fan ducted into the attic space instead of through the roof or the wall, pumping warm wet air at the underside of the deck every morning. You get rusty nail points, damp insulation, a musty attic, sometimes mold on the sheathing, and a homeowner convinced the roof leaks. It does not. The duct needs to go somewhere, and that is a two-hour job with a proper roof jack and a boot.

What we check

Attic floor area, actual net free area at the ridge, actual net free area at the eave — which means going out and looking behind the soffit, not counting panels — whether the baffles exist, whether the insulation is blocking the bays, whether exhaust types are mixed, and where every fan in the house terminates.