Attic ventilation: the part of your roof nobody looks at

How it is supposed to work

Cool air enters at the soffits, travels up under the roof deck and leaves at the ridge or through roof vents. That keeps the attic close to outdoor temperature year round, which is exactly what you want.

Intake and exhaust need to be roughly balanced, with at least as much intake area as exhaust. Adding more roof vents without open soffits does very little, and can make things worse by pulling conditioned air up out of the house instead of outdoor air in through the eaves.

The general rule of thumb is one square foot of net free vent area for every 300 square feet of attic floor when there is a proper vapour barrier, split about evenly between intake and exhaust.

Signs it is not working

Frost on nail tips or sheathing in winter, which melts during a thaw and drips onto the insulation below. Damp insulation or dark staining on the underside of the deck. Ice dams along the eave. A shingle roof that aged out years earlier than it should have.

In summer, an attic that feels like an oven is the same failure in the other direction. Heat trapped against the underside of the deck cooks the shingles from below and pushes your cooling costs up.

Musty smells in upstairs closets and peeling paint on the top-floor ceiling are indirect signs worth chasing.

Where the moisture comes from

Ordinary living. Showers, cooking, laundry, houseplants and breathing put a surprising amount of water into the air, and warm air holds more of it than cold.

That moist air rises and looks for a way out: pot lights, the attic hatch, plumbing stacks, gaps at interior wall top plates. When it hits the cold roof deck it condenses and freezes.

Bathroom and kitchen fans that vent into the attic instead of through the roof or wall are a common and very fixable cause. So is a dryer duct that has come apart.

The usual fixes

Air seal first, then check attic insulation levels. Sealing the leaks below is more effective than any amount of venting above, and it lowers your heating bill at the same time.

Clear the soffit vents. Insulation pushed tight into the eave blocks them, and baffles hold it back so air can pass. Then add or correct exhaust so it matches the intake.

Do not mix exhaust types on the same attic space. A ridge vent combined with box vents or a powered fan can short-circuit, drawing air in through one and out the other instead of pulling it up from the soffits.

Do it during the reroof

Ventilation work is far cheaper while the roof is already open for a replacement and the crew is on site. Cutting in vents later means disturbing shingles that were just installed.

If a quote does not mention ventilation at all, ask what was found in the attic. Manufacturers can and do deny material warranty claims on roofs that were poorly ventilated, so it is not a small detail.