How do you vent an attic properly, and what mistakes should you avoid?

Mistake 1: intake buried in insulation

Blown insulation often fills the eaves. Install rafter baffles and keep at least a 1 inch air space between insulation and the roof deck at each vent.

Mistake 2: two or more exhaust types

A ridge vent plus box vents, a turbine, a power fan, or open gable vents lets air loop between exhaust openings. Choose one and close the rest.

Mistake 3: exhaust without intake

Adding ridge vents or fans to an attic with solid soffits pulls air from the house through ceiling gaps. Fix intake first.

Mistake 4: exhaust fans dumping into the attic

Bath fans, kitchen hoods, and dryers must vent outdoors through a proper cap. The code specifically prohibits discharging exhaust into an attic, soffit, or ridge vent.

Mistake 5: vented panels over solid wood

New perforated vinyl soffit installed over old plywood looks vented but is not. Open the wood behind the vented panels.

Mistake 6: forgetting separate spaces

Attics over garages, additions, and knee wall areas may not connect to the main attic. Each needs its own intake and exhaust.

Mistake 7: guessing the amount

Calculate net free area from the attic floor area: 1/300 with balanced, high exhaust, or 1/150 otherwise. Use the ratings printed on the vents.

Mistake 8: venting a sealed attic

If spray foam was applied to the underside of the roof deck, the attic is designed to be unvented. Leaving vents open undermines that design.

For the fundamentals, see roof ventilation basics and ridge vent vs box vents.