Should an attic have ventilation, and what is attic ventilation?
How it moves air
Two natural forces do the work. The stack effect: warm attic air rises and leaves through high vents, pulling cooler air in at the eaves. The wind effect: wind passing over the roof creates low pressure at the ridge and on the downwind side, drawing air out. Powered fans are optional extras, not the foundation.
How much ventilation
The residential code sets net free area at 1/150 of the attic floor, or 1/300 when the system is balanced between high and low vents. A 1,200 square foot attic in a balanced system needs about 576 square inches, roughly 288 of intake and 288 of exhaust. That is only about 4 square feet of real opening, which is why vents must be well placed and unobstructed.
How to get it
- Keep intake clear, with open soffit vents and baffles in each rafter bay.
- Choose one exhaust type near the ridge.
- Route bath and kitchen fans outdoors.
- Seal air leaks in the ceiling so house moisture stays downstairs.
What it costs
Ventilation is usually a small part of a roof replacement budget when it is handled during the project. Standalone retrofits cost more per vent because of setup and shingle repairs. Get line items and the vent math in writing. Read roof ventilation basics and heat, humidity and roof life.