Short answer: the common standard is one square foot of net free vent area per 150 square feet of attic floor, split roughly half intake at the soffits and half exhaust at the ridge. With a proper vapor retarder, 1 per 300 may apply. Balance matters more than total.
Running the number
Take your attic floor area. Divide by 150. That is your total required net free vent area in square feet. Split it half and half between intake and exhaust.
A 2,000 square foot attic needs roughly 13.3 square feet of net free area total, so about 6.7 at the soffits and 6.7 at the ridge.
Net free area is the actual open area after screening and louvers, not the size of the hole. Vent products publish it, and it is always less than the gross opening.
Balance is the part that matters
Exhaust without intake does not ventilate. If there is more exhaust than intake, the system pulls air from the easiest available source, which is conditioned air leaking up from your house through ceiling gaps. You end up paying to cool air that exits through the ridge.
If anything, intake should slightly exceed exhaust. The opposite is the common defect.
How to check your own attic
- Look at the soffits from outside. Are there vents, and do they run the length of the eave or appear occasionally?
- Look at the soffits from inside the attic. This is where the problem usually is. Insulation pushed into the eaves blocks intake completely, and it is invisible from outside.
- Check for baffles, the channels that hold insulation back from the eave. Many homes do not have them.
- Identify the exhaust. Ridge vent, turbines or box vents, and whether more than one type is mixed.
- Feel the temperature on a hot afternoon. A properly ventilated attic is hot. One at 140 plus with still air is not ventilating.
Symptoms of a shortfall
- Upstairs noticeably hotter than downstairs
- AC running continuously in the afternoon
- Shingle blistering or premature aging on the roof above
- Rusted nail tips in the attic, which indicates condensation
- Ice damming in a freeze
Do not mix exhaust types
Ridge vent plus turbines lets one short circuit the other, pulling air in through the turbine rather than from the soffits. Pick one exhaust strategy. Our ventilation comparison covers the options.
Fix it during a replacement
Ventilation correction during a re-roof is inexpensive, because the roof is open and soffits are accessible. Retrofitting later costs several times more for the same result. If you are getting quotes, ask specifically whether intake is being assessed and corrected.
Get a free inspection that includes the attic, or call (210) 245-4125.
