How do you calculate attic ventilation, starting with the attic space?
Short answer. Start by measuring the attic floor area, which is usually the footprint of the ceiling below the attic, not the roof surface; break odd shapes into rectangles and add them up. Then divide that area by 300 (or 150), multiply by 144 for square inches, and split the result between intake and exhaust.
Measuring the attic space
- Simple rectangle: length times width of the ceiling below the attic. A 36 by 60 foot ranch has a 2,160 square foot attic.
- L or T shaped homes: divide the footprint into rectangles, measure each, and add them.
- Overhangs: leave out the eave overhangs; they are outside the attic floor.
- Garage attics: include them if they connect to the main attic, or calculate them separately if a firewall or separate roof divides them.
- Bonus rooms and cathedral ceilings: the sloped ceiling sections need their own vent channels, and the small attics around them are calculated separately.
- From the outside: measuring the exterior walls is a good approximation for a single story house. For two story homes, use the top floor footprint plus any lower roof sections that have their own attic.
Applying the ratio
Using the 2,160 square foot example:
- At 1/300: 7.2 square feet, or about 1,037 square inches, split into about 518 intake and 518 exhaust.
- At 1/150: about 2,074 square inches, if the exhaust cannot be placed high on the roof.
Common mistakes
- Using the roof area from a roofing estimate. Roof area is larger than attic floor area because of slope and overhangs, which overstates the need.
- Treating several disconnected attic spaces as one.
- Forgetting that existing vents may be blocked or painted over, so their rated area is not real.
Once you have the numbers, compare them with the vents you have. See roof ventilation basics and how roofers measure a roof.