How do you calculate attic ventilation per code?
Short answer. Per the International Residential Code, calculate attic ventilation by dividing the attic floor area by 150 to get the minimum net free area; you may divide by 300 instead only if 40 to 50 percent of that area is in upper vents no more than 3 feet below the ridge, with the rest in the lower part of the attic. Then confirm the vents are screened, installed per the manufacturer, and not blocked by insulation.
Step 1: decide which ratio you may use
- 1/150 is the default.
- 1/300 is allowed when the upper vents provide 40 to 50 percent of the required area and sit within 3 feet of the ridge or highest point, measured vertically, with the balance in the lower part of the attic.
- In Climate Zones 6, 7, and 8 the code also ties the 1/300 option to a vapor retarder in the ceiling. Georgia is in warmer zones, so that condition does not apply here.
Step 2: run the numbers
Example: a 1,600 square foot attic with a ridge vent and soffits.
- 1,600 divided by 300 is about 5.33 square feet, or 768 square inches.
- Upper vents must supply 40 to 50 percent: between about 307 and 384 square inches.
- Lower vents supply the rest: between about 384 and 461 square inches.
If the exhaust were box vents placed halfway down the slope, they would not count as upper vents, and the attic would need 1,536 square inches at 1/150.
Step 3: check the other code items
- Openings larger than 1/4 inch protected with screening that has openings between 1/16 and 1/4 inch.
- At least 1 inch of air space between insulation and the roof sheathing at eave vents.
- Vents installed per the manufacturer's instructions.
- Each separate enclosed attic or rafter space ventilated on its own.
Step 4: confirm the local edition
Georgia's residential code is based on the IRC with state amendments, and local building departments enforce the edition currently adopted. Check with yours if a permit is involved. See roofing permits in Georgia and roof ventilation basics.