What do building regs say about roof ventilation?
Short answer. Under the International Residential Code, which Georgia uses with state amendments, a vented attic needs net free vent area of at least 1/150 of the attic floor area, or 1/300 when 40 to 50 percent of it is in upper vents near the ridge and the rest is low at the eaves. The code also sets screen opening sizes and requires an air space above the insulation at the vents.
The numbers in plain terms
- Base ratio: 1 square foot of net free vent area for every 150 square feet of attic floor (IRC Section R806.2).
- Reduced ratio: 1 for every 300 square feet when 40 to 50 percent of the required area is in upper vents no more than 3 feet below the ridge, with the balance at the eaves. A vapor retarder condition in the same exception applies only to very cold climate zones, not Georgia.
- Every space counts: each separately enclosed attic or rafter space needs its own cross ventilation.
- Openings: at least 1/16 inch and no more than 1/4 inch in their smallest dimension, or covered with screening, hardware cloth or perforated material in that range (R806.1).
- Air space: at least 1 inch between the insulation and the roof sheathing at the vent, which is what eave baffles provide (R806.3).
A worked example
A 1,800 square foot attic at 1/300 needs 6 square feet, or 864 square inches, of net free area: roughly 432 square inches of exhaust up high and 432 of intake down low. Net free area is the open area rated by the vent maker and printed on the product, not the vent's outside size, so a vent that looks big may count for much less.
The other option
The code also allows unvented attics (R806.5), where insulation such as spray foam goes against the underside of the roof deck and the attic becomes part of the conditioned space. That is a whole assembly with its own requirements, not a fix for a poorly vented attic.
How the pieces fit together is in roof ventilation basics, and why it matters for shingle life is in attic heat and shingles.