What are the attic ventilation code requirements?
Short answer. For homes, attic ventilation rules come from the International Residential Code, which Georgia uses as the base for its residential code: 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 exhaust high in the attic. Vents must be screened against weather and pests, and insulation cannot block them.
Here is what a building inspector or plans reviewer is generally looking for. Your local Georgia building department enforces the edition and amendments it has adopted, so confirm details there.
The checklist
- Cross ventilation: each separate enclosed attic or rafter space needs its own ventilation.
- Minimum area: 1/150 of the vented space, or 1/300 with 40 to 50 percent of the required area in upper vents no more than 3 feet below the ridge (measured vertically) and the balance low in the attic.
- Opening size: vent openings larger than 1/4 inch must be covered with corrosion resistant mesh, perforated vinyl, or similar material with openings between 1/16 inch and 1/4 inch, to keep out rain, snow, birds, and rodents.
- Insulation clearance: blocking, bridging, and insulation cannot obstruct airflow, with at least a 1 inch space between insulation and roof sheathing at the vent.
- Manufacturer instructions: vents must be installed as the maker specifies.
- Exhaust fans: bath, kitchen, and dryer exhaust must go outdoors, never into the attic, a soffit, or a ridge vent.
- Attic access: attics of about 30 square feet or more with at least 30 inches of height need an access opening of at least 22 by 30 inches.
The unvented alternative
IRC R806.5 allows an unvented attic if specific conditions are met, typically air impermeable insulation such as spray foam directly against the underside of the roof deck. It is a legitimate design, but it has to be done as a whole system, not by simply closing vents.
Shingle warranties usually expect ventilation that meets code as well. See roofing permits in Georgia and roof ventilation basics.