ATTIC AIRFLOW & CODE COMPLIANCE

Roof Ventilation Calculator

Calculate balanced attic Net Free Vent Area (NFVA) for soffit intake and ridge exhaust under IRC §R806 standards.

Roof Ventilation Calculator

Calculate balanced attic airflow requirements (Net Free Vent Area) under IRC §R806 standards.

sq ft

Use horizontal ceiling/attic square footage (length × width).

IRC §R806 Ventilation Formula

$$text{Total Required NFVA (sq ft)} = frac{text{Attic Floor Area}}{text{Code Ratio (150 or 300)}}$$

$$text{Intake NFVA (sq in)} = text{Total NFVA (sq in)} times 50%, quad text{Exhaust NFVA (sq in)} = text{Total NFVA (sq in)} times 50%$$

Under IRC §R806, attics require 1 sq ft of Net Free Vent Area for every 300 sq ft of attic floor space when balanced between lower eave intake and upper ridge exhaust.

Building Code Ventilation Rules (2021 & 2024 IRC §R806.2): The model code establishes a baseline minimum net free ventilating area of 1:150 (1 sq. ft. of net free area per 150 sq. ft. of vented attic floor area). The code permits a reduction to 1:300 under specific conditions:
  • Climate Zones 1 through 5: The 1:300 reduction applies provided that not less than 40% and not more than 50% of the required ventilating area is provided by ventilators located in the upper portion of the space to be ventilated (at least 3 feet above eave or cornice vents), with the remaining balance provided by eave or cornice vents.
  • Climate Zones 6, 7, and 8: The upper/lower distribution condition (40% to 50% in the upper portion) applies, and a Class I or Class II vapor retarder is installed on the warm-in-winter side of the ceiling.