Attic & Roof Ventilation Calculator
U.S. Department of Housing and Urban Development Statutes typically require one sq foot of net free vent area per 150 square feet of attic area to be vented (1/150 rule).
Certain scenarios change the requirement to one sq foot of net free vent area per 150 square feet of attic area to be vented (1/150 rule).
The scenarios allowing for the change are summarized as follows:
- No less than 40% & no more than 50% of the required free air space is placed in the upper portion of the attic with the remainder being found in the lower (eave) And/or
- Included in the ceiling assemby on the warm in winter side is a Class I or II vapor retarder in climate zones 6, 7, and 8.