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.
Our Calculator is based on the 1/150 rule.