International Code Council
IRC R806.2
Attic ventilation — minimum net free vent area
How much ventilating opening a vented attic must have, and the narrow conditions under which that requirement can be halved.
What does the test involve?
The minimum net free ventilating area is 1/150 of the area of the vented space. An exception permits 1/300 only when BOTH conditions are met: in Climate Zones 6, 7 and 8 a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling; and not less than 40 percent and not more than 50 percent of the required ventilating area is provided by ventilators in the upper portion of the attic, not more than 3 feet below the ridge, with the remainder in the lower third.
| Vented attic area | At 1/150 | At 1/300 (exception only) |
|---|---|---|
| 1,000 sq ft | 960 sq in | 480 sq in |
| 1,500 sq ft | 1,440 sq in | 720 sq in |
| 2,000 sq ft | 1,920 sq in | 960 sq in |
| 2,500 sq ft | 2,400 sq in | 1,200 sq in |
Common misconceptions
- The 1/300 figure is an exception with two simultaneous conditions, not a free choice between two ratios. Most consumer pages present it as a choice. It is not.
- Exhaust without intake does not ventilate an attic — it short-circuits, pulling air from the nearest exhaust vent instead of from the soffits. Balanced intake and exhaust is the whole point.
- Adopted IRC edition varies by state and often by county. Confirm which edition applies where you are before treating any of this as binding.
Sources (3)
- UpCodes (2021 IRC R806.2) — Attic ventilation — minimum net free vent area (accessed 2026-08-17)
- International Code Council — 2021 IRC R806.2 — Minimum vent area (accessed 2026-08-17)
- IIBEC — Attic Ventilation 101 (accessed 2026-08-17)