New 30 by 50 house with a vented attic, soffit vents and a ridge vent.

Code requirement (IRC): For a vented attic you need net free vent area (NFVA) of 1 ft² per 150 ft² of attic floor (IRC M1602.3). The 1/300 figure (1 ft² per 300 ft²) is only allowed if both of these are true:

1. The total NFVA is split 50 % high, 50 % low (e.g., soffit + ridge), and
2. The low vents are protected with an approved air-permeable screen (typically 1/8 in. hardware cloth or equivalent) to keep out rodents and debris.

Your 30 × 50 house has a 1,500 ft² attic floor.

| Option | NFVA required | |--------|---------------| | Standard 1/150 | 1,500 ÷ 150 = 10 ft² total | | 1/300 (if balanced & screened) | 1,500 ÷ 300 = 5 ft² total |

So you can use the smaller 1/300 number only if your soffit vents are screened and you maintain the 50/50 high-to-low split.

Quick check for your setup:

• Ridge vent (high) + soffit vents (low) = balanced.
• Make sure soffit vents have 1/8 in. screening behind them.
• Verify the listed NFVA of your ridge vent and soffit vents; some products under-rate in the field.

If either condition is missing, you must meet the full 1/150 (10 ft² total).

Code Minimum (IRC R806.1):

• 1/150 rule (Default): 1 sq. in. NFA per 150 sq. ft. of attic floor area.
• 1/300 rule (Exception): Allowed only if both conditions are met: 1. Climate Zone 6, 7, or 8 (cold climates). 2. Class I or II vapor retarder installed on the warm-in-winter side of the ceiling (e.g., polyethylene sheeting or kraft-faced batts).

Your Calculation:

• Attic Area = 30 ft × 50 ft = 1,500 sq. ft.
• 1/150 Rule: 1,500 ÷ 150 = 10 sq. ft. (1,440 sq. in.) Total NFA required.
• 1/300 Rule: 1,500 ÷ 300 = 5 sq. ft. (720 sq. in.) Total NFA required.

Balancing Requirement (IRC R806.2): You must split NFA roughly 50/50 between intake (soffit) and exhaust (ridge).

• At 1/150: 720 sq. in. intake / 720 sq. in. exhaust.
• At 1/300: 360 sq. in. intake / 360 sq. in. exhaust.

Can you use 1/300? Likely no. Most of the US (Zones 1–5) requires 1/150. Even in Zones 6–8, you need a verified, continuous Class I/II vapor retarder on the ceiling plane—rare in modern construction without poly. *Default to 1/150 unless you are in a cold zone and have the vapor retarder.*

Product Check: Verify NFA ratings on your specific vent boxes.

• Typical Ridge Vent: ~18 sq. in./lin. ft. → Need 40 lin. ft. (1/150) or 20 lin. ft. (1/300).
• Typical Continuous Soffit Vent: ~9–12 sq. in./lin. ft. → Need 60–80 lin. ft. (1/150) total (both eaves combined).

Pro Tip: Install baffles (rafter vents) at every bay to prevent insulation from blocking soffit intake.