Reference

Attic Ventilation NFA Table

How much net free vent area an attic needs under the code's two rules, and what typical vents deliver.

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The rules

The International Residential Code (R806.2) requires a net free ventilating area of 1/150 of the attic floor area, or 1/300 when 40 to 50 percent of the vent area is exhaust in the upper attic (at least 3 ft above the eave vents), or when a Class I or II vapor retarder is installed on the warm side of the ceiling in climate zones 6 to 8. Net free area (NFA) is the open area after screens and louvres.

NFA (sq in) = attic floor area (sq ft) ÷ 150 (or 300) × 144

Required NFA by attic size

Attic floor 1:150 total 1:300 total 1:300 intake 1:300 exhaust
600 sq ft 576 sq in 288 sq in 144 sq in 144 sq in
800 sq ft 768 sq in 384 sq in 192 sq in 192 sq in
1,000 sq ft 960 sq in 480 sq in 240 sq in 240 sq in
1,200 sq ft 1,152 sq in 576 sq in 288 sq in 288 sq in
1,500 sq ft 1,440 sq in 720 sq in 360 sq in 360 sq in
1,800 sq ft 1,728 sq in 864 sq in 432 sq in 432 sq in
2,000 sq ft 1,920 sq in 960 sq in 480 sq in 480 sq in
2,500 sq ft 2,400 sq in 1,200 sq in 600 sq in 600 sq in
3,000 sq ft 2,880 sq in 1,440 sq in 720 sq in 720 sq in

Under 1:150 split the total roughly in half the same way, with at least as much intake as exhaust.

Typical vent ratings

Vent Typical net free area Check
Ridge vent (shingle-over) 12 to 20 sq in per linear ft; 18 is common Printed on the product
Box (low-profile) roof vent 50 to 61 sq in each Printed on the product
Rectangular soffit vent, 16 × 8 in about 50 to 65 sq in each Varies by louvre design
Continuous soffit strip, 2 in about 9 sq in per linear ft Printed on the product
Vented vinyl or aluminum soffit panel about 5 to 10 sq in per sq ft of panel Often much less than it looks
Round soffit plug, 2 in about 2 sq in each Small; many needed

Powered fans

A powered attic ventilator needs about 1 sq ft (144 sq in) of net free intake per 300 CFM of fan capacity. Do not combine one with a ridge vent.

Sources

IRC R806.2 (ventilation ratios and the 1:300 conditions), R806.3 (vent clearances), R806.5 (unvented attic assemblies). Vent ratings from manufacturers’ published net free area data; individual products vary, so use the figure on the product.

Calculators that use this table

The guide attic ventilation and the 1:150 rule explains balance and the mistakes to avoid.