Attic Ventilation: Net Free Area, the 1-to-300 Rule, and Intake vs. Exhaust Balance
October 21, 2026
A ventilated attic helps remove moisture in winter, preventing condensation, mold, and rot, and reduces heat buildup in summer, which can lower cooling costs and extend the life of roof shingles. Ventilation is specified as net free area (NFA), the actual open area through which air can flow after accounting for screens and louvers.
A widely used code rule is 1 square foot of NFA for every 150 square feet of attic floor area, which can be relaxed to 1 for every 300 square feet if a vapor retarder is installed on the warm side of the ceiling or if at least 40% to 50% of the ventilation is located high in the attic (near the ridge) with the balance low (in the eaves). For a 1,500-square-foot attic floor, the 1:300 rule requires 5 square feet, or 720 square inches, of NFA in total.
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Step 1: find the attic floor area, usually the same as the ceiling area below, such as 1,500 square feet. Step 2: divide by 300 (or 150) to get the required square feet of NFA: 5 (or 10). Step 3: convert to square inches by multiplying by 144: 720 (or 1,440). Step 4: split roughly half to intake (low, at the soffits) and half to exhaust (high, at the ridge): 360 square inches each for the 1:300 rule.
Check the manufacturer's NFA rating on every vent product. A standard 16-by-8-inch soffit vent may have only 50 to 65 square inches of NFA, not the 128 square inches of its overall face area. To supply 360 square inches of intake at 60 square inches per vent, you need 6 vents; a continuous soffit vent strip may offer 9 to 12 square inches of NFA per linear foot, so 30 to 40 feet would do the job.
| Attic floor area | Required NFA at 1:300 | In square inches | Intake (half) | Exhaust (half) |
|---|---|---|---|---|
| 1,000 sq ft | 3.3 sq ft | 480 | 240 | 240 |
| 1,500 sq ft | 5.0 sq ft | 720 | 360 | 360 |
| 2,000 sq ft | 6.7 sq ft | 960 | 480 | 480 |
| 2,500 sq ft | 8.3 sq ft | 1,200 | 600 | 600 |
Balance Intake and Exhaust
Ventilation works by letting cooler air in low and allowing warm air out high, driven by the stack effect and wind. If exhaust capacity exceeds intake, the attic pulls conditioned air from the house through ceiling leaks or draws rain and snow in through the exhaust vents. A good rule is to have equal or slightly more intake than exhaust. Make sure that soffit vents are not blocked by insulation: install baffles (rafter vents) that maintain an air channel from the eave up into the attic.
A ridge vent combined with continuous soffit vents provides smooth, even airflow along the whole roof. Mixing different types of exhaust vents, such as a ridge vent and gable fans, can short-circuit airflow.
Signs of Poor Ventilation
In winter: frost on the underside of roof sheathing, damp insulation, mold, rusty nails, and ice dams at the eaves. In summer: an attic that is much hotter than the outdoor air (temperatures of 140°F or higher), blistering or curling shingles, and high upper-floor temperatures. Ice dams are caused by heat leaking from the house melting snow on the roof; air sealing and insulation are as important as ventilation in preventing them.
Air Sealing Comes First
Ventilation cannot compensate for a leaky ceiling. Seal gaps around wiring, plumbing penetrations, recessed lights, and attic hatches before adding insulation. Good air sealing reduces the moisture load entering the attic and reduces heating and cooling energy use. Unvented (conditioned) attics with spray foam on the underside of the roof deck are an alternative design with different requirements, so follow local code for that approach.
Frequently Asked Questions
How much attic ventilation do I need?
Typically 1 sq ft of net free area per 300 sq ft of attic floor (or 150 if the conditions for 300 are not met).
What is net free area?
The actual open area of a vent after accounting for screens and louvers.
Should intake and exhaust be equal?
They should be balanced, with at least as much intake as exhaust.
Do I need baffles?
Yes, where insulation could block the soffit vents.
Can ventilation alone prevent ice dams?
No. Air sealing and insulation to keep the roof cold are essential.