The basic definition
Free area is the total unobstructed area through a louver, divided by the wall-opening area. Blades, frame, and supports subtract from the numerator. The result is a percentage.
If a 48"×48" (16 sq ft) wall opening has 8 sq ft of unobstructed area after subtracting blades and frame, the free area is 50%.
Why it's not a constant
Free area changes with size. As louvers get smaller, the frame eats a larger portion of the opening — so a 12"×12" louver might be 35% free area even if the same model is 50% at 48"×48". When comparing models, always compare at the same test size (48"×48" is the industry standard).
Free area also changes with options. Bird screens, insect screens, and mullion supports all reduce it. Ask for the value after options when comparing.
Why higher isn't automatically better
Higher free area means more air through a smaller wall opening — which is great. But pushing more air through a smaller opening means higher velocity, which means more water entrainment and more pressure drop. The right number depends on the application:
- Sheltered exhaust: push free area as high as possible. Try E6JN at 69%.
- Coastal intake: moderate free area, high first-point-of-water. E6DP or wind-driven rain models.
- Hurricane zones: free area takes a back seat to wind-load and impact rating.
How to use the number
Multiply your wall-opening area by the free-area percentage to get net free area in square feet. Multiply that by the safe design velocity (75% of first-point-of-water) to get safe CFM through that louver.
Example, E6JN at 48"×48":
- Wall opening: 16 sq ft
- Free area: 69.1% → net free area = 11.1 sq ft
- First point of water: 915 fpm → design velocity (75%) = 686 fpm
- Safe airflow = 11.1 × 686 = 7,615 CFM
Need help? Pick up the phone — this is exactly the kind of math we do every day.