How louvers work.

Free area

Free area is the total unobstructed open area of a louver — the wall opening minus all the blade and frame obstructions — divided by the overall wall opening. It's expressed as a percentage. Common louver free areas range from 35% to 60%.

A high free-area percentage matters because it lets more air through a smaller wall opening — reducing the cost of both the wall cut and the louver itself. But some obstruction is necessary to keep water out. A fully obstructed opening allows zero water in (and zero air); a fully open hole allows everything in. The job of a well-designed louver is to maximize free area while minimizing water entrainment.

First point of water penetration

This is the threshold air-intake velocity at which a louver begins to let water through. It's measured in feet per minute (fpm) of free-area velocity, and it ranges roughly from 300 fpm (poor) to over 1250 fpm (excellent) on traditional louvers.

Wind-driven rain testing is more stringent — wind is applied to the louver face simultaneously with water, at standardized rates (typically 3" of rain at 29 mph wind, or 8" of rain at 50 mph). Performance is expressed as an efficiency percentage. Our E2WV, E4WH, and E6WH wind-driven rain models reach 99%+ efficiency.

Pressure drop (resistance to airflow)

Every obstruction in an airstream creates resistance — louvers, ductwork, filters, coils. The louver's resistance is measured by running air through it at known free-area velocities and recording the pressure differential across it, in inches of water gauge (in. w.g.).

For most applications, you want pressure drop below 0.2 in. w.g. If your louver runs higher than that, increase the opening size or pick a model with higher free area, higher first-point-of-water, or lower pressure drop at the same velocity.

Putting it together

To evaluate two louvers fairly, you have to combine free area and water-penetration limit into a single number: maximum allowable airflow without entraining water.

Example, at a 48"×48" wall opening:

• Louver 1: 45% free area, first-point-water 1190 fpm. With a 25% safety factor, design velocity is 893 fpm. Allowable volume = 7.2 sq ft × 893 fpm = 6,426 CFM.
• Louver 2: 53% free area, first-point-water 750 fpm. With a 25% safety factor, design velocity is 563 fpm. Allowable volume = 8.5 sq ft × 563 fpm = 4,781 CFM.

Louver 1 wins — by 25% — despite having lower free area. Always combine the two numbers before deciding.