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A louver is a ventilation product that allows air to pass through it while keeping out unwanted elements such as water, dirt, and debris. A number of fixed or operable blades mounted in a frame can provide this functionality. The basic considerations for selecting louvers are Free Area, Water Penetration, and Resistance to Airflow (Pressure Loss). Once these concepts are understood, they can be used to properly apply a louver.

Free Area

Free area is derived by taking the total open area of a louver (after subtracting all obstructions - blades and frame) and dividing by the overall wall opening. This gives a comparison of a louvered opening to an unobstructed opening. Common louver free areas range from 35% to 60% of the wall opening (65% to 40% obstructed). A high percentage free area is beneficial because more air can enter into a smaller wall opening, reducing the cost of the wall opening and louver.

louver isometric louver airflow

Obviously some obstruction is required in order to keep undesirable water out. A fully obstructed opening would allow no water in, while a totally unobstructed opening would allow water to enter unimpeded. A properly designed louver will maximize free area while allowing a minimal amount of water to enter.

For more information about free area, please visit the linked page Louver Free Area.

Water Penetration

First Point of Water Penetration is the point at which a louver allows the passage of water through the louver. It is a threshold measurement of air intake velocity at which the louver will begin leaking (in feet per minute or fpm).

Traditional Louvers: The typical method of testing for water penetration is to intake air through the louver while applying a measured water content into the airstream. The velocity of airflow through the louver free area is increased until the louver allows water to enter. The result of this test is the first point of water penetration - ranging from 300 fpm (a very poor resistor to water entrainment) to 1250 fpm (a very good resistor to water entrainment).

louver water

Wind Driven Rain Louvers: Testing is done similar to traditional louvers, but with wind simultaneously applied to the face of the louver. The wind is applied at a fixed rate, while the air intake velocity is increased from 0 feet per minute to a predetermined value. Instead of a "first point of water penetration" value, efficiency of the louver is measured instead. Basically, "how good is the louver at stopping the water?" The efficiency is rated as a percentage, determined from the amount of water that passes through the louver divided by the total water applied during the test. A very efficient louver will have a value from 99-100%. Inefficient louvers will have values below 85%, meaning they allow over 15% of the water applied to pass through the louver.

Testing procedures have fixed values for water volume and the wind speeds applied. Two tests are common - 3" per hour rainfall combined with 29 mph wind speeds, and 8" per hour rainfall combined with 50 mph wind speeds. As described earlier, the air intake velocity is the only variable.

Obviously this testing is more stringent and requires a special louver design to perform well in this environment. Several designs are available throughout the market, but few have surpassed the capabilities of our E2WV, E4WH, and E6WH louver models.


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Resistance to Airflow

Every obstruction in the airstream creates resistance - louvers, ductwork, filters, coils, building structure, etc. The resistance of the louver can be measured by running air through the louver and measuring the pressure differential at various free area velocities (measured in water gauge or wg). Every louver will create resistance based on the frame and blade shapes. Lower blade angles or more aerodynamic shapes create less resistance. We must know the free area velocity through the louver in order to properly evaluate the overall resistance to airflow. For a majority of applications, we can calculate the pressure loss of the louver at the required free area velocity and determine if it is acceptable. The resistance created can be detrimental to the application of fans and other air movement equipment, so we should attempt to minimize it.


Applying the Principles

To properly evaluate a louver's capability, we must have a method to include both the Free Area and the First Point of Water Penetration in a meaningful way. Since the overall objective is to get as much air as possible through the louver, we want to evaluate the allowable volume of air through the louver (cubic feet per minute or cfm). Test methods for these principles are covered in AMCA Standard 500-L Laboratory Methods of Testing Air Louvers for Rating. The following example compares two louvers for a wall opening size of 48" wide x 48" high with different performance characteristics:

Free Area (percentage)
Free Area for 48" x 48" (square feet)
First Point Water (fpm)

Since our objective is to get more air through the louver, we might assume that Louver 2 is better than Louver 1, since it has a higher free area. However, more evaluation is required. The real question is, "How much total air can I get through the louver without entraining water?"

Free Area (percentage)
Free Area for 48" x 48" (square feet)
First Point Water (fpm)
Design Velocity (fpm)
Volume of Air (cfm)

View performance of Architectural Louvers

Louver 1 has a free area of 45% for a size 48" wide x 48" high wall opening. The total square feet of free area is 7.2 ( = 45% x 16 sq ft of wall opening). The tested First Point of Water Penetration for this louver is 1190 feet per minute free area velocity. We should build in a safety factor for some variation in our airflow through the louver - we have chosen 25% safety factor. The design velocity including the safety factor would be 25% less than 1190 fpm, or 893 fpm (1190 x .75). We can now determine how much air can safely be run through the louver by multiplying the louver free area by the design velocity ( 7.2 sq ft x 893 fpm). The resulting Volume of Air for Louver 1 is 6424 cfm.

If we go through the same calculations for Louver 2, the result (with a 25% safety factor) is only 4781 cfm. This is 25% less air through the same opening size. Louver 1 is a better choice - IF we can live with the pressure drop from the higher airflow rates! back to top

Most Manufacturers publish air flow resistance for their louvers. Each louver will have slightly different resistance based on the blade and frame shapes and angles. These characteristic can be expressed by a formula and graphed, such as:

Louver 1
Louver 2

Simplifying things a bit - most louvers do not fluctuate dramatically from these graphs (about 15% in this example), unless the louver is designed for very high air velocities, like wind driven rain louvers. However, if you have the data, use it! Here we can calculate the resistance at the design velocities for each louver and determine that:

Louver 1 - at 893 fpm free area velocity, will create 0.090 inches w.g. of static pressure

Louver 2 - at 563 fpm free area velocity, will create 0.055 inches w.g. Of static pressure

Both of these values should be acceptable for HVAC system design, and would mean that our Louver 1 is the better choice, even though the free area is lower. A good rule of thumb is to stay below 0.2 inches w.g. static pressure for most applications. If your values exceed this rule, we recommend you increase the opening size or select a louver model with higher free area, higher first point of water penetration, lower pressure drop, or a combination of these factors.

The following table represents the performance capability of all louvers by Architectural Louvers. Table is based on a test size of 48" wide x 48" high for comparison purposes:

Product Model Free Area First Point of Water Penetration (free area velocity) Overall Performance (C.F.M.) Pressure Loss at this velocity (inches water gauge)
E6JN 69.1% 915 fpm 9014 cfm 0.12
E4DP 59.3% 930 fpm 8826 cfm 0.12
E4JP 58.4% 960 fpm 8976 cfm 0.13
E6DP 57.7% 1046 fpm 9655 cfm 0.13
E6JP 57.3% 1123 fpm 10298 cfm 0.18
E4DS 56.0% 930 fpm 8333 cfm 0.13
E4WS 56.0% 346 fpm 3100 cfm 0.02
E6JF 54.4% 1020 fpm 8884 cfm 0.18
E2WV 53.8% 889 fpm 7645 cfm 0.24
E6WH 51.4% >1250 fpm 10275 cfm 0.21
E6WF 51.1% 1081 fpm 8832 cfm 0.14
E4WH 50.6% >1250 fpm 10113 cfm 0.25
E4JS 50.4% 888 fpm 7157 cfm 0.15
E2DS 49.4% 889 fpm 7032 cfm 0.12
E2JS 48.7% 725 fpm 5648 cfm 0.08


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