Louvers by the Numbers

In this article, we review the important numbers to consider when choosing a louver for air performance or weather protection.

Louver Design, A Game of Numbers

Louver selection is a game of numbers. You need to know how potential louvers will perform and whether they will provide the necessary airflow for your intended application. You should also understand how the louver will protect your ventilation opening from rainfall.

The key to choosing a louver is balancing protection with performance. The louver should protect from rainfall or severe weather, as required, while also maintaining the necessary amount of airflow. To strike this balance, you need to know your louver by the numbers. 

Check out these Newsstand articles for more on louvers:

• Ten Important Traits of Highly Effective Louvers
• Four Questions on Louver Location
• Drainable Louvers for Rainy Days

The Numbers - Important Stats for Louvers

In this article, we will explain some of the data points involved in louver selection, why each is important to the louver’s application, and what to look for on the louver's submittal. We start our list with an important number for determining a louver's air performance.

Free Area (sq.ft. or %)

• What is it?: Free area is the free space between blades measured in square feet or square meters. For adjustable louvers, free area is measured with the blades fully open. It can also be stated as a percentage of louver’s total area.
  • Ex. A 48" x 48" sample louver has 10.06 sq. ft. of free area or 63% free area.
• How does it work?: Higher free area means more space between louver blades. Free area will always increase with the size of the louver. Louvers with fewer blade features – such as straight non-drainable blades - will have more free area than blades with features.
• Why is it important?: Free area is important to figuring both the louver’s air performance and its ability to reject rainfall. More free area will translate to greater air performance. 
• Where do you find it?: Free area can be found on a louver’s submittal, along with other important performance metrics. The louver’s total free area will be listed in a table by the louver’s width and height.
• Manufacturers calculate the total free area of each louver at various dimensions, up to the louver’s maximum dimensions.

Pressure Drop (in. w.g. at FPM)

• What is it?: Pressure drop is the amount of pressure lost as air passes from the face of the louver to the ductwork behind it. This loss is measured in inches of water gauge at a given air velocity.
  • Ex. A sample louver's pressure drop is 0.05 in. w.g. at 1000 FPM.
• How does it work?: As pressure drop increases, the airflow rate will decrease. The system can overcome this by increasing the speed of its fans. In other words, the system will consume more power to achieve the necessary airflow rate.
• Why is it important?: Pressure drop is important to the louver’s air performance. All louvers will cause some amount of pressure drop, but lower pressure drop will increase airflow. The HVAC system will use less power to draw in air.
• Where do you find it?: Pressure drop can be found on a louver’s submittal. Pressure drop data is displayed as a straight line graph, with pressure drop (in. w.g.) along the Y axis and velocity through free area (FPM) along the X axis.
• Louvers are tested in accordance with AMCA Standard 500-L to determine the pressure drop at various air velocities. Louvers tested in this manner will display an AMCA seal for air performance on their submittals.

Learn how pressure drop affects airflow with this quick read: Air Performance and Keeping the Flow.

Water Penetration (FPM)

• What is it?: Water penetration refers to the amount of water that passes through the louver at a given air velocity. For selection purposes, louver manufacturers will list the beginning point of water penetration.
  • Ex. The beginning point of water penetration for a sample louver is 1103 FPM.
• How does it work?: The beginning point of water penetration is the point, in air velocity, when an excessive amount of water will infiltrate the louver. The beginning point of water penetration will be listed in FPM, or in metric as m/s.
• Note: AMCA testing limits the maximum beginning point to 1250 FPM. For details, read our article on Water Penetration testing. 
  • Ex. If a sample louver's beginning point of water penetration exceeds 1250 FPM, it will be listed as "Above 1250 FPM".
• Why is it important?: The beginning point of water penetration is important for basic weather protection. A higher beginning point of water penetration will provide more protection.
• Where do you find it?: Water penetration is displayed as a curved line graph on the louver’s submittal. The amount of water infiltration through free area (in 15 minute intervals) will be on the Y axis and the velocity of airflow through free area will be on the X axis. The beginning point of water penetration will be listed near this graph in FPM.
• Louvers are tested in accordance with AMCA Standard 500-L to determine their beginning point of water penetration. Louvers tested in this manner will display an AMCA seal for water penetration on their submittals. Read AMCA Testing - Water Penetration for more information.

Wind Driven Rain

• What is it?: Wind driven rain refers to rainfall carried towards a louver by strong winds during a storm. Louvers designed for wind driven rain are categorized as severe weather louvers.
• How does it work?: Wind driven rain data measures the amount of water that infiltrates a louver during a severe weather event. Severe weather louvers must block heavy rainfall driven towards the louver at high air velocities, while accepting the necessary volume of airflow.
• Why is it important?: Areas prone to severe weather will require louvers tested and rated for wind driven rain protection. This rating is considered a step above water penetration protection, but a louver can be rated for both types.
• Where do you find it?: Wind driven rain data is listed as a table on the louver’s submittals, which includes the percent of test water rejected by the louver during the test and a corresponding letter grade.
• Wind driven rain data is measured through testing in accordance with AMCA Standard 500-L. Tests are conducted at two simulated rainfall rates under specified wind velocities. Each test measures the amount of water rejected by the louver.
  • Louvers tested in this way will receive a letter classification – Class A (100% - 99%) through Class D (Below 80%) – that signifies its effectiveness to reject wind driven rain.
  • Class A is considered the best classification for wind driven rain, since the louver rejected 99 to 100 percent of simulated rainfall.

Want to know more about wind driven rain testing? Read how AMCA-approved labs test WDR louvers on the Newsstand: AMCA Testing - Wind Driven Rain

Face Velocity (FPM)

• What is it?: Face velocity is the speed of airflow as it passes through the face of the louver. Face velocity will be measured in feet per minute, or FPM. In metric measurements, face velocity is measured in meters per second (m/s).
• How does it work?: Face velocity increases as intake/exhaust air velocity increases. Increasing face velocity will also increase pressure drop.
• Why is it important?: Face velocity will help you determine the pressure drop of the louver and whether the louver will maintain its water penetration protection. Face velocity should never exceed the louver’s beginning point of water penetration.
• Where do you find it?: Your project’s specifications will list the louver’s required face velocity for its intended application.

Airflow Rate (CFM)

• What is it?: Airflow rate is the amount of airflow passing through the louver, measured as a volume. This is expressed in cubic feet per minute, also called CFM. For metric measurements, use cubic meters per second (m³/s).
• How does it work?: More air will flow through the louver as the airflow rate increase. The louver should be designed to accommodate the necessary airflow rate for its application.
• Why is it important?: HVAC systems require a certain amount of air circulating through the ductwork. Your louver should provide enough free area to allow the necessary volume of airflow.
• Where do you find it?: Review the specification documents for your project to determine the louver’s required airflow rate.

Air Leakage (CFM / sq.ft.)

• What is it?: Air leakage is the amount of air that passes through a closed adjustable louver. Leakage is measured as air volume over square feet of louver face area (cfm/sq.ft.).
• How does it work?: Air can leak through a closed louver, which makes it more difficult to achieve the necessary airflow rate. System fans will work harder to make up this difference.
• Why is it important?: Leaky adjustable louvers will affect air performance across the HVAC system. Leaks will require more power to maintain adequate airflow. learn more in "Air Leakage - The Performance Killer". Eliminate this inefficiency by choosing adjustable louvers with the least air leakage at the recommended size.
• Where do you find it?: Air Leakage will be displayed as a curved line graph on the louver’s submittal, with pressure drop on the Y axis and air leakage on the X axis.
• Adjustable louvers are tested in accordance with AMCA Standard 500-L to determine the louver's air leakage at various static pressure differentials, or pressure drops. Louvers tested in this manner will display an AMCA seal for air leakage on their submittals.

Applying the Numbers

Now that you know the numbers, you can apply them to your project! Below are three ways to calculate the necessary values to determine whether a louver will work for your project.

You will need the specification documents for your project and the submittal for the louver under consideration. These two documents will help you determine if a louver works for its intended application.

Don't know about specifications? Learn about these important documents in our Newsstand article: Specifications - The Plans to Your Building

Finding the required total free area based on pressure drop

• Review the specifications for your project. These documents will provide the necessary airflow rate and the maximum allowable pressure drop for the louver.
• Find the pressure drop graph on the louver’s submittal. Use this graph to find the louver’s air velocity through free area at the maximum allowable pressure drop from the specification.
• Divide the necessary airflow rate (on the spec) by the louver’s air velocity through free area (on the submittal).
• The result will be the louver’s required total free area, or the total amount of free space between blades, head, and sill required to achieve the necessary airflow rate and velocity.
• Check the free area chart on the submittal. This chart will list the total free area of the louver at various widths and heights.
• Compare the required total free area calculated above with the sizes listed on the submittal's free area chart. The louver’s total free area should be greater than or equal to the required total free area at the desired width and height.

Finding the required total free area based on water penetration

• Review your specification documents to determine the necessary airflow rate for the louver.
• Find the water penetration graph on the louver’s submittal. This graph will display the louver’s beginning point of water penetration, expressed as velocity through free area.
• Consider a velocity through the free area that will be lower than the louver’s beginning point of water penetration. This will be the louver’s desired air velocity.
• Divide the volume of airflow by the desired air velocity. The result will be the louver’s required total free area.
• Check the free area chart on the submittal. Compare the required total free area you calculated above with the sizes listed on the chart. The louver’s total free area should be greater than or equal to the required total free area at the desired width and height.

Finding the required minimum free area based on air volume

• Locate the Water Penetration graph on the louver’s submittal. Use this graph to determine the louver’s beginning point of water penetration.
• As a rule, calculate an air velocity that is 80% of the louver’s beginning point of water penetration or less. You can also work with a baseline of 500 FPM, as an average. This will be your working air velocity. 
• Review the project’s specifications to find the required airflow rate for the louver.
• Divide the required airflow rate by the working air velocity. This calculation will give you a minimum free area for the louver.
  • In other words, you are determining the minimum free area required to achieve the necessary airflow rate without exceeding the louver’s beginning point of water penetration.
• Review the louver’s free area chart to find dimensions that provide a total free area close to the minimum free area you calculated above. Find the closest match to the calculated minimum free area.

All louvers should meet your project’s requirements first. Otherwise, the best louvers will be no more than fancy metal on your wall. Review your project's specifications when deciding on louvers. Your louvers should provide the necessary airflow rate at the correct face velocity.

Once requirements are met, then you can begin looking for louvers that perform the best within their application: louvers that protect against severe weather, louvers that provide the most airflow, or a balance between the two. It all starts with knowing your louvers by the numbers!

Do you know your louvers by the numbers? Did you ever think that choosing louvers would require this much math? Let's talk about louver numbers in the Comments below. MCDLG wants to hear from you!

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