Percentage Free Open Area: Is It Relevant to Louvre Performance?

Although many commercial and industrial buildings require louvres, either for ventilation or simply as a vision screen, there is often insufficient consideration given to exactly what the system is required to achieve. This is particularly common when it comes to “Performance Louvres” and the need to exclude wind-driven rain.

Ultimately, everyone wants 100% rain defence and 100% air flow. However, this is not always achievable and there will always be a compromise between the two performance factors. In Australia, AS/NZS 4740:2000 sets out the test method and standard required for testing natural ventilators. When specifying a louvre, the information below must be taken into consideration.

Measuring ‘Percentage Free Open Area’

AMCA, ADB and ‘Industry Standard’: each of these standards measure free open area differently, so how do we know which is correct, and most importantly, whether they are even relevant to louvre performance?

To add to the confusion, there are many varying terms and test data published by
competing louvre manufacturers when describing louvre performance. Free area velocity, throat velocity and face velocity are a few of the many, making it unclear and difficult to compare like for like.

MethodSingle StageTwo Stage

Without being a louvre expert, how do we ensure that what is calculated is accurate and beneficial to us when selecting the correct louvre?

No matter which of the testing methods you choose to use, none of the above take into consideration any specific air flow rate or pressure drop (Pa) unique to that louvre profile, nor do they consider the weather performance characteristics of the louvre. On this basis we can easily conclude that Percentage Free Open Area is not the most accurate way to measure louvre performance.

In order to accurately compare like with like, a louvre must be tested to the Australian standard, AS/NZS 4740:2000. AS/NZS 4740-2000 sets out all the guidelines for performance testing and provides the classification system for natural ventilators. This method of testing and performance classification provides “comparative” performance data for both Rain Defence and Airflow, offering protection to the specifier and a clear guide to the contractor regarding project requirements and performance expectations.

This test can be conducted as either a physical test or through Computational Fluid Dynamics (CFD) modelling. The AS/NZS 4740:2000 test requires the dimensions of the test louvre to be 1m x 1m, which is then tested to at least five different air velocities. From this, we assess the effective aerodynamic area and the coefficient of discharge (Cd) for that louvre profile. For the rain defence test, the louvre panel is then subject to 75 L/hr m2 of wind driven rain at a velocity of 13m/s.

Part 1: Louvre Rain Resistance Effectiveness (or Penetration Class)

This classification allocates the “Rain Resistance Effectiveness” Class of each weather louvre against water (rain) penetration. Each class covers a specific range, and it can be seen from the table that Class A is the highest rating achieving up to 99% effectiveness, which is significantly more effective than Class B below it.

NB: Louvre performance is dependent on the intake velocity. i.e., a Louvre may be class A with an intake velocity of 0m/s, but at 3.5m/s it might be a D.

AS/NZS 4740:2000 Standard:

CharacteristicPerformance LevelSummary
Rain ResistanceClass A
1 to 0.99% effectiveness
Class B
0.989 to 0.95% effectiveness
Class C
0.949 to 0.80% effectiveness
Class DBelow 0.80% effectiveness

Part 2: Louvre Effective Aerodynamic Area Class

This classification rates the louvre’s ability to allow air to pass through it and is determined by establishing the Discharge Loss Coefficient (DLC) at various airflow velocities. Each class covers a specific range, as can be seen in the table below. The higher the DLC the less resistant to air the louvre is, with a DLC of 1 being ideal. In simple terms, a hole in the wall with no louvre would have a DLC of 0.7 or above depending on the size of the hole. This airflow class provides a guide for mechanical consultants and building designers on how the louvre performs at various ventilation rates, while the DLC figure is used to establish the correct actual area of louvre required.

AS/NZS 4740:2000 Standard:

CharacteristicPerformance LevelSummary
Discharge Loss Coefficient (Cd)
Effective Aerodynamic AreaClass 1Cd = 0.7 & Above
Class 2Cd = 0.5 to 0.699
Class 3Cd = 0.3 to 0.499
Class 4Cd = 0.1 to 0.299

BS EN 13030: The British Standard

Please note that although the test method used in the British Standard (BS EN 13030) is the same, the parameters of the classification system are significantly different when it comes to the effective aerodynamic area.

As you can see, what may be a Class 3 according to AS/NZS 4740:2000 standard would simultaneously be a Class 1 according to BS EN 13030. Therefore, it is advised you check which standard the louvre has been tested to in order to fully understand the louvres performance.

BS EN 13030 Standard:

CharacteristicPerformance LevelSummary
Discharge Loss Coefficient (Cd)
Effective Aerodynamic AreaClass 1Cd = 0.4 & Above
Class 2Cd = 0.3 to 0.399
Class 3Cd = 0.2 to 0.299
Class 4Cd = Below 0.2

We can now prove that the Percentage Free Open Area of a louvre is not the most accurate way to measure louvre performance.

To ensure you specify the right performance louvre for your project, you can use the following process:

1. Confirm that the louvre is tested to AS/NZS 4740:2000.

2. Mechanical engineer defines the required Volume Flow Rate (m3/s) for
mechanical plant or passive ventilation.

3. Mechanical engineer defines the maximum allowable pressure drop (Pa)
across a louvre before fan performance suffers.

4. The architect and engineer balance the louvre façade area (m2) against the
effective aerodynamic area of any louvre selections and the required rain
resistance rating, to get a mutually workable outcome.

5. Specify the louvre that works for your design aesthetically while also
achieving the performance Classification required for both Aerodynamics
and Rain Defence.

For example: Jupiter Series 2 Stage Louvre – Class A3

(Example schedule available on draft specification)

For more information, please call Louvreclad and one of the team will be happy to answer any further questions you may have.

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