Updated: 11 December 2023

Let’s talk ventilation – Free area vs Airflow

At Kingfisher we’re regularly asked for louvres with a very high free area. There’s a common misunderstanding within the industry that a high free area will provide you with better ventilation, this is not always the case. Here are the facts:

Free Area is a nominal figure, based on the smallest gap between the blades. This is not a measure of airflow. Free Area does not consider the shape/profile of the blade and this is essential when considering the ventilation performance of a louvre system. Free Area can be easily manipulated and some companies will express a “Visual” free area and a “Physical” free area; Visual Free Area relates more to the louvre pitch (The distance between the blades) whereas Physical Free Area is the smallest gap between the blades divided by the blade pitch and this is usually expressed as a percentage.

When discussing ventilation performance, we should be referring to Cd Values and Pressure Drops as these figures are actual measures of ventilation performance. Cd Value (also known as Discharge Loss Coefficient) is the measurement of ease with which air will flow across a louvre; this is where the shape/profile of the blade becomes important! The Cd value of a louvre system is used for ventilation classification during the BSRIA louvre testing. Kingfisher’s louvres are tested to BS EN 13030:2001, the Cd value measured throughout the testing relates to the ventilation classification, expressed as Class 1-4.

A Pressure Drop is measured in Pascals (Pa) and is the calculated change in air pressure from the front to the back of a louvre. If you have a higher pressure drop this generally relates to lower ventilation performance as the louvre isn’t allowing the air to pass through. On the other hand, if you have a lower pressure drop, the louvre has a higher ventilation performance as it’s allowing more air to pass through the louvre. Usually there’s a maximum allowable pressure drop for a louvre and Kingfisher have a range of systems we can run calculations for to meet pressure drop requirements.

Example of Free Area differences on our single bank louvres systems.

A great example of the unreliability of Free Area as a measurement of air flow can be shown using our single bank 75mm pitch louvre systems; KW75HPG, KW75Z and KW75S.

Our KW75HPG is our high performance single bank louvre and has the lowest free area of our three 75mm pitch systems at 47%, but has a Cd value of 0.311, which equates to a Class 2 on the BS EN 13030:3001 BSRIA testing. You’ll see from the above detail that the KW75Z single bank louvre has a free area of 58% but only a Cd value of 0.273, which equates to a BSRIA class 3. The reason the KW75HPG outperforms the other louvres at the same pitch is thanks to its aerodynamic design, the air is actually “pushed” through the face of the louvre, meaning better ventilation and lower pressure drops. In addition, the better the ventilation performance of a louvre the smaller overall louvre area is required, which provides a cost saving.

Kingfisher are here to discuss your ventilation requirements and can provide you with calculations and support for technical submissions. Contact our technical team on 01773 814 102 or email info@kingfisherlouvres.com.