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February 12, 2016

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Free Area % . . . . . forget it!

April 12, 2016


One of the worst ‘simplifications’ when designing a Performance Louvre system, is to use Free Area as the means to calculate the required louvre area.  This can lead to both an overestimation of louvre area or worst still, an underestimation of the area required.  This in turn can cause a shortage of ventilation area, leading to overheating and premature failure of mechanical plant.


Free area is the measured area, taking the louvre components from the total area of the louvre.  This is quoted as percentage of the total louvre area.  Typically, 50% is commonly adopted as the standard to follow.  The origins of this form of evaluation, stem back to planning consents where façades are typically required to highlight appearance, of which louvres feature.  At that stage, the mechanical services requirements of the building (louvre areas) are unknown, so by taking an air change rate for the type of building usage, a typical open area can be determined.  And by assuming louvres restrict airflows, this area is doubled to achieve the required louver area required.


HOWEVER, this percentage type of evaluation takes no account of whether the louvres provided are single, double or triple stage, profile, thickness, depth, spacing, airflow efficiency, etc.  A typical single stage and triple stage louvre can have a similar percentage free area, but the airflow efficiency (the ease at which air can pass from one side of the louvre to the other) can be vastly different!


The only way louvres can be sized, is to accurately know the louvre co-efficient of airflow (Ci/d), which in turn can only be determined by wind tunnel testing at varying velocities.  Adding this value into known formula, the resulting pressure drop is determined and the louvre sized correctly.


A full evaluation of Performance Louvre requirements should always be carried by a specialist with expertise in this field of design and the 50% Free Area ‘rule of thumb’ should always be avoided.


The Author:  Paul Paffett is Director of INNIVATE PTE LTD, specialists in Performance Louvres. Find more at


© INNIVATE 2016.  All Rights Reserved


No part of this article or any of its contents may be reproduced, copied, modified or adapted, without the prior written consent of the author.






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