Free open area size has long been used when selecting louvres. The way it works is simple. When comparing louvres, the assumption is a larger free open area results in a decrease in pressure drop. In other words, the bigger the hole for air to get through, the more air will get through. Simple.
The problem is it’s too simple. Louvres with a free open area of 48% can have a lower pressure drop (and let in more air) than alternative louvres with a free open area of 60%. This presents a risk when selecting louvres and doing so based on the free open area alone might end up costing you.
Why the Assumption?
It’s hard to pinpoint why and when louvres were judged based primarily on their free open areas to determine how much air they allow through. But the logic is easy to understand and simple ideas tend to spread more rapidly than complicated ideas.
Maybe the assumption comes from opening a window. It would make sense to say a sash window opened to 50% allows for 50% of the air to flow in when fully open. Extrapolating this to louvres is easy enough and it’s a simple concept for everyone to understand.
But let’s say a casement window has the same free open area to the sash window. The amount of air coming in would be dependent on the direction of airflow, as that casement window might be blocking some air from getting in. So there are complications.
In fact, there’s a few of them.
The Complications
For a start, there’s the shape of the louvres, something which every calculation method used to calculate the open area fails to factor in. Two louvre profiles might have the exact same free open area of 50% but if one is streamlined and the other isn’t, there’ll be a significant difference in the amount of air which makes it through. Sure, technically they do have the same open area, but if that is being to make a judgement of the amount of airflow getting through - it’s wrong. There’ll be instances where a louvre profile with a free open area of 33% can have a lower pressure drop then a louvre profile with an open area of 50%.
So, shape and design has a profound impact. Think of it this way - you could take a 1m by 1m sheet of aluminium and pierce it with thousands of little pin pricks until it had an effective open rate of 50%. Then take a second aluminium sheet and cut a single big hole that covers 50%. Which one will have a lower pressure drop?
The next complication comes from insects. Well, screens that keep them out. It’s pretty common to have an insect screen next to a louvre for the simple reason no-one wants flying nasties munching down on them. Those insect screens will increase the pressure drop. So suddenly the louvres (with insect screen) ordered with a 50% open area have a higher pressure drop than the louvres with a 35% open area. It’s another complication that needs to be taken into account.
It doesn’t end there either. Two stage louvres are great for advanced weather protection but will almost always have a higher pressure drop. The air simply has greater resistance and a more difficult path to entry (or exit).
The Cost of Getting it Wrong
If the free open area is used to make a judgement on the amount of air getting through louvres, then the consequences can be uncomfortable at best. At worst, they might be fatal. That sounds sensationalist but we’re aware of instances where the free open rate has been used to make a choice of louvre which is a critical part of the fire safety of a building. In this instance, if the pressure drop is too high, smoke and heat is contained in the building, beyond the amount mitigated for.
In most instances the cost of getting it wrong is far less dramatic. It might just be a case of the interior of a building becoming a little hot and stuffy on a summer's day.
How to Get it Right
The way to avoid all of the above is to dispense with the notion that the free open area is an indicator of the pressure drop or the performance of a louvre. We’d advise seeking information on the louvre based on test results from testing against BS/EN:13030 or AS/NZS 4740:2000. This standard sets out the guidelines for testing and classification of all weather and performance louvres. Louvres are subjected to 75mm/hour of water at a velocity of 13 m/s. The louvre is then given a 3 part classification based on; effectiveness, discharge loss coefficient and wind loading.
The discharge loss coefficient will assign an Airflow class from 1 to 4. The higher the class, the lower the pressure drop. This is a far better method for assessing pressure drop than using the free open area as an indicator.
The testing also comes with the bonus of the effectiveness classification providing an indication of water penetration. After all, increasing airflow is simple but it has to be balanced against water ingress.
The final part of the guidelines, concerned with wind load, provide some confidence (or not) the louvres can withstand the Wellington winds and take a blowy battering should they need to.
In conclusion, using the free open area as a method for determining pressure drop is wrong. As is using the free open area for assessing the overall performance of a louvre. We’ve not mentioned water ingress much here. Yet, it might be assumed a higher free open area is more likely to result in water penetration. There are some flaws to that assumption too.
This leads us back to BS/EN:13030. When making a determination about the performance of a louvre, ask to see the results. Or you can talk to us. We’re here to help and cut through the misleading maths.
