The 100% rainproof louvre, under all environmental conditions, is a common ‘myth’. Whenever an opening is formed in a building’s façade, for the purposes of air inlet or exhaust, there is always a risk of water entry.
It is true, that at certain airflow velocities; rain defence louvres can prevent total (100%) rain water entry. But there will be a point, as the air velocity increases through the Louvre system, that traces of rain water will infiltrate inside the building. Factors such as rainfall rate, wind velocities, mechanical plant demand, exposure and even ‘macro’ factors such as, surrounding topography and site location, all play a part in when this will happen. There is no doubt that if you increase the airflow through the louvre, during a rain storm, water will enter through the louvre. Generally regarded as the more severe and ‘threatening’ condition, air intake is where the wind pressure, combined with the air demand of the mechanical plant, put the louvre at its most vulnerable condition. With 1.5 to 2.5m/s being widely regarded as the accepted design airflow velocity range, it is possible to design a system that will perform to the requirements of being resistant to rain entry. However, one must accept that, however severe, this is based upon a test protocol that simulates wind forces and rainfall rates and should these rates be exceeded, there is always the risk of some water ingress. The only recognised European Standard for testing Rain Defence Louvres (Performance Louvres) is BS EN 13030:2001. This dictates a defined test protocol and includes a simulated Wind velocity of 13 m/s and a simulated rainfall rate of 75mm/hour. This is a constant velocity ‘wind’ force and constant rainfall rate and therefore does not fully simulate a real life condition (which would be gusting winds and variable rainfall). However, this does offer a worst case scenario.
Under the test scenario louvres of a 1m2 core area are subjected to wind and rain and a variable intake velocity that is increased from 0m/s up to 3.5m/s, in 0.5m/s increments. Test readings are taken at each intake velocity, and the relative effectiveness of the louvre system, between 0 to 100%, is recorded. Louvres attaining an effectiveness of between 99 to 100% are rated as Class A, 95 to 98.9% Class B, 80 to 94.9% Class C and those below 80% are all Class D. It should be noted these classifications must be reviewed in relation to the intake velocities they are achieved against, with as stated 1.5 to 2.5m/s being the design norm.
In summary, Specifiers need to adopt the correct design and realistic expectation of a Performance Louvre’s performance and avoid the notion of a 100% rainproof louvre. Why? It’s a myth!