Weather Performance - what you need to know.


Demystifying Window Ratings: Air, Water & Wind Performance Explained!

Ever wondered what those window performance ratings mean? We break down air permeability, water tightness & wind resistance tests to help you choose the right windows for your home.

When you buy new windows and doors, you’ll see they come with weather performance classifications for air permeability, water tightness and wind loading. These give you a good indication of the durability and thermal efficiency of the product you’re buying and are the results of standard tests carried out at accredited, independent test centres. Our 58BW HI window system, for example, is rated a maximum Class 4 for air, a maximum Class 9a for water and a maximum E2400 for wind. What do the tests actually involve though, and what do the figures mean in practice? 

Air permeability
Air permeability is all about leakage and draughts and is a measure of the volume of air that can pass through a window or door when it is fully closed.(It obviously has a  major impact on thermal efficiency.) The BS EN1026: 2016 test involves air being forced at the window from one side and sucked in from the other to see how much air leakage takes place. The results are classified from 0 to 4, with 0 being the worst and 4 the best. Like our 58BW window, our 58BD single open-out door, for instance, is rated 4.

Water tightness
This is a measure of how well windows and doors can withstand the elements. The test to BS EN1027 involves water being sprayed at steadily increasing pressure to simulate the effect of rain, sleet and snow. As soon as there is any leakage on the inside, windows or doors are deemed to have failed. Classification for water tightness starts at 0, which means leakage occurred within just 15 minutes, and it goes up in 5 minute and approximate 50Pa increments to a top score of 9 (This indicates that the window or door system withstood 600Pa of water pressure for the maximum 55 minutes). There are also A and B ratings added, which just show whether the test simulated the window or door on its own (A) or with a balustrade in front (B). As an indication, our 58BD door is rated 7A, while our new S140 sliding door is 6A and our popular F82 bifold is 5A.

Resistance to wind load
There are actually a series of tests within EN 12210 which determine how well windows, doors and glazing perform under wind loading, without serious deflection or damage. This includes a deflection test, a repeated pressure test, an air permeability test and a safety test. The standard classification includes both a number 1-5 for the pressure test and a letter A-C which shows the amount of deflection. There is also an E classification which shows that the windows exceeded the boundaries of the test, and this includes the pressure the window or door was actually tested to (as for our 58BW and 58BD products which are rated E2400). 

It's worth pointing out that there are no fixed rules on the ratings and specifications you need for your home. It depends almost entirely on the location and exposure category of your property. If you’re working with an architect, they will normally tell you the classification required but if not, you need to consider things like: the basic wind speed at your location, the exposure of your property (ie the height above sea level, whether it is on a hill, the distance from the coast and whether it is in a town or in open country), the height and positioning of the window themselves (ie if they are on the ground or upper floors) and if they are being fitted in a dormer because then they can be subject to more wind turbulence. Your installer should be able to help.