Peter Body MRICS FCIOB
It is a technical requirement for modern windows to be tested, to determine their performance rating, prior to their introduction into the marketplace.
The foremost window test facility in Scotland is provided by the Department of Civil Engineering at the University of Paisley.
They issue what is known as a ‘Macdata Certificate’. This records the Performance of Windows Classification for Weathertightness, in accordance with British Standard 6375:Part 1:1989
The highest test performance classification achievable is an Exposure Category of 2400. Typically, this will provide:
§ Air Permeability Pressure Class of 600 Pa or better.
§ Watertightness Pressure Class of 300 Pa or better.
§ Wind Resistance Pressure Class of 2400 Pa or better.
A window achieving this test standard is generically known as ‘High Performance’.
Within the UK, the further north you go the more severe wind speed design criteria become, varying from 38 metres per second (m/s) in London to 56 m/s on the Isle of Lewis. These maximum gust speeds are only likely to be exceeded once in 50 years. The annual mean wind speed across the north of Scotland is about 10 m/s.
A wind speed of 56 m/s is equivalent to 125 miles per hour (m/h) or just over 2000 Pa (Pascals – a measure of pressure). So, a window achieving a Wind Resistance Pressure Class of 2400 Pa is unlikely to suffer structural failure under any circumstances.
However, a Watertightness Pressure Class of 300 Pa is equivalent to a wind speed of only 21 m/s or 48 m/h – Force 9 (47-54 m/h) on The Beaufort Wind Scale – classified as a severe gale during which slight structural damage occurs (chimney-pots and slates removed). Similarly, an Air Permeability Pressure Class of 600 Pa is equivalent to a wind speed of 30 m/s or 68 m/h – Force 11 (64-72 m/h) – classified as a violent storm resulting in wide-spread damage.
This doesn’t mean every window is going to leak immediately the wind exceeds Force 9. But windows facing the storm, often from a south-westerly (prevailing) wind direction are vulnerable.
All High Performance Windows should have a Macdata Certificate, or equal test certification, proving they have achieved an Exposure Category of 2400 or better. However, as has been shown above, the weather to which windows are subjected in the Highlands & Islands (particularly on the west coast) often exceeds that upon which these tests are based. Consequently, in such instances, it is not inconceivable for any window, from any manufacturer, to suffer temporary failure.