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Air Permeability Classification

• Introduction
• Current issues
• Differences between the standards
• Worked examples

Introduction

• The requirement to match the weather performance of the windows with the environmental requirements of the building has been important for as long as walls have had windows. This led, eventually, to the idea of testing windows for their performance, and then classifying them, such that architects and specifiers can select an appropriate window without the necessity to test it for the specific building into which it is to be fitted.
• In the UK the weather performance of windows has traditionally been classified by reference to BS 6375-1, the current version being BS 6375-1:2004.
• With the rapid increase in the use of curtain walling systems during the 1980s it became apparent that the BS 6375-1 tests were inappropriate for curtain walling and this led, eventually, to the publication of the first CWCT Standard for Curtain Walling in August 1996. This introduced test methods, and a classification system, for curtain walling although the test methods were more frequently applied to the project-specific testing of bespoke systems than the classification of proprietary systems.
• A large number of BS EN standards relating to curtain walling have been published since 1996 (none existed prior to this date) with the result that the CWCT Standard for Curtain Walling was superceded by the CWCT Standard for Systemised Building Envelopes in 2006. This references the many BS EN standards, including BS EN 12152 for the air permeability classification of curtain walling, and it is anticipated that classification of curtain walling systems will increase, and the amount of project-specific testing will decrease, over the coming years.
• The guidance on this page deals solely with the classification of windows subject to general performance testing and does not cover the performance of windows subject to project-specific air permeability testing.

Current issues

• Due to the current boom in high rise residential buildings, window systems tested to BS 6375-1 are now being specified in more exposed situations than has been the case in the past.
• At the same time, some manufacturers are choosing to test their windows to the relevant BS EN curtain walling standards, rather than BS 6375-1, in order that the same window systems can be used both as individual windows and as opening lights in curtain walling systems.
• Although certain requirements of BS 6375-1 and BS EN 12152 may appear similar, there are significant differences which need to be understood by those specifying windows and curtain walling. This applies particularly to air permeability where the requirements of the two standards diverge at the higher design pressures appropriate to more exposed situations. The differences between BS 6375-1 and the equivalent BS EN standards are less significant for water tightness and structural resistance to wind.
• Specifiers need to be aware, in a price-competitive market, that they may not be comparing like-for-like if one window is tested to BS 6375-1, and an alternative to BS EN 12152, even if the pressures at which they have been tested are the same.

Differences between the standards

• BS 6375-1:1989 : Although now superceded, some window manufacturers still refer to the (1989) A/B/C classification in their trade literature. Figure 1 of the 2004 Standard shows an approximate comparison between the two standards. In principle:
• 1989 Class A is slightly "better" than 2004 Class 1.
• 1989 Class B is slightly "worse" than 2004 Class 2.
• 1989 Class C is very close to 2004 Class 3.
• There is no 1989 equivalent of 2004 Class 4 (the "best").
• BS 6375-1:2004 : Measures the air leakage of the overall area of the window, and the air leakage along the total length of the opening joint, at the specified test pressure. These figures are then compared with a standard table (Figure 1 on page 4 of the Standard) and the window given a classification based on the better (i.e. lower leakage) of the two values. The table shows a sliding scale of air leakage relative to test pressure.
• BS EN 12152:2002 : This covers only fixed panels/lights and sets two acceptable air leakage rates, one by area and one by joint length, with compliance required for both criteria at the specified test pressure.
• CWCT Standard:2006 : This refers to BS EN 12152, and uses the same permissible air leakage rate of 1.5m³/hr/m² (i.e. by area) for fixed lights and introduces a CWCT-specific requirement of 2.0 m³/hr/lin.m (i.e. by length of opening joint) for opening lights.

Worked examples

These are all based on a 1.5m x 1.5m opening light:

At 300 Pascals

• BS 6375-1:2004 Class 3/BS 6375-1:1989 Class C:
  Permitted air leakage by area: 18.0m³/h/m² = 40.5m³/h.
  Permitted air leakage by length of opening joint: 4.5³/h/linear m = 27.0m³/h.
• BS 6375-1:2004 Class 4:
  Permitted air leakage by area: 6.0m³/h/m² = 13.5m³/h.
  Permitted air leakage by length of opening joint: 1.5³/h/linear m = 9.0m³/h.
• CWCT Standard:
  Permitted air leakage by length of opening joint: 2.0³/h/linear m = 12.0m³/h.
• Conclusion : At 300 Pascals, windows complying with BS 6375-1:2004 Class 4 have a comparable performance to windows tested to the CWCT Standard. It is, however, worth noting that some manufacturers are still offering windows tested to BS 6375-1:1989 Class C as their "best product" and this is significantly worse than a window complying with either BS 6375-1:2004 Class 4 or the CWCT Standard.

At 600 Pascals

• BS 6375-1:2004 Class 3/BS 6375-1:1989 Class C:
  Permitted air leakage by area: 30.0m³/h/m² = 67.5m³/h.
  Permitted air leakage by length of opening joint: 7³/h/linear m = 42.0m³/h.
• BS 6375-1:2004 Class 4:
  Permitted air leakage by area: 10.0m³/h/m² = 22.5m³/h.
  Permitted air leakage by length of opening joint: 2.5³/h/linear m = 15.0m³/h.
• CWCT Standard:
  Permitted air leakage by length of opening joint: 2.0³/h/linear m = 12.0m³/h.
• Conclusion : At 600 Pascals, windows tested to both Classes 3 and 4 of BS 6375-1:2004 have a significantly higher air permeability than windows complying with the CWCT Standard.

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