Air permability or air tightness is defined as the window capability to prevent air from passing through it.
Air permability is measured on a scale, from class 1, less tightness, to class 4, higher tightness.
There are several standards that relate to the Air Permeability and weatherproofing of windows and double and triple glazed units. These include:-
EN 12207:2000 Air Permeability which replaces BS 6375-1:1989 and partially replaces BS 6375-1:1989. Class 3 at 600Pa. or better.
Classification and Guidance for Weather Tightness BS 6375 Part 1: 2004
Windows & Doors, Air Permeability test method BS EN 1026: 2000
Windows & Doors, Air Permeability classification BS EN 12207: 2000
Windows & Doors, Classification for weather tightness and guidance on selection and specification BS 6375-1
For systemised building envelopes the standard is CWCT Standard:2006