Weather performance

Experience has shown that steel windows are suitable for the majority of sites and exposure conditions in the UK, where an exposure rating of 1200 Pascals satisfies most locations. Extensive laboratory prototype and production audit testing give typical results tabulated below, classified in accordance with BS 6375-1. The extra weatherseals introduced in W40 to provide an external rainscreen, internal airseals and a pressure equalised cavity have significantly improved resistance to air and water penetration in severe exposure conditions, particularly for windows that open inwards.

BS 6375-1:1989
Air permeability was measured in terms of opening joint length (m³/h/m) against progressively increasing test pressures through 200Pa (class A), 300Pa (class B) up to 600Pa (class C). Class B, or a maximum value at 300Pa of about 16 m³/h/m, was the UK standard requirement. Watertightness was measured in resistance to leakage at progressively increasing test pressures, 300Pa being considered the most severe UK requirement. Wind load resistance entailed deflection and gusting tests at pressures ranging from 1200Pa to 2400Pa.

BS 6375-1:2009
Air permeability is measured in terms of both area (m³/h/m²) and opening joint length (m³/h/m) against progressively increasing test pressures through 300Pa up to 600Pa for classes 3 and 4. Class 2, or a maximum value at 300Pa of 13 m³/h/m, is the UK standard requirement. Watertightness is measured in resistance to leakage at progressively increasing test pressures, 300Pa being considered the most severe UK requirement. The values tabulated in Pascals can also be expressed in classes ranging from 2A (50Pa) through 8A (600Pa) to exceptional resistance up to a maximum of E1050. Wind load resistance entails a deflection test (at 2000Pa for class 5), a repeated pressure test of 50 positive and 50 negative gusts at half the deflection test pressure, and a safety test at 150% of the deflection test pressure (i.e. 3000Pa for class 5). The prefix C means that deflection of the longest frame member was less than 1/300 with less than 1/150 (prefix A) considered adequate in the UK. The 2009 edition of BS 6375-1 clarifies and supersedes the 2004 edition but does not change either of the test methods.

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