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Calculated U-values

Selectaglaze has worked with the Centre for Window Cladding and Technology (CWCT) to calculate U-values for their full range of secondary windows in combination with existing single glazed windows constructed from different materials and at various air spacings (cavities).  For comparison purposes, the calculations are based on the British Fenestration Rating Council's standard window size, 1230mm wide and 1480mm high.

Results

• Secondary Glazing with Standard glass in combination with a primary window can reduce heat loss by around 50%.
• Secondary glazing with low emissivity glass exceeds the performance required by Part L of the Building Regulations.
• Secondary Glazing with sealed glass units offers significant insulation levels.
• The effect of glass thickness is negligible whilst larger air spacing will marginally reduce the insulation level.

Note 1: Limiting value for new build dwelling and non-dwelling when other building elements offer higher insulation.  Total energy loss from building to be within regulation.

Note 2: Area weighted average for new-build dwelling and non-dwelling and standard for replacement window in non-dwelling.

Note 3: Standard for replacement window in dwelling.

Why Secondary Glazing helps Building Energy Performance

Building Regulations and U-values