Technical Booklet F1 Conservation of Fuel and Power in Dwellings·Page 48·3.48
Window Energy Rating (WER) Calculation Formula
The WER is calculated using a specific equation that incorporates frame factor, glass solar transmittance, window U-value, and air leakage to determine window energy performance.
The Window Energy Rating (WER) is given by the following equation –
WER = 196.7 x {(1 – f) x gglass} – 68.5 x {U + (0.0165 x AL)}
where –
f is the frame factor (i.e. the percentage of the window obscured by frame and gaskets);
gglass is the normal total solar energy transmittance of the glass as determined by BS EN 410;
U is the whole window U-value as given in paragraphs 3.46 and 3.47.
AL is the air leakage through the window in m³/(h.m²) at 50 Pa pressure difference based on testing to BS 6375-1.
Note – AL is based on the whole window area, not per unit length of opening light.
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