Window glazing and frames
The entire window, ie: glazing and frame, should have a U-value of 0.80W/(m²K) or less (this value may have to be more stringent in more extreme climates, whereas milder climates may manage to meet the criteria with higher U-values) and the installed window should have a total U-value of no more than 0.85 W/(m²K). It is therefore essential to use well-insulated window frames with multiple lip packing. Glazings should have a high total solar transmittance (g-value) of at least 50% making a net heat gain possible during the winter (although lower g-values may be appropriate for extremely warm, sunny climates). The windows themselves must be airtight and the spacers in the glass seal edge must be thermically separated, ie: not aluminium. Windows should be installed in a thermal bridge free manner in the insulation layer.