Uw = heat transfer coefficient for the entire window (w for window). The Uw value is influenced by four factors. The size of the window, the glazing, the frame and the linear heat transfer coefficient ψg . The Uw value often quoted by manufacturers usually refers to a standard window size of 1.23 m x 1.48 m. The smaller a window is, the more the Uw value deteriorates, as the area of the frame, which is usually poorer in terms of thermal insulation, becomes larger compared to the better glazing.
The Uw value is calculated using the following formula:
UW = (Ag x Ug + Af x Uf + Ig x ψg) / (Ag +A f)
Ug = Heat transfer coefficient of the glazing
Uf = Heat transfer coefficient of the frame
ψg = Linear heat transfer coefficient of the Insulating glass edge composite
Ag = glass surface
Af = frame surface
Aw = Ag + Af
lg = circumference of the inner edge of the frame profile
(or visible circumference of the glass pane)
Unit of measurement: W/ m²K
Source: bauwion
