What is the Uw value and how is it calculated?
The heat transfer coefficient of a window (Uw) expresses the loss of heat through a building element to the outside air. The lower the value, the better for the structure's energy efficiency!
As the formula for calculating the coefficient shows, the Uw value depends on several factors: window frame, insulating glass unit and glazing edge are the 3 components that influence the Uw value.
The proportions of the window frame and the insulating glass unit are each considered as a product of their specific U value (Uf and Ug) and their respective area (Af and Ag). Because the heat loss obviously occurs across the entire surface area of the window separating inside and outside.
The formula additionally considers the glazing edge as a third factor – i.e. the heat loss at the edge of the insulating glass unit. This is calculated as the product of the length of the glazing edge (lg) and the so-called length-related linear heat transfer coefficient Psi (Ψg). The so-called Psi value stands for the heat loss caused by the interaction of frame, glass and spacer bar.