The thermal transmission coefficient U [W/m2 •K] specifies what heat quantity in [J] is transferred by a component in a steady state over a period of 1 [s] over an area of 1 [m2] at a temperature difference of 1 [K] (from air to component to air). It is the inverse value to the total thermal resistance. The U-value thus defined is a dimension for the "thermal transmission" or the thermal insulation properties of components.
The smaller the thermal transmission coefficient U, the greater the thermal insulation property of the component. In Germany, regardless of whether the project is a new build or a renovation, the requirements of the currently valid EnEV must be considered.
According to the European standard DIN EN ISO 10077-1, the insulation values of a window are calculated by splitting it into three zones: frame, glazing and glass composite edge. The components for the calculation are named accordingly: Uf (f for frame), Ug (g for glazing) and ψ (psi = glass composite edge).
The values Uf and Ug are factored into the calculation with their respective area. By contrast, the glass edge psi is considered according to its length.
From these components, the total thermal insulation value Uw (w for window) is calculated according to the following formula:
Uw = (Af • Uf + Ag • Ug + lg • ψg) / Aw (W/m²K)
Uw: U-value of the window [m²]
Aw: area of the entire window, including the frame
Ag: visible glazing area [m²]
Ug: uninterrupted glass U-value [W/m²K]
Af: frame area [m²]
Uf: U-value of the frame [W/m²K]
lg: circumference of the glazing (length of the glass composite edge
ψg (psi): thermal bridge loss coefficient of the glass edge in W/mK
The calculation formula shows that the geometry of the window plays an important part. With the same window construction, different U-values are possible with different window sizes. Note, however, that due to the large frame portion, small window sizes are particularly unfavourable, and narrow and long windows evince unfavourable thermal insulation properties.
In most cases, the Uw value of the test window size is specified in the documentation. This is 1230 x 1480 mm.