U value – the choice of warm windows

For those who want to save heat energy, when choosing windows should pay attention to the heat transfer coefficient of the window, which is denoted U_w. The heat transfer coefficient (U_w) s the main criterion that shows how much heat is transferred through parts of a building from the inside of the building to the outside.

The value of the thermal conductivity coefficient U_w is determined by the values of the individual window elements – the values of wood (Uf) and glazing (Ug).

There are three different U values:

 U_w (w = window) – heat transfer through the whole window.

 U_g (g = glass) – heat transfer through the window glass.

 U_f (f = frame) – heat transfer through the window frame.

The value of U_w is determined by the linear coefficient of thermal conductivity – ψg (g = glass) and the size of the window.

We note:

The larger the window dimensions, the better the U_w value. The better U_w is determined by the large glass surface, which has good thermal insulation properties. Up to a 10 % better U_w value can be achieved by applying a warm glazing frame, which is characterized by the parameter ψg, for comparison, the value of the aluminum frame is 0.081, and the value of the warm frame is up to 0.029.

The lower the value of U_w obtained, the better the thermal insulation, the less heat is transferred to the outside.

The heat transfer coefficient (U_w) is a key criterion for the energy efficiency of windows, strictly regulated by EU regulations.