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Choice of the system depending on Uw


The U value, that is a thermal insulation measure, plays a very important role in construction as it describes how much thermal energy is transported through a building component. This coefficient is expressed in units (in/m²K) and it depends on the type and thickness of the material from which it is made, as well as fitting method.

Total thermal insulation value of the windows - Uw - shall determine the loss of the heat in watts (W) on 1 square metre of the surface separating the external and internal environments with a temperature difference on both sides of the window of 1 K (ºC) within one hour.

There is lot of factors impacting on the Uw value to include every part of the window from glazing, frames to the connectors themselves. Each and every window has its own thermal insulation value, which is depending on the type of the window and its size. Bigger windows have a better Uw value due to the fact that the glass and frame have different U values. More glass means a warmer window, never the other way around.

  • A lower thermal transmittance value (Uw) means less heat loss
  • Better quality windows and fitting techniques means more energy savings and lower heating costs.
  • When choosing windows the Uw factor is the most important value to look for (separate glass value Ug or frame value Uf are not so important).

To call a window an energy saving window they are require to have a Uw value below 1 W/m2K. Passive windows have even higher requirements, the value must be below 0,8 W/m2K and for a built-in window no more than 0,85 W/m2K.

Table values for total thermal insulation coefficient of the window (Uw) is based on window size of 1230x1480mm and it also depends on the type of the window itself, its profile and frame type where:

  • Uw - Total thermal insulation value of the window
  • Ug - Thermal insulation value of the glazing
  • Uf - Thermal insulation value of the window frame

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