The type of frame affects the level of thermal insulation it can provide. Thus, as mentioned above, opening frames usually offer higher thermal insulation compared to sliding ones, as their design allows for higher tightness, thanks to the perimeter insulating rubbers around the leaf and the frame. For the same reason, namely the level of tightness, a double-leaf sliding frame in which one leaf is sliding and the other fixed, offers greater thermal insulation than a corresponding one in which both leaves are moving.
However, to be sure that the frame we choose has a high level of thermal insulation, we should examine two basic indicators: the Uf and Uw of the frame.
Uf is the indicator that indicates the level of thermal insulation of the frame without the glass, that is, it clearly measures the thermal loss of the frame and the sash. Through Uf, we can directly compare two different frames and draw useful conclusions about the level of thermal insulation they offer. You should also always ask for the Uf certifications of the system company, because only in this way can you be sure that the product has been objectively tested by a third independent body. However, in order to reach a final decision, we must examine the Uw index, which measures the thermal losses of the window as a whole, including the glass that will be placed in the frame.
Taking into account the fact that the glass usually constitutes 70%-80% of the surface of a window frame, we understand that the Uw index plays a decisive role in the thermal insulation we will have.
For example, a simple aluminum frame without thermal insulation can have, with the application of an energy-efficient glass, a level of thermal insulation similar to another more expensive thermally insulating aluminum frame in which, however, no energy-efficient glass was installed. We should therefore pay attention to both indicators, but giving more emphasis to Uw which is essentially the final thermal insulation index of the window. It should be noted that in entrance doors the corresponding indicator is called Ud and measures the level of thermal insulation of the door in combination with the panel that has been applied.
In addition, the Ug index determines the thermal insulation of the glass. In essence, Uf combined with Ug form the final Uw (for windows/balcony doors) or Ud (for exterior doors).
It is important to remember that all of the above indicators measure heat loss, which is why they are also called thermal transmittance indicators. The lower the heat loss, the greater the thermal insulation offered by the frames in question. Therefore, the lower the Uf, Ug, Uw and Ud indicators, the better the energy performance of your frames. For example, a sliding frame with Uw =1.6 W/m2K will offer higher thermal insulation compared to a sliding frame with Uw =1.9 W/m2K. Finally, we emphasize that Uw and Ud should always be calculated and compared for specific frame dimensions.