The thermal transmittance of a window Uw, is the flow of heat through the window, in W/m2K. The closer the U is to zero the better the insulating performance of the window. On the other hand, the bigger the U, the larger the heat loss through the window and the more the energy needed to heat or cool the interior.

The heat transferred through a window or the required energy to keep the same temperature inside is E(Watts)= Uw x A x ΔT, where E is the heat transferred or the energy input, Uw the thermal transmittance of the window and ΔΤ the difference between the outside and the desired inside temperature.
Example:
An aluminum window, with WxH=1.0mx1.0m and single glass has Uw=6.1W/m2K. Let us say that the outside temperature 6oC and the desired inside temperature is 22oC. Then heat transferred through that window or the required energy input to keep the interior temperature is E(Watts)= U x A x ΔT, or
E = 6.1 x 1.0 x (22-6) or E=98W or 2.4KW in 24hrs. When the Baufens PVC tilt and turn window is used, the heat lost is only 0.45KW!! instead.

As glass comprises the highest percentage of the window overall area, the choice of glass is the No.1 energy decision. The best type is Low-E glass. The more the glass panes the better the thermal insulation. In addition, more panes and thicker or laminated glass provide better sound insulation. All Baufens windows come with Low-E glass.