Window U-value (Uw) is a measure of the heat gain or loss through glass due to the difference between indoor and outdoor air temperatures. The heat transfer coefficient Uw relates to the entire window.
A lower U-value indicates better insulating properties. The U-value of our windows can be as low as 0.72. The U-value worsens as the size decreases, larger windows features better values. This is because U-values achieved in glazing are better that in the frame material therefore a larger glass area is able to produce a better thermal insulation value.
U-value also incorporates the U-values for the glazing and the frame Uf. The overall value Uw is also influenced by the linear heat transfer coefficient (g – glazing) and the size of the window.

U-value of window frame (Uf). The Uf-value for the frame-sash combination is defined by means of measurement or calculation. The area for the calculation of the Uw-value is the cross-section of the profile.

U-value of window glazing (Ug). The Ug-value is a function of the type of gas filling of the intermediate space between the glass sheets, the distance between the sheets and the number of sheets.
Glass unit consists of 2 or 3 glasses connected with spacer. Gaps between glasses are filled in with Argon gas, which is in sufficient amounts completely harmless to a living creatures health, but is a very good material to ensure our windows a best possible U value.