The influence of the inclination of the roof window on its overall value of the heat transfer coefficient Uw is a neglected feature from the customer’s point of view, but this does not apply to designers who are aware of this negative influence and fight with roof windows from the point of view of the energy demand of the entire building, or rather against them.

The problem of the very value of the roof window heat transfer coefficient Uw is already based on the requirements of the standard itself, namely that it is measured or calculated from a relatively large standardized window size of 1.23×1.48m or from the largest manufactured size, which on the one hand does not generally represent commonly sold window sizes , but mainly thanks to the large area of the glazing compared to the area of the frame, it provides better heat transfer results than the dimensions actually available on the market. Another pitfall arising from the standard is the indication of the value of the thermal transmittance of a vertically installed window, which would not be harmful if the value of thermal transmittance did not deteriorate more or less with a change in the inclination of the installation with regard to the type of glazing. This effect is due to the fact that the gas flow in the cavity changes as the angle changes. Considering that the glazing has a significant influence on the resulting heat transfer coefficient Uw, therefore even the deterioration of the properties of the glazing by changing the slope of the installation is prescribed negatively in the total value of Uw. In the following graphic, you can see the influence of the slope on the glazing itself by calculation according to ČSN EN 673. The values are determined for the ideal location of the glazing, which means that the edge of the glazing, the type of spacer frame, sealing, etc. are not taken into account. The double-glazed glass has a composition of 4-16-4 , triple glass 4-16-4-16-4, inert gas Argon 90%.

From this point of view, it is clear that the deterioration of the heat transfer coefficient is more pronounced with double glazing than with triple glazing. Given that the glazing of larger windows has a relatively greater effect on the overall heat transfer through the window, a significant deterioration of this parameter can be inferred compared to the value given by the standard in the vertical position, which of course is definitely not worth presenting to window manufacturers. To get a better idea of the actual overall coefficient of heat transmission through the window at a given slope without calculation, we can simply apply the correction coefficient as stated by the British Center for Building Science BRE. It should be noted that the coefficients are indicative only and do not reflect the heat transmission coefficient for individual window sizes, which is usually worse for smaller windows than the one declared by the manufacturer for the size 1.23×1.48m.


Inclination of roof               Uw value adjustment (W/m2K)
Double glazed Triple glazed
70° – 90°    0.0    0.0
<70° and > 60° + 0.2 + 0.1
≤ 60° and > 40° + 0.3 + 0.2
≤40° and > 30° + 0.4 + 0.2
≤30° – 0° + 0.5 + 0.3


From the above, it quite logically follows that the use of double-glazed windows in roof windows is currently excluded, not to mention the tragic properties of double-glazed windows from the point of view of internal surface temperatures at the edge of the glazing, which are far from the values recommended by the ČSN 730540-2 standard. The result is dew or freezing of the glazing, especially in the corners. However, it cannot be universally said that this does not apply to triple-glazed windows, because even with them, if the glazing is structurally poorly solved and the conditions are unfavorable, dew can also occur.


Notify of
Inline Feedbacks
View all comments