The thermal transmittance of windows: what is it and how is it calculated
(UNI EN ISO 10077-1 UNI EN ISO 10077-2)
The thermal transmittance U is the average heat flux that passes, per square meter of surface, through a structure that delimits two environments at different temperatures (for example a heated environment from the outside, or by a non-heated). The unit of measurement is the thermal transmittance W / m 2 K.
Why is it important?
Since the aim is to save on energy, one needs to minimize heat dispersion. It becomes necessary then that the elements forming the building casing have a low thermal transmittance value, so as to reduce the amount of heat that is dispersed. The Uw thermal transmittance of windows for windows and doors, the thermal transmittance of the window represents the weighted average of the thermal transmittance of the Uf frame and that the Ug glazing, plus an additional contribution, the linear thermal transmittance Ψg, due to ‘interaction between the two components and the presence of the spacer, is applied along the perimeter of the window that is visible.
How do you determine the thermal transmittance value of the fixtures
Typically, the thermal transmittance Uw of a window frame, can be determined either by calculation of, or through an experimental test on a physical sample. Here are patterns that can be used for:
a) the estimated values of Uw of the existing fixtures, in the absence of accompanying documentation used in a stringent calculation; b) the simplified calculation, according to the norm EN 10077 to determine the value of Uw in reference to all types of frames, c) the method of stringent Uw calculation, according to the formula of the literature; d) the methodology of calculating Uw in reference to the standard fixture and its relative extension rules, according to the UNI EN 14351-1
The simplified calculation of the thermal transmittance Uw
Below, we report on the more specific estimation methods more for the different types of windows and doors. In the absence of specific data of the project in relation to the elements, it is possible to estimate the original thermal parameters and following the indications contained in – Recommendation CTI – R03 / 3 Energy performance of buildings. Energy certification. Handing out the energy performance certificate. Data for the building. The norm- UNI EN ISO 10077-1, which provides a simplified calculation method for the estimation of thermal performance of frames and tabulated values of the thermal transmittance of the main types of glazing. The characterization of the fixtures are: – the component type – the type of glass; – the type of frame. The thermal transmittance Uw (W / m 2 K) is determined according to the UNI EN 10077-1 with the simplified calculation method, or deriving it from the prospects F.2 and F.3 referring to the window sash vertically positioned, with sizes – considered representative – equal to 1.23 mx 1.48 m, depending on the value of the Ug glazing, the Uf value of frame, and the percentage of the area of the frame with respect to the entire window. Vital for the purposes of calculating the Uw value of the finished window : – choose the thermal characteristics of the Uf frame, – choose the thermal characteristics of Ug glass, crosscheck the values of Uf and Ug, (choosing the % of the frame with respect to the whole window between 30% and 20%), and find the Uw value of the casing with the chosen features.
Product norm UNI EN 14251-1 Windows
The norm UNI EN 14351-1 Part 1 “Product standard, performance characteristics of windows and external pedestrian doors without the characteristics of resistance to fire and / or smoke leakage” is the Italian normative reference for the CE marking of transparent windows . Such legislation specifies that the value of the thermal transmittance of the window including the frame can be obtained either by laboratory testing or by simple calculation according to UNI EN ISO 10077-1 or still more using finite elements according to UNI EN ISO 10077-2 in combination with the simplified calculation.