If you are choosing window fixtures for your house, or if you have decided to change that one that you already have, you might need to look into certain variables such as thermal transmittance. If it’s true that the objective of energy savings is to minimize the dispersion of heat, is necessary that all the elements composing a building have a low value of thermal transmittance, to reduce this heat dispersion. Thermal transmittance, also known as U-value, is the rate of transfer of heat (in watts) through one square meter of a structure, divided by the difference in temperature across the structure. It is expressed in watts per square metre kelvin, W/m²K, or W.m−2.K−1.
This is the technical definition of thermal transmittance and the arithmetic formula. Obviously the lower is the value, the higher is the isolation of the building. The opposite of the transmittance is the thermal resistance, which is the capacity of a material to resist the warm transmittance:
However, to simplify this definition, we can say that thermal transmittance is a value that, calculated with an equation, determinates the insulating power of a window fixture, based on its thermal performance. For windows thermal transmittance (Uw) of the window fixture can be expressed by the average between the thermal transmittance of the frame and the one of the glass. After that, we need to add up the effect of linear thermal transition (Ψg), given by the interaction of the two components and at the presence of the frame spacer along the perimeter of the glass. Window fixtures producers need to declare the values of thermal transmittance by law in order to use the mark CE (as prescribed by the product regulation UNI ISO 9869). The tests and/or calculations for the CE certification of window fixtures need to be performed by official public authority or by a certified company.