The image below, acquired with an infrared thermal camera, illustrates all anomalies in the distribution of heat of a general window, thanks to which thermal bridges can be seen.
WHAT IS THE THERMAL TRANSMITTANCE OF WINDOWS?
Thermal transmittance is indicated by “U” and is the flow of average heat that passes, per square metre of the surface area, through a structure that divides two environments at different temperatures (for example an environment heated from the outside or an environment not heated).
The unit of measurement of thermal transmittance, in the INTERNATIONAL SYSTEM is W/m² K.
WHY IS THE "U" TRANSMITTANCE IMPORTANT?
As the objective of energy savings is to reduce heat dispersion, all of the elements that form the building must have a low value of "U" thermal transmittance, so as to reduce the quantity of heat scattered.
Basically, the lower this value, the greater insulation of the window.
With this news we will not focus on the real calculation of transmittance, but those who are interested in receiving further information on this topic may request informative material from the NURITH Technical Department (email@example.com).
For the "U" calculation, therefore, 4 possible cases exist:
a) an estimate of the Uw values of existing windows, in the absence of supporting documentation that can be used for strict calculations;
b) simplified calculations, according to the norm EN 10077 for definition of the Uw value with reference to all kinds of windows;
c) rigorous calculations of Uw, according to the literature formula:
d) calculation of Uw with reference to a normalised window and the relative rules of extension, according to the norm UNI EN 14351-1.
We will now summarise letter c), therefore the "rigorous calculation" according to use of the literature fomula;
Thermal transmittanc Uw is the average between thermal transmittance of the frame Uf and that of the glass Ug, plus an additional contribution, the linear thermal transmittance Ψ, due to interaction between the two components and the presence of a spacer, applied along the entire visible perimeter of the glass.
c) Rigorous calculation
Calculation of thermal transmittance of the Uw window component consisting of one single window and relative glass (or panel) is carried out, as indicated, using the mathematical formula from which the Uw value is elaborated, always expressed in W/m²K (Watt per square metre Kelvin).
Uw = (Ag Ug + Af Uf + IgΨg) / (Af +Ag)
This value is affected by various factors, some of which are dictated by norms, others are geometric, others still are dictated by the manufacturer of materials:
Ag is the area of the glass;
Ug is the value of thermal transmittance refered to the central area of the window and it does not include the spacer effect of the glass along the entire edge of the glass itself;
Af is the area of th frame;
Uf is the value of thermal transmittance of the frame applicable in the absence of glass;
Ig is the length of the perimeter of the glass;
Ψg is the value of linear thermal transmittance regarding the conduction of additional heat due to interaction between the frame, glass and spacer of the glass according to the thermal properties of each of these components.
WHAT ARE THE VALUES OF "U" TRANSMITTANCE IN ITALY?
As years go by, norms are stricter and more severe and always protect internal thermohygrometric comfort. Italy is currently divided up into six climatic sections, according to external thermohygrometric conditions.
The Decree of 26th January 2010 (Updated by the decree of 11th Marrch 2008) related to energy requalification of buildings, paragraph 2, specifies the limit values of "U" thermal transmittance that windows should observe in general, valid as of 1st January 2010.
THE VALUES OF "U" TRANSMITTANCE OF NURITH WINDOWS
Naturally, the lower the limit is, the "colder" the climatic area is.
By examining the most rigorous of climatic areas, it has a limit transmittance value (obviously this figure refers to Italy) of U = 1,6 W/m²K. By comparing this value with that of the one achieved through tests on our windows, we can guarantee a safety margin of more than 30 %, with adequate glass thanks to our windows DOMUS and PR1MA, we can even reach U = 0,80 W/m²K, exactly half the limit in Italy.
For further information please take a look at our product catalogue.