The total thermal transmittance (Uw value) of doors and windows is expressed in W/m2K. It is obtained by dividing the sum of the thermal transmittance values of its components by the total area of the door or window, where:

Uf = heat transmittance of the frame profiles

Af = area of the frame profiles

Ug = thermal transmittance of the glass

Ag = area of the glass

I = thermal transmittance of the material of which the spacer bar between the panes of glass is made

L = perimeter of the spacer bar(s)
according to the formula

**Uw = (Af*Uf + Ag*Ug + I*L) / Af + Ag**

The Uw value provides an indication of the quality of heat that is able to pass through the door or window as a whole, from the interior (hot zone, assumed to be 20°C) to the exterior (cold zone, fixed at 0°C), per unit of area and per 1° of temperature difference.

Therefore, the lower the thermal transmittance value Uw, the higher the thermal performance of the door or window and the greater the energy saving it yields.

The above formula for calculating thermal transmittance (Uw) complies with the formula laid down in UNI EN ISO 10077-1:2007 and is used to determine the thermal performance of the door or window for the purposes of CE marking and checking compliance with Italian Legislative Decrees 192/05 – 311/06, and as evidence of eligibility for the tax relief established in the Italian budget, which was renewed until the end of 2010.

Although it is not complex, the formula has to be used for each door or window and each type of system, making it time-consuming to apply.

The concept of extendibility of results and of specimen doors and windows, introduced by UNI EN 14351-1 (CE marking of doors and windows) in Annex E, is a useful way of avoiding repetitive calculations and estimating the real performance of the door or window as a combination of system+glass+spacer bar.

The standard allows you to calculate the thermal transmittance for each
type of door or window and then extend it to ALL doors or windows of THE SAME TYPE within the limits set down in the standard.

For example: you can calculate the thermal transmittance Uw of a window with two sashes measuring L 1230mm x H 1480mm (specimen window) with the Gold 600 TT system, with low-E glass with a rating of Ug 1.1 and a traditional metal spacer bar, and then extend the value thus calculated (Uw= 2.2 W/m2K) to all the windows having the same characteristics and with an overall area of less than or equal to 2.30m2.

A window measuring L 900 x H 1000 (Area 0.900m2 < 2.3) will take on the Uw value of 2.2 W/m2K.

As specified in Annex E of UNI EN 14351-1, the specimen doors and windows that can be used for calculating thermal transmittance Uw and then extended, for windows and patio doors, are as shown in the table:

If the glass has a Ug value of less than or equal to 1.9 W/m2K, you can extend the Uw rating calculated on the type of door or window measuring 1230×1480 to all doors or windows of the same type, regardless of the area and hence dimensions of the actual door or window to be evaluated.

The software application called Uw-Facile by Indinvest takes account of the possibility of using specimen doors and windows, and the latest version of it is kept available on our website at all times.