U-values explained

The lower the U-value, the better the thermal performance

Thermal performance is judged by the heat loss through a substance – in our industry this is the glass and window frame as a combined product. This heat loss which is the thermal conductivity of the glass and window products, is known as the U-value. It is expressed: W/m2.K.

This is explained as:

Watts – the amount of energy conducted per (/) metre squared – the area of the substance (.) multiplied by the temperature difference on either side of the substance being measured in Kelvin (Kelvin is the same structure as Celsius).

E.g. The U-value figure 1.2 W/m2.K, means the amount of energy (1.2W) that will pass through 1m2 area of a substance for each degree of temperature difference from one side to the other. In our case heat passing through sealed units/windows.

In our market there are two commonly quoted U-value measurements.

Centre pane U-values. This is the measurement of energy conductivity through the middle of a pane of glass whether it is single glazed, double glazed or triple glazed, etc. It does not take into account anything at the edge of the glass such as the spacer bar or window frame.

Window U-values. This is the measurement of energy conductivity through the window that is made up of glazing and frame. Here spacer bar plays an important role, as may sealant.
This is why window U-values are improved using warm edge spacer bar but centre pane U-values are not.

The higher the U-value of glazing or windows, then the higher the energy conductivity through them.
This means that lower U-values mean better performance in terms of insulation. This is why our customers often want lower U-values as they are proof of better performing products.

Simple U-value calculation

A window measures 1.2m x 1.2m = 1.44 m2, the U-value is 1.6W/m2.K – a typical ‘C’ rated window. This window will conduct 1.6 W of energy per m2 for every degree (K) difference in temperature from one side to the other.So, if it is the depths of winter and say 20 ºC inside and -1ºC outside, this is a 21ºC difference. This window will have conductivity rate of 1.6 (U-value of window) x the 1.44 m2 area of window x 21 (difference in temperature)

= (1.6 x 1.44) x 21 = 48W of potential energy loss

The same size window with a U-value of 1.4W/m2.K – a typical ‘A’ rated window would work out:

= (1.4 x 1.44) x 21 = 42W of potential energy loss

The same window with a U-value of 2.0W/m2.K – typical old windows with hard coat sealed units

= (2.0 x 1.44) x 21 = 60.48W of potential energy loss.

Thus: the lower the U-value the lower the energy loss.