The U value is a measure of energy efficiency that is, how effective this particular element is at retaining heat, and is used for any kind of home construction. The lower the U Value, the more effective the unit is at saving energy because of its greater insulating quality.
So, in terms of double glazing the U Value can be used as a method of comparison of different types of unit in terms of energy efficiency
Double glazed windows and double glazed doors have a much lower U Value than single glazed windows have. The benefit of using the U Value to compare different kinds of glazing is that it gives an index of energy efficiency. Cutting down heat loss, reduces heating bills which in turn reduces carbon dioxide emissions overall by as much as 50% when replacing single glazed windows by double glazed units.
The recognised U rating method for double glazing is for the whole window including all components: glass, frames and spacers, th9ough sometimes a different measure – the centre of glass U Value is given which indicates the energy efficiency of the glazed units alone, and is usually used by double glazing companies (but also ask them about the other components as well).
A guide for the common U Values are: For a single glazed pane U Value = 5.6 watts per sq m Kelvin, which falls to a more efficient U Value for a standard double glazed unit of 2.8w/sq m K, to 2.6 w/sq m K for argon gas in the cavity.