A U-Value, or “thermal transmittance” value, measures how good a material is at insulating.
It is calculated by how much energy is transmitted through the material. So, the lower the U-Value the more energy efficient the material is.
U-Values are expressed by a figure such as “1.2W/m2K”, which stands for the amount of watts per square meter per kelvin.
The calculation assesses the amount of heat energy lost through the material (eg through a window) when there is a 1 kelvin (1°C) difference in temperature between the internal and external sides. The heat energy transferred is measured over 1m2 and expressed as watts.
To calculate the U-Value, the thermal transmission of all the layers of the element (eg the various layers which make up a composite door) over 1m2 are added together.
For consumers, U-Values are a good way of comparing different options in terms of their energy efficiency. If you’re fitting new double glazing, ask your supplier about the U-Value of the various options. This will give you a good indicator of how energy efficient each one is.
In the construction industry, the U-Values of all materials being used (windows, bricks, plaster, etc) are combined to give the building an overall U-Value. Buildings with lower U-Values tend to be cheaper to heat, more comfortable and better for your health.
There are two primary contributors to achieving a good U-Value.
The first is to use the most energy efficient materials, ie materials which have a low U-Value.
The second is the standard of installation. Even the most energy efficient window can be let down by poor installation, allowing far more heat to be transmitted through the window than is necessary.
This can vary hugely depending on the material being measured.
For a brick cavity wall, for example, a U-Value of 0.1W/m2K would be excellent.
Windows and doors will, naturally, never achieve this level of insulation! An energy efficient door should achieve a U-Value of 1.6W/m2K or less. High quality glazing would have a U-Value of around 1.0W/m2K.