All construction materials have a measure of how they conduct heat (Lambda value), used together, various materials will determine the overall thermal capacity of a wall; this is known as the U-value. The lower the U-value, the more thermally efficient the wall construction is.
A U-value will take into consideration the wall thickness & material and any render or insulation used i.e. a typical solid wall (9 inch brickwork) used in a Victorian property has a U-value of 2.1 W/m2K. Through the use of different types of insulation materials and thicknesses used in webertherm systems, the thermal performance of a property can be dramatically improved and U-values of 0.30 W/m2K (which is the current target for building regulations in England) and below can be easily achieved.
If the U-value for a wall is calculated to be high it means it is easier for the heat to escape through the structure of the building so will be more difficult to keep warm and expensive to heat.
External Wall Insulation will lower a U-value meaning less heat escapes from the building and continues to keep your house warmer for longer.