U-value is also often referred to as «thermal transmittance», which refers to the rate at which a structure allows the transfer of heat energy, divided by the temperature difference on either side of the structure.
Therefore, U-values are essentially a measure of the effectiveness of a material’s insulation. When this value is lower, it means that the material is considered as performing more efficiently as an insulator against heat loss from an internal space, than when it is a higher value.
U-value is measured in W/m²K. The average cavity wall will have a U-value of around 1.6 W/m²K, whereas a wall built of solid brick will be lower at around 2.0 W/m²K. For a window, this can rise to around 2.8 W/m²K. This is typical for existing dwellings, but many countries now have guidelines regarding the maximum allowed U-value that a new build window may have.