The U-value describes the heat loss per sq. metre in relation to the temperature difference between the interior and exterior.

The U-value or transmission coefficient [W/m2K] is used in connection with the calculation of thermal energy. It is used when calculating the heat loss or gain within buildings and technical installations, as well as when calculating the dew point within a construction. 

Heat loss per unit area
The U-value of a building element, insulation or other construction is the relationship between the heat flux density and difference between the temperatures of the interior and exterior of the construction. Therefore, the U-value describes heat loss per unit area in relation to the temperature difference of a construction.

A low U-value is indicative of a small amount of thermal energy transfer through the construction.