Now often quoted in building or building part specification, a U-value is placed upon an assembly of components to rate how well that assemblage performs in terms of energy efficiency.
It is important to note that the U-value relates to the performance of the assemblage as a whole. For example, the U-value of a window unit will be calculated with reference to the glass panes, frame, sealants etc. and their performance as a whole.
The U-value gives a numeric rating of the amount of heat that passes through the building part, therefore the lower the value the better the energy efficiency.