A measure of how effective glass (and window) is in preventing heat escaping from the inside to the outside of a building. The lower the U Value, the greater resistance the glass has to heat flow and therefore the better insulator it is. U Value is measured in W/m2k.
For example, 4mm VFloat™ Clear glass has a U Value of 5.9, whilst ComfortPlus™ has a U Value of 3.6. So ComfortPlus™ is a better insulating product than 4mm VFloat™ Clear glass.