The most popular measure of energy efficiency in your home is the U-value. This describes how effective a material is as an insulator. The lower the value, the less heat is able to pass through a given area of material in a given amount of time. It’s typically measured in watts per metre squared. A standard double-glazed window will have a U-value of just under three. This measure of efficiency, while accurate, tends not to be terribly useful for homeowners. The figure advertised tends to only account for one part of the door (the glass) and only in a specific area (typically the dead centre). It’s therefore worth considering the colour-coded energy ratings offered by the British Fenestration Council, which vary from A++ right down to E (the lowest legally available). Not only are these ratings more understandable, they cover the performance of your entire unit - whether that's a window, door or something else. Wooden French doors tend to be more efficient than their uPVC equivalents, because the material is much denser. Heat therefore has a far more difficult time passing from one side of the material to the other.