Part L1B (October 2010) of the Building Regulations deals with the conservation of energy in our homes. The thermal efficiency is measured as a U Value, the lower the U Value the better the thermal properties of the product in question. As described within the Energy Efficiency Frames section the U Values are banded to indicate their performance - the information depicted for various products provide a guide - it is understood that with every scenario type of dwelling it is the overall majority (combination of product ratings) U Value that is generally accepted and not an individual.
L1B for Dwellings -Domestic/Retail & New Build
The minimum requirement for Windows: WER - is to be Band C (or better): 1.6W/m2 K (U value)
The minimum requirement for Doors: DSER - is to be: 1.8W/m2 K (U value)