The U-value (or U-factor) measures the rate of heat transfer through a building element over a given area, under standardised conditions; in our case, the frames and glass of our doors and windows. A smaller U-value is better at reducing heat transfer. To comply with building regulations on a new build a window must achieve a U-value of 2.0 W/m2K and in an extension or alteration 1.6 W/m2K. A door has to achieve a U-value of 2.0 W/m2K in a new build and 1.8 W/m2K in an extension or alteration. The only other way to pass building regulations for insulating for a window only (does not apply to doors) is to have a BFRC (British Fenestration Rating Council) energy rating of A-C.