The changes to the Building Regulations that have increased the requirements for the thermal performance of doors and windows came into effect in 2010. These can be boiled down to a need for replacement windows to have a U-value of 1.6 W/m2K and replacement doorsets to have a U-value of 1.8 W/m2K for England and Wales, and 1.6 W/m2K in Scotland. Also, for Scotland, where the external walls of an extension have a U-value above 0.7 W/m2K, which would be a poor figure, the windows and doorsets will need to have a U-value of 1.4 W/m2K.
One phenomenon of which to be aware is of condensation on the outside of the newly installed windows fitted with double glazing units with centre pane U-value of 1.2 W/m2K. Predicting when windows would be susceptible to external condensation is difficult, and if it does occur, it may not occur on all windows in a property. This is due to particular conditions of high humidity, no wind and low overnight temperatures need to happen together.