BWF members such as yourselves frequently use our Member and Technical Helplines for assistance on a wide range of Employment, Health and Safety, Pay and Technical issues. We’re putting up a frequently asked question every Tuesday in order to help members and illustrate the type of advice we are able to provide. If you have any questions that you would like to see featured, we would be happy to assist. This question and many others like it can be found in our Question Centre. If you want further advice on any of the issues, don’t hesitate to ring the helpline or browse our extensive publications library, which includes our Fact Cards and guidance on installing timber windows. This week: Won’t a Window With a Centre Pane U-Value of 1.2 W/m2K Potentially Cause Exterior Condensation? The changes to the Building Regulations that have increased the requirements for the thermal performance of doors and windows are now in effect. While any existing contracts can be completed under the old values, any new work needs to be upgraded to the new levels. 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, low overnight temperatures and no wind need to happen together. The higher insulating properties of the improved glazing units reduce the amount of heat escaping from the property and the external face of glass may not be warmed to a temperature above the ‘dew-point’ of the outside air and thus condensation occurs. A householder waking to new windows through which their view is obscured by condensation may think that the glazing units have failed. The condensation will disappear as the outside conditions change or the surface temperature of the glass reaches a higher temperature as the property is warmed. If the glazing units are fitted with traditional aluminium spacer bars, heat conducted through the spacer bar will warm the edges of the glass. The external condensation may only form in the centre of the glass with a clear band around the edge. Where the thermal properties of the glazing units are enhanced with the use of a ‘warm-edge’ spacer bar, this clear band would be reduced or eliminated altogether. We recommend that you either brief the client that this may occur during installation, or at least be ready to explain should they query the window’s performance, that the condensation will disappear in a short period of time when the conditions change.