From 1st October 2010, window manufacturers needed to show that their windows complied with the energy efficiency requirements in the latest revision of Part L of the Building Regulations and the Approved Documents L1A (new dwellings) and L1B (existing dwellings).
Obtaining a precise U-value for a manufactured window can be extremely expensive especially for smaller manufacturers or architects with bespoke designs. U Value Calculator is able to calculate the u value for timber framed windows and doors at a very reasonable cost.
Architects and specifiers are now increasingly asking for the thermal transmittance values of windows, so that they can calculate the SAP ratings of the buildings where the windows are to be fitted. Clients require that value for completion certificates.
As a builder if you cannot provide U-values for your products, then your client may have to use the windows of a competitor who has that information to hand.