Methods of Compliance

Whatever the particular U value required by a Regulation, the window supplier will need to demonstrate what the U value of the product is, in order to prove compliance.  There are basically three alternative ways of demonstrating a window's U value.

The first is by testing a complete window in a "Hot Box", to BS EN ISO 12567-1.  This is the most accurate, but also the most time-consuming and expensive method.  Pilkington has undertaken joint testing with a number of window manufacturers to obtain the U values of a range of windows incorporating Pilkington K Glass™.

The second is by calculation, using BS EN ISO 10077.  That Standard in itself has two alternative parts; the first is a simplified method which chiefly involves reading from tables and performing simple arithmetic, the second enables the U value to be obtained through a computer simulation of window performance, using advanced software.  The window configuration which should be evaluated is given in GGF Data Sheet 2.2 "Window and Door System U values: Provision of Certified Data".

Finally, in documents supporting some of the Regulations, there are tables of indicative U values for various frame and glass permutations.  Using these tables it is possible to read off directly, or interpolate, the U value of a particular window.  The advantage of this method is simplicity; the disadvantage is that it tends to underestimate a window's performance.

The Table of Default U values gives figures for windows based on the table of indicative values in England & Wales Approved Documents L1 and L2.