Window replacement conservation of fuel and power

Part L

The glazing specification should provide an appropriate thermal performance.

Typically accepted standard - Current Requirements

The replacement windows will need to meet the new thermal insulation requirements of the regulations. If the replacement windows have wooden or plastic frames, then the glazing needs to achieve a maximum U-value of 2.0W/m2K while for metal frames a slightly higher U-value of 2.20W/m2K is allowable. Please note that these values are very difficult to achieve and many double glazing units currently on sale will not meet these new standards. Take great care when ordering new windows that your supplier can prove the glazing units used will satisfy this requirement, as the Building Control Officer will need to see this proof before issuing a completion certificate. We would particularly recommend that you leave any labels on the glazing in place until after a satisfactory inspection has been carried out by the Building Control Officer.

In some cases it may be permissible to use glazing units which do not meet the above specifications, but to do so you (or your supplier) would have to submit calculations to prove that the overall insulation requirements of the regulations would still be met. This may be possible if other insulation measures are undertaken at the same time as the window replacement - for example, installing cavity wall insulation or "topping up" loft insulation. The benefits gained by installing this extra insulation can be used to offset the higher losses through the glazing, but this should not be undertaken lightly. We would strongly advise you get any such calculations checked by the Building Control Officer well before the replacement windows are installed, so that expensive mistakes can be avoided.

Note: For installations ordered before 1st April 2002 and installed before 1st July 2002 the ‘U’ value of the glazing need only meet current standards (3.3 W/m²K).