PVC U windows have always been energy efficient. But now thanks to new government building regulations, there are minimum standards of energy efficiency that window, door and conservatories fitted in the UK must meet.
The regulations are known as "Document L" (Document J in Scotland) and they set energy efficiency targets that reduce heat loss from frames and glass to help Great Britain meet its commitment to reduce green house gases under the Kyoto accord.
To ensure that you fit replacement windows that meet these new standards, the Glass and Glazing Federation (GGF) and other industry bodies. With government encouragement, have set up FENSA - the Fenestration Self-Assessment scheme. Companies in the FENSA scheme will provide you with a certificate confirming that their windows comply with the new regulations. Installations by FENSA - registered companies are subject to random inspections by the schemes representatives.