The Zero Stress Guide to Double Glazing


What You Need To Know Before You Buy Double Glazing Windows

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Replacement windows building regulations

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  • Replacement windows must meet minimum insulation levels
  • The lower the ‘U’ value, the better the insulation
  • Proof of compliance required when you sell your home

Thanks to world-wide agreements to reduce energy consumption, there are now strict regulations covering replacement windows.

All replacement window installations in England and Wales are subject to Document L of Building Regulations which affect, in particular, the minimum levels of insulation that replacement windows must have when fitted in the home.

These levels of insulation are measured as U values. The lower the U value, the better the insulation level. In future what was in the past referred to as ‘normal’ double-glazing - i.e. two pieces of glass separated by a spacer bar – is very unlikely to conform to building regulations. To get the required level of insulation it is almost certain some sort of Low E glass (typically Pilkington K in the UK – although there are other brands) will have to be used. It may also be necessary for the sealed double-glazed units to be gas-filled (probably Argon).

FENSA self-certification scheme

With so many replacement windows being installed it was agreed that the industry could adopt a self-assessment method for administering the many thousands of window installations that are now subject to building regulations. What this means is that it is not always necessary for a building control officer to inspect each installation or for companies to make separate Building Regulations applications.

A contractor registered with the FENSA (Fenestration Self-Assessment) self-certification scheme is approved to carry out the work in accordance with relevant regulations without inspection by the council and will inform FENSA when installation has been completed. Random inspections of completed work are carried out.

If you are not using a FENSA registered contractor or if you are doing the work yourself then you will need to arrange for building regulation approval. This will mean that you must be able to confirm that you have met a number of criteria. You will be responsible for paying for the building regulations application and should check with your local council for costs. The time taken to obtain approval will depend on the local authority concerned.

In addition to Building Regulation consent you must ascertain whether or not planning permission or conservation area consent is required. Your local council can advise you on this.

For further details see

Scotland’s Building Standard Part J requires an even higher performance level from windows than that specified in England and Wales.

Replacement windows and doors in Scottish homes are expected to achieve U values 10% lower than those in England and Wales, which means a difference of 0.2 of a U value. Soft coat Low E glass with a 16mm cavity containing an inert gas will be necessary in most cases.

This means that in most existing dwellings, replacement windows will need to have a U value of not more than:

  • 1.8 for windows made of plastic or wood
  • 2.0 for windows with metal frames

Windows also need to comply with all other appropriate aspects of the technical standards to the Scottish building regulations – eg emergency escape, safe-cleaning, safety glass and ventilation.

More information is available at

Selling your home

When you sell your property, surveyors will ask for evidence that any replacement glazing installed after April 2002 complies with the new Building Regulations. There are two ways in which you can prove compliance:

  • a certificate showing that the work has been done by an installer who is registered under a FENSA, CERTASS Limited or the British Standards Institution self-certification scheme, or
    a certificate from the local authority saying that the installation has approval under the Building Regulations.

If there is any doubt, a glass analysis gauge can be used to establish whether or not the correct glass has been used.

Note that this only applies when windows have been replaced – original windows are not subject to this scrutiny.

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