Double Glazing Specifications

Turning to thedouble glazed sealed unit itself, the insulating effect of double-glazing can be affected by a number of factors, including the cavity width, the use of argon instead of air in between the panes or by utilising a low emissivity coating that reflects heat back into the room.

The following table illustrates some typical U values (the measurement of heat transfer through glass) using various combinations of cavity widths, air or argon and Pilkington K Glass, which has a low emissivity coating.

Optifloat is the name given to 'normal' float glass by its manufacturer - Pilkington. The company's K-Glass has a microscopic coating on the surface, which is a poor radiator. This means that heat absorbed by the coated glass is inhibited from radiating across the air gap and then through the outer pane to the open air. It acts rather like silver foil placed behind a central heating radiator to improve its effectiveness and reflects any heat gained from the sun back into the room.

In appearance, it is virtually impossible to tell K-Glass apart from clear float glass apart from a minor effect it has on white light transmission. In certain circumstances such as strong oblique lighting, the coating may sometimes be seen as a transparent film, but this visual effect is only transient.

Double Glazing Regulations
With effect from 1st April, 2002, all replacement window installations became subject to Document L of the Building Regulations. This states that PVC-u or timber replacement windows should not have a U value higher than 2.0 W/m 2K and metal windows (aluminium) should not have a U value higher than 2.2 W/m 2K.

This new regulation particularly affects the minimum levels of insulation that replacement windows must have when fitted in your home. Levels of insulation are measured as U values. The lower the U value, the better the level of insulation. What was previously referred to a 'normal' double-glazing, ie two panes of glass separated by a spacer bar is very unlikely to conform to building regulations. To obtain the required level of insulation, it may be necessary to utilise low emissivity glass, Pilkington K for example, and fill sealed double-glazed units with an inert gas (probably argon).

With so many replacement windows being installed in British homes, it has been agreed that the double-glazing industry can adopt a 'self-assessment' method for administering the many thousands of installations that take place each week. The self-assessment scheme is referred to as F.E.N.S.A. - Fenestration Self Assessment Scheme. Providing the company you are dealing with is registered with the FENSA scheme, they will certify that the work complies with the relevant regulations and you will not need to notify your local council.

If they are not a member of FENSA or you are carrying out the work yourself on a 'DIY' basis, then approval will have to be obtained from the local authority.

It is most important to conform to these new regulations as it is entirely possible that you will need to provide proof of compliance with Document L if you wish to sell your property after having installed replacement windows.