Double Glazing

What exactly is double glazing?

Double glazing is today’s general term used for uPVC windows / replacement windows. The double glazing part of the window is the glass, or sealed unit, this comprises of two panels of glass, i.e. double glazing or double glazed.

uPVC windows / replacement windows glass panes are kept apart by a spacer bar which contains hundreds of tiny silica balls. These same balls can be found in the little bags you find in the packages when you buy electrical goods to keep then dry and free from moisture. It is these balls which soak up any moisture which exists in the air gap between the two panes of glass.

Normal float glass or safety toughened (tempered) glass?

uPVC windows / replacement windows glass itself will either be normal float glass for small double glazed sealed units or safety toughen also known as tempered glass for larger double glazed sealed units as required by building regulations and the Glass and Glazing Federation.

What is tempered or safety glass? Tempered glass when broken, shatters into many small fragments, which prevent major injuries. Due to these characteristics tempered glass is used traditionally in applications requiring increased strength such as Patio doors, French doors or Upvc windows, which reduces the likelihood of injury in the event of breakages. However tempered glass should receive the same care as annealed glass.

uPVC windows / replacement windows Insulation

Turning to the sealed unit itself, the insulating effect of double glazing can be affected by a number of factors.

• Double glazing insulation is affected by cavity width of the sealed unit.
• The use of argon instead of air in between the panes.
• Utilising a low emissivity coating that reflects heat back into the room.

The latest uPVC windows / replacement windows now offer a greatly improved insulation compared to traditional single glazing openings. In addition to their better insulation, uPVC windows / replacement windows are also draught-proof, therefore reduces the amount of heat that can escape, so you can cut down on your heating and save on fuel bills without sacrificing comfort.

The insulation of Upvc windows / replacement windows is measured in U values (the measurement of heat transfer through the window it's self). The following table illustrates some typical sealed unit U values using various combinations of cavity widths, air or argon and Pilkington K Glass, which has a low emissivity coating.

Diagram showing heated reflected 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.

Argon Gas double glazing sealed units

Argon is a colourless, odourless, non-flammable, non-reactive, inert gas. Argon gas fills are used to reduce heat loss in double glazing sealed units by slowing down convection inside the air space. Argon gas is extremely cost-efficient, and works well with Low-e coated glazing.

When we talk about insulation glass without low-e coating, we refer to glass that uses air between panes as a primary source of insulation. As air itself is a good insulator, filling the gap between the glass panes with a low-conductivity gas such as argon improves window performance by reducing conductive and convective heat transfers. This phenomenon results from the fact that the density of the gas is greater than the density of the air. Argon is the most commonly used fill gas, due to its excellent thermal performance and cost-efficiency in comparison to other gas fills.

All uPVC windows / replacement windows and doors now come under the requirements building regulations and document L. Now requires that all uPVC windows / replacement windows and doors must meet a high standard of insulation.

uPVC windows / replacement windows Building Regulations

This new uPVC windows / replacement windows regulation particularly affects the minimum levels of insulation that replacement double glazing 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 double glazing 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).

uPVC windows / replacement windows FENSA Scheme

With so many uPVC windows / replacement windows being installed in British homes, it has been agreed that the uPVC windows / replacement windows 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 double glazing windows.

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