Double glazing has changed dramatically over the last 40 years since its first domestic manufacture to today. From the original double glazed sealed units of two sheets of float glass with a very small spacer bar to to-days high spec double glazed sealed units with toughened safety glass, wide spacer bars, argon fill sealed units, Low-E glass, also a self cleaning process, a film on the glass which allows the double glazed unit to clean itself. This makes for a complete new Hybrid double glazed sealed unit.
With effect from 1st April, 2002, all replacement double glazing window installations became subject to Document L of the Building Regulations. This states that new uPVC or timber double glazed windows should not have a U value higher than 2.0 W/m 2K and metal double glazed windows (aluminium) should not have a U value higher than 2.2 W/m 2K (upgraded in 2010 see window energy ratings). With so much double glazing 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 FENSA - 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 the double glazing company is 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 double glazing regulations as 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.
This double glazing window regulation particularly affects the minimum levels of insulation that replacement double glazed windows must achieve 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, i.e. 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 use low emissivity glass, Pilkington K for example, and argon gas filled sealed units.
Double glazing for the technical minded
Double glazing what exactly is it? Double glazing is today’s general terms 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.
The double glazing 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.
Float glass or safety toughened (tempered) glass?
Double glazed sealed units will either be normal float glass for small double glazed sealed units or toughened 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 uPVC Patio doors, French doors and windows, which reduces the likelihood of injury in the event of breakages. However tempered glass should receive the same care as annealed glass.
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.
Utilizing a low emissivity coating that reflects heat back into the room.
The latest double glazed sealed units offer a greatly improved insulation (as required by FENSA) compared to older traditional double glazed sealed units and single glazing. In addition to their better insulation, double glazed windows are also draught-proof, therefore reducing the amount of heat that can escape, so you can cut down on your heating and save on energy without sacrificing comfort.
The insulation of a double glazed unit 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.
Double Glazing Pilkington K or Low E
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 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 glazed Argon gas sealed units
Argon is a colourless, odorless, 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 double glazing 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.
Double glazed windows, patio doors, French doors and uPVC doors now come under the requirements building regulations and document L. Now requires that all these products must meet a high standard of insulation.