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How does double glazing work?

Double glazing is a great investment. Not only will it save you money on energy bills, but the improvement to your home’s energy performance can also improve your property’s value. But how does it work? Well, as Bristol’s leading installer of double glazed windows, we thought we’d shed some light on the subject.

How do double glazed windows work?

Double glazed windows are constructed from two panes of glass. One pane is on the outside and the other on the inside. These two panes of glass are separated by a spacer bar, leaving an air pocket between them. Having a cavity of air between the two panes of glass provides the window with additional thermal insulation. This is because the air that is trapped in between the glass is unable to circulate, meaning it conducts heat poorly. This leads to a reduction in the rate of heat loss from inside the house. However, an inert gas, such as Argon or Krypton, is usually fitted inside the cavity instead of air. These gases conduct heat poorer than air, meaning even less heat is lost from inside the house.

What is a spacer bar?

The spacer bar is a small bar that is used to separate the panes of glass on a double glazed window. As well as creating an air cavity between the two panes, they hold the desiccant. Spacer bars also have small holes in their surface to absorb water vapour from inside the sealed cavity.

Since double glazing became popular, aluminium was used to construct spacer bars. However, because aluminium is a highly conductive metal, the insulating qualities of the window were hindered somewhat. Modern spacer bars utilise something called warm edge technology. Warm edge spacer bars are insulated at the edge of the sealed unit where most heat is lost, which lowers the amount of heat lost through the window.

Double glazing units thickness vs triple glazing thickness

The overall thickness of a standard double glazed unit started at 20mm before expanding to 24mm. Nowadays however, the standard thickness of an Insulated Glass Unit (IGU) is 28mm. The current industry standard unit consists of one 4mm pane of low emissivity glass, one 4mm pane of plain annealed glass and a 20mm cavity of argon gas (4mm+4mm+20mm=28mm).

A triple glazed unit is fitted with a central pane of glass, 4mm in thickness. However, if the overall thickness of the IGU remains at 28mm, the thermal performance of the window will be affected negatively. Therefore, the overall thickness of the unit is increased by expanding the size of the gas cavities between the panes, which also improves the unit’s performance. The optimum size of a triple glazed unit is 44mm (4mm+16mm+4mm+16mm+4mm=44mm).

Double glazing U-value

U-values are a way of measuring heat transmittance through a structure. The unit for measuring them is W/m2k and the lower the U-value, the less heat is transferred through the structure. A standard 28mm double glazed unit will provide a centre pane U-value of 1.1 W/m2k. However, bear in mind that this measurement is only for the glass unit and not the whole window. The overall U-value of a double glazed window is affected by a number of factors, such as insulation quality and framing material.

Any replacement windows in existing homes must have an overall U-value of at least 1.6 W/m2k. Although, high-performance double glazing, like Residence 9 windows, can achieve an overall U-value of 1.2 W/m2k. The typical performance of a 44mm triple glazed unit is 0.9 W/m2k. However, if a second pane of low emissivity glass is used, U-values as low as 0.6 W/m2k can be achieved.

Benefits of double glazed windows

Double glazing offers homeowners a host of benefits, some of which are well-known and others which are not. The better-known benefits of double glazing include:

  • Improving the energy efficiency of your home. This is important for a number of reasons, such as lowering your energy bills.
  • A warmer home in the winter and a cooler one in the summer.
  • Less outside noise making its way into your home, resulting in a more peaceful home environment.
  • Increased security levels, especially compared to single glazing.

Taking advantage of these benefits is also easier than ever. Many companies, including us, offer a range of finance options so the costs of double glazing can be spread out over time.

Types of double glazed windows

Nowadays, double glazing is available in a variety of forms to suit the requirements of virtually any homeowner. Common applications for double glazing includes:

If you’re unsure whether or not the windows in your home are single or double glazed and would like to identify them, why not check out our short guide on how to tell if a window is double glazed?

Associated Windows are double glazing specialists located in Bristol. For more information on our products and services, please call 0117 9311777 or contact us online.

Infographic how does double glazing work

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