DESCRIPTION
Double glazing cuts heat lost through windows by half and can save £80 - £100 a year on your heating bills and about three quarters of a tonne of CO2 a year. Double glazing works by trapping air between two panes of glass creating an insulating barrier that reduces heat loss, noise and condensation.

Typically, the gap between the panes is filled with air, and it is the air, not the glass, that provides the insulation. It is important that the air remains as immobile as possible to prevent convection currents transferring heat across the insulating gap. This limits the thickness of the air gap used and is the reason for triple glazing.

The space between the panes may be filled with air or an inert gas like argon or krypton which provides better insulating performance. (Argon has a thermal conductivity 67% that of air).

LOW-EMISSIVITY COATING
Low emissivity (Low-E) glass has a thin coating, often of metal, on the glass within its airspace that reflects thermal radiation or inhibits its emission reducing heat transfer through the glass. A basic low-e coating allows solar radiation to pass through into a room. Thus, the coating helps to reduce heat loss but allows the room to be warmed by any sunshine. The low-e coating is usually on the inside pane of glass; if solar control is required then the outside pane of glass would have either a film or a body tint to reflect or absorb solar radiation. The principle of operation is similar to the greenhouse effect in which short wavelength radiation is transmitted through the pane, but longer wavelength radiation is absorbed. However, low-e glass reflects the radiation rather than absorbing it, improving performance compared to the glass in a simple greenhouse.

ALTERNATIVES
For those who want to be as ecologically friendly as possible and do not wish to use PVC-U frames, there are three alternatives: recycled PVC-U frames, wooden window frames, and secondary glazing.

Recycled PVC-U
Optimum are partnered with the only company in Europe to own and run a dedicated recycling plant making PVC-U frames from 90% recycled PVC-U. This means that you would be actively contributing to the recycling of all those petrochemical-based windows already out there that would otherwise go off to landfill.

Wood
The second alternative is to have wooden windows fitted. The natural beauty and versatility of wood are fundamental to its universal popularity. When properties such as high strength, light weight and good insulation are required, wood becomes the ideal choice for many applications. Unfortunately, these compelling attributes are offset by the susceptibility of many wood species to deterioration, particularly when used outdoors.
Historically, the use of inherently durable tropical hardwoods and the application of toxic chemicals to non-durable woods have provided partial solutions to the problem of durability. However, the wood’s dimensional stability is not improved and toxins create a disposal problem.
If an alternative existed which was sourced from sustainable forests, had zero toxicity and provided dimensional stability and durability that exceeded even the best tropical woods, a perfect material would have been found.
Accoya™ wood is such a material - a “new wood species” whose performance credentials have been extensively researched and repeatedly demonstrated. Accoya™ wood has properties that match those of the best tropical hardwoods, yet is manufactured using a non-toxic treatment and wood from sustainable sources. In fact, it may seem too good to be true… but Accoya™ wood really exists and is so good that it comes with a 50-year guarantee.

Secondary Glazing
For those on a budget or who like the appearance of their current windows but would like to make them more efficient, secondary glazing could be the answer. It costs less than double-glazing and will still reduce heat loss and draughts, although the savings will be around half of those achieved by double glazing. Secondary glazing works by installing supplementary glazing on the inside of an existing single-glazed window. No planning consent is required and it can provide a solution for listed buildings or homes in conservation areas if replacing existing windows is not a practical option.
Secondary glazing shouldn't prevent windows being opened easily for ventilation, but can restrict escape in an emergency, so it may be best to leave one window untreated. In summer, secondary glazing may not be wanted, particularly if it restricts the opening of windows. It can be taken down, but will need to be stored. When fitted, secondary glazing should be draught stripped, but to prevent condensation the original window frame should not be.

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