Choosing your windows

Double glazing for thermal insulation.

What is Low-e Glass?

Glass used in double glazing window for thermal insulation is known as Low E, or low-emissivity glass. It has a transparent metallic coating that works in two ways to economise heating energy. The dual action coating reflects heat back into the room, whilst allowing heat and light from the sun (known as passive solar heat gain) to pass through. Thermal insultion glass should be used on face 2 or 3 of a double glazing unit (see double glazing units)

Window energy ratings

As government building regulations become more stringent and heating costs rise, it is becoming ever more important to choose the most effective thermal insulation glass. The British Fenestration Ratings Council (BFRC) has set out an official window energy ratings table, ranging from A to G, with A being the most energy efficient. For more detailed information see Window Energy Ratings section). Windows with a C rating or higher are endorsed by the government-backed Energy Saving Trust.
The energy efficiency of a thermal insulation glass is measured by its U (formerly K value) and its Solar Heat Gain Coefficient (g).
In addition to the low E glass used, energy ratings can be further enhanced by the variety of Spacer Bar, Cavity and Edge Spacing used in the double glazing unit.

U Values

The ‘U’ value of a double glazing window is the measure of its ability to transfer heat – so double glazing windows with the lowest U value are the most efficient insulators against heat loss from a room (see Window Energy Ratings).

Solar Heat Gain

The Solar Heat Gain Coefficient (SHGC), which measures a thermal insulation glass' ability to transmit solar energy into a room, is measured in value from 0 to 1. The SHGC is commonly referred to as the g-value, or solar factor. The higher the g value, the greater a thermal insulation window’s ability to transmit solar heat (see Window Energy Ratings) and thus increase its energy efficiency.

Types of Low E Solutions

The most efficient thermal insulation glass use a unique manufacturing process which builds up microscopic layers coating, using a technology known as sputtering, under vacuum conditions. (See online and offline coatings). This advanced process builds up a highly resistant, but imperceptively thin coating which gives it a much clearer appearance than other thermal insulation glass. The coating also allows maximum daylight and heat into the room for optimised solar gain. Some products have been shown to reduce heat loss by 24% more than traditional online coated thermal insulation glass, and by 40% compared to standard double glazing window. Further energy savings can be made by using warm edge ‘thermal break’ spacer bars. These can reduce heat lost around the edge of the window by up to 65%.

Combining thermal insulation glass with other solutions

Some ranges of thermal insulation glass can be toughened and laminated and used in double gloazing window in various combinations with solar control, self-cleaning, decorative and acoustic glass. (see Which Combination).

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