Anyone giving their home a green makeover will discover that there are more environmentally friendly alternatives to most of the fixtures and fittings that we take for granted. Windows are no exception. The problem they present is that after the roof, they are the number one reason for heat escaping – which means extra heating costs. Low E glass windows are one option to help limit excessive heat loss.
What is low-e glass? Technically, it’s about low thermal emissivity. That’s how much heat a surface reflects and emits heat. On the standard scale, aluminium foil would score 0.3. Asphalt scores 0.88 and ordinary window glass 0.91. That means it’s a poor insulator. Low-E glass is designed so that it has improved insulation qualities.
Depending on its components, ordinary glass varies in how good or bad it is at emitting heat. Typically, glass that is naturally low-e gains in some areas but loses in others. Though it’s low-e, it has a high reflective capacity. As window glass it isn’t great because it’s not good at reflecting near infrared (NIR); in fact it’s quite the opposite, meaning that it lets heat out too easily.
Low-e windows are designed to deal with this problem of near-red reflectivity. It’s not the glass itself that makes the difference. Instead it’s the film coating that is applied to ordinary soda-lime glass, to improve its NIR efficiency. That’s the key thing. These include metal oxide layers, although various types of thermal barrier materials can be used. Different methods produce ‘soft coat’ or ‘hard coat’ finishes; soft coat finishes have to be used in double glazed window casings. Both have pros and cons, with soft coat windows unsuitable for single panes, but more efficient and less likely to haze and impede incoming light.
The effect of film coating is that heat stays on the same side it comes from in the first place, without limiting light passing through it. The result is that the heat you build up inside in winter by turning on the heating stays indoors. Similarly, in summer, hot sunlight is reflected away, limiting heat build-up indoors and reducing the need to use the air-conditioning.
There are alternatives to low-e window glass, including solar screens and films, which can be a quarter of the total cost, but they are not as efficient as investing in proper low e-windows. Against this, you can factor in replacement costs. Low-e windows are durable and should come with a good guarantee. The extra costs should soon be recouped as your heating and cooling costs are curbed.
So successful have low-e windows become that some anticipate they will become the standard window glass for new build homes. Your main problem will be deciding on hard or soft coat single or double glazed units. Generally, consumers who have switched from standard to low-e glass windows have few complaints after making the change.
Posted December 26, 2013
by Gabriel Posternak.