There are two main different types of low-e glass coatings. Some types of low e glass are designed for heat gain and some for heat rejection.
The heat-gain types- also known as High Solar Gain, or Hard coat Low E- work by reflecting far infrared low temperature radiation. Heat rejection types- also called Low Solar Gain- or Soft coat Low E work by reflecting both long wave and short wave infra-red.
Low-emittance, or Low-E glass, is glass coated with microscopically thin, and effectively invisible, layers of metal or metallic oxide. It was developed to reduce a windows U-factor or coefficient of heat transmission through the suppression of radiant heat flow. The lower the U value, the higher the amount of insulation.
Hard coat Low E is created by spraying a tin coating at high temperatures onto the glass surface. This type is more durable, and can be used in single glaze windows.
Soft coat Low E manufacture involves the application of numerous layers of silver sandwiched between layers of metal oxide while the glass is in a vacuum. The coating is delicate and if exposed to air is subject to oxidization, so soft coat low E must always be sandwiched between two layers of glass.
As a result, this type is usually more expensive than hard coat low e glass. The trade-off for all this, however, is that soft coat low e enables superior transmission of visible light, and features very low levels of emissivity for maximum insulation in winter winter – up to 70% less UV transmission than standard clear glass.
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What this all means is that windows with low-E glass reflect both light and heat. The result is that things like your furniture, draperies and carpeting don’t fade due to sunlight as fast, and your house stay cooler in the summer (heat from the outside does not get in through the glass) and warmer in the winter (heat inside does not get out).
So, which type of Low e Glass should you use?
In general, get Soft coat low-E glass if your house is located in sunny, hot part of the country. It will not lock heat into your house like hard coat low E glass will. This will help you save on air conditioning energy costs Conversely, if you happen to live in a cold climate, Hard coat low-e glass will help keep your heating costs down.