The Emissivity of a material is it’s ability to emit energy as radiation and has a value of between zero and one. In other words materials with high emissivity (like tinfoil) does not heat up when the sun hits it unlike a material with a low emissivity (like a black shirt).
Standard glass is high E, which is the reason greenhouses are so hot. This property translates into a loss of heat during the winter and an increase in heat during the summer. This is clearly not ideal for comfort and energy efficiency.
Low E glass has been treated with a microscopic film that allows visible light to pass through the glass while blocking Infrared (heat) light.
At Tashman Home Center we carry Milguard windows that feature various energy saving types of glass, including Low E. Recently we installed Heritage doors with high performance Cardinal Low E 366 glass