Low E, What is it ?
Windows and doors may have Low-E glass.


Low emissivity (Low E) - actually low thermal emissivity - 
is a quality of a surface that radiates, or emits, low levels of radiant thermal (heat) energy. All materials absorb, reflect and emit radiant energy. 
This article is primarily about material properties within a special wavelength interval of radiant energy - namely thermal radiation of materials with temperatures approximately in the interval -40..60°C. 

Emissivity is the value given to materials based on the ratio of heat emitted compared to a blackbody, on a scale of 0 to 1. A blackbody would have an emissivity of 1 and a perfect (White, Silver) reflector would have a value of 0. 

Window glass is by nature highly thermal emissivity of 0.9 (That is bad news). 
There few ways to improve thermal efficiency, 

Here are three ways to apply LOW E
Method 1...A thin film coatings are applied to the raw soda-lime glass. 
Method 2. A typical double-glazed window with a high-solar gain low-E is filled with argon gas 
Method 3. Method one plus method two.