What is Low E?

Low emissivity(Low E) is a quality of a surface that radiates, or emits, low levels of radiant energy. All materials absorb, reflect and emit radiant energy. 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 reflector would have a value of 0.

Reflectivity is inversely related to emissivity and when added together their total should equal 1. Therefore, if asphalt has an emissivity value of 0.90 its reflectance value would be 0.10. This means that it absorbs and emits 90% of radiant energy and reflects only 10%. Conversely, a low-e material such as aluminum foil has an emissivity value of 0.03 and a reflectance value of 0.97, meaning it reflects 97% of radiant energy and emits only 3%. Low-emissivity building materials include window glass manufactured with metal-oxide coatings as well as housewrap materials, reflective insulations and other forms of radiant barriers.

 

More about Low-emissivity windows…

To make Low E glass, certain properties such as the iron content may be controlled. Also, some types of glass have naturally low emissivity, (such as borosilicate or Pyrex). Specially designed coatings, often based on metallic oxides, are applied to one or more surfaces of insulated glass. These coatings reflect radiant infrared energy, thus tending to keep radiant heat on the same side of the glass from which it originated, while letting visible light pass. This often results in more efficient windows because radiant heat originating from indoors in winter is reflected back inside, while infrared heat radiation from the sun during summer is reflected away, keeping it cooler inside.

 

low e efficiency chart

Low solar gain, or southern climate, low-e windows do a better job of blocking heat from the sun and provide the greatest energy savings in this region; they generally have an SHGC between 0.20 and 0.40.

High solar gain, or northern climate, low-e windows are better at reducing heat loss in winter and allow more heat from the sun to enter the home; they are advantageous for south-facing glass in a passive
solar home.

Spectrally selective low-e: Specialized coatings are available that can cut heat gain without sacrificing the view and visible light. This allows for a low SHGC with a higher VT, to preserve daylight without excess
heat gain.

-eXtension Research 2010