Every day we hear questions about energy efficiency and how it pertains to windows (glass specifically). Low E glass (Low Emmisivity) is a term you might hear a lot when shopping for windows and although the explanation can be quite complex, essentially it is the amount of thermal energy a window does or does not let through. The following are some of the technical terms associated with Low E
Low E – How does it work?
Low E glass coatings work by reflecting or absorbing IR light (heat energy). The thickness of the Low E coating and the position in the window (#2 or #3 surface) dictate how the window will perform.
When installed on the #3 surface of an insulated glass unit (IG), the Low E coating will reflect IR heat from inside the room to help reduce the energy loss during the cold months, thereby reducing heating costs.
When installed on the #2 surface of an IG unit, the Low E coating will reflect or absorb IR heat from the outside, thereby reducing solar gain and cooling costs during the warm months.
Low E Terminnology
With Low E, solar selective Low E, hard-coat, soft-coat, sputtered, and pyrolytic terminology to deal with, you need the facts on Low E.
CVD, or Chemical Vapor Deposition, is one of two main technologies used to manufacture Low Emissivity glass. In the CVD process, vapor directed to the hot glass surface reacts to form a ceramic coating. The resulting Low E product is often referred to as “hard coat”.
Emissivity refers to the ability of a surface to absorb or reflect heat. An energy efficient glazing technology, Low Emissivity glass is a poor absorber of heat!
Also known as “pyrolytic”, hard coat refers to Low E glass manufactured via the CVD process. Because the coating is covalently bonded to the glass, hard coat Low E is extremely durable.
MSVD, or Magnetron Sputterering Vacuum Deposition (also known as “sputtering”), is one of two main technologies used to manufacture Low Emissivity glass. In MSVD, a metal or ceramic target bombarded with ions releases atoms to form a thin coating on a sheet of glass. The resulting Low E product is often called “soft coat” or “sputtered”.
See “Hard Coat” or “CVD”
Also know as “sputtered”, soft coat refers to Low E glass manufactured via the MSVD process. Less durable than its hard coat counterpart, soft coat Low E requires special storage and handling to prevent damage to the coating.
Solar Heat Gain Coefficient (SHGC)
The Solar Heat Gain Coefficient (or SHGC) refers to a window’s ability to transmit solar radiation. The SHGC ranges from 0-1. A value of 0 indicates that the window functions like a wall, essentially preventing any solar energy from entering the building. A value of 1 indicates that the window functions like an opening, allowing all solar energy in. In cold climates, a high SHGC can lower heating costs by using passive solar heating. In warm climates, a low SHGC is desired to keep unwanted heat out and reduce cooling costs.
Solar Selective Low E Specifically designed to enhance comfort in warm climates, solar selective Low E is a Low E glass with a low solar heat gain coefficient, like Arkema’s Sun ETM.
Also known as the K-value, the U-value is a measure of a window’s ability to transfer heat (usually given in W/m2K or Btu/hr ft2 oF). Windows with a low U-value, like Low Emissivity windows, are efficient insulators against heat loss.