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Low-emissivity coating (low-e)

LOW-E COATING, DUAL PANE, & THERMAL EFFICENCY

By Jerry Kahn of A Plus Windows 

 
 
Light Science:

Electromagnetic energy l at a particular wavelength has an associated frequency  f and photon energy E (c is the speed of light & h is Planck’s constant).

l = c/f

 or

E = hf

The relevant point for the discussion below is that light carries energy.  Depending on the frequency of vibration of the bonds in the atoms making up a molecule certain frequencies of light will be absorbed—this gives the object the color we observe.  The other frequencies will be reflected.  Absorption of light energy is converted to heat energy.  Also the light energy can increase the energy of the molecules and cause chemical reactions.  In addition, one way in which objects loose energy is through infrared light.  The warmer the body, the more infrared light energy is emitted.  A principle source is our sun.  Infrared light accounts for about 50% of the heating of the earth surface by the sun.

 

Some animals have evolved sensors.  The pit viper has two infrared sensory pits on its head which aid in detection of prey.  Several families (boas, common vampire bat, jewel beetles, and a few other insects) have like the viper evolved infrared sensors.

 

The Earth’s surface and the clouds absorb visible and invisible radiation from the sun and re-emit much of the energy as infrared back to the atmosphere.  Certain substances in the atmosphere, chiefly cloud droplets and water vapor (and to a lesser extent carbon dioxide) absorb this infrared, and re-radiate it in all directions, including back to the Earth.  This process has been termed the greenhouse effect. 

The visible band (380-780 nanometers), seen with our eyes, is the only a small part of the solar electromagnetic spectrum . Approximately 1,370 watts per square meter are deposited on the earth.    

In the home we want to be able to see outside and have the house illuminated naturally.  However, light carries energy, and direct light shining through the window much more energy.  Objects which absorb some of this light will warm up, and thus warm up the house.  Moreover, glass is an efficient conductor of heat and cold.  To reduce the transfer of the surface temperature dual pane glass system was developed, and to reduce the transfer of solar energy low-e coatings were developed. 

 

Low-e glass:

Low-E Glass has a thin layer of oxidized metal or metals on one side.  It is used almost exclusively on the inner surface of a dual pane sealed unit[i].  It functions to reduce solar heat gain in the summer and retain heat in the winter.  Low-e coating works by blocking both the shorter UV light and the longer infrared light.  Soft coat low-e two coatings blocks about 70% of the infrared and 80% of the UV light from entering through the glasss.  It also, unfortunately, blocks about 30% of the visible light, thus low-e coating is not clear and there is reduced lighting.  On a bright day this difference is hardly noticed.  The current formulations have a slight greenish or grayish tint.  It is most obvious from the outside looking in against a white background.  Manufactures of such coating seek to find formulas which maximize the blocking of UV and infrared light, but not visible light.   

 

INFRARED LIGHT

The near infrared (700-2400 nanometers) part of the spectrum is the heat band. It cannot be seen, but we experience it as heat. 53% of solar energy is near infrared.   You can feel infrared radiation when you sit next to an electric heater, or place your hand under a heat lamp. Another band, the far infrared is a tiny section beyond the near infrared region. While solar energy does not contain far infrared, you can feel it as heat that is re-radiated from objects exposed to the sunlight. Windows, furnishings, and even your skin will give off far infrared heat after absorbing solar energy.[i][i][v]

In the winter the low-e coating acts to keep heat in by reflecting back the heat carrying far infrared light.  The far Infrared light carries energy away from a body.  Low-e coating blocks the warming infrared light from entering your home, but it also keeps in your home the far infrared light.  The net effect is a warmer home in the winter. 

Low-e coating also contributes to energy savings by reducing the thermal efficiency of glass.  Thermal efficiency is the ability of a substance to conduct heat and cold.  Aluminum has a high and Styrofoam a low thermal efficiency.  Glass with low-e coating has a lower thermal efficiency.  Thus in the summer the inner pane of your glass will be cooler than clear glass, and in the winner it will be warmer.  For all of the above reasons, the net effect of low-e coating is a warmer home in the winter and a cooler home in the summer. 

 

UV LIGHT

The UV band makes up 3% of the solar spectrum. The low-e coating reflects around 84%[ii] of the longer ultraviolet light—dual pane clear glass reflects only around 45%.  Ultraviolet light contributes to most of the bleach of fabrics and wood exposed to sun light inside your home[iii].  Though UV is only 1-3% of solar radiance, it is important because below 300 nm, it is absorbed by diatomic oxygen molecules to form single oxygen atoms, which almost instantly react with other oxygen for form ozone, a very reactive molecule, unstable molecule consisting of 3 oxygen, which is a free radicals [iv][iv].  They are extremely reactive and last but a short time.  Over 99% of our oxygen in our lower atmosphere is diatomic. But over time, a small amount of free radical oxygen will cause bleaching.  The UV band (100-400 nanometers) is part of the spectrum that is the leading cause of fading for interior furnishings, carpets, and draperies. It also tans the skin, and increase the risk of skin cancer. 

 

THE MARKET PLACE

Glass companies are driven to carry the most effective coatings. That is because window manufacturers are required to post on their new windows a label indicating the thermal properties of their window—a sticker like those found on refrigerators.  When an improved formulations of low-e coating becomes available, the window companies will order glass with this new coating.  They don’t want to confuse the consumer with too much information, they just one the consumer to buy their product.  The better rating (better coating) increases sales,      

 

 

[i][i][v]   From http://www.solarshieldaz.com/solar.htm

[ii][ii][ii]   Cardinal low-e 171 and clear glass for the inner of a dual glaze unit. 

[iii][iii][iii]    Fading accelerated by light occurs through energy of activation.  Most chemical reactions to occur require the input of energy, usually in the form of heat, but sometimes in the form of light.  Oxygen atoms, the principle cause of fading (it is the active atom in anions used in bleaches such as HClO, O3, and SO3).  At the correct frequency of light, one attuned to the vibration frequency of the oxygen bond in O2, the oxygen will absorb the light energy causing its outer electron to move to the next higher electron orbit.  In this higher orbit, the O2 can easily split and become two single oxygen atoms—termed a free radical.  Free radical oxygen is very reactive.  Some of the molecules which the free radical Oxygen can react with are responsible for the color, and this reaction changes that color.   HClO, O3, and SO3 all readily yield free radical oxygen.   Thus for example green oranges have been commercially bleached by SO3 gas so as to turn them orange.  Fortunate ultraviolet light produces very little free radical oxygen, but over the period of years, it can be enough to fade fabrics and woods. 

[iv][iv][iv]  The Solar Spectrum and Terrestrial Effects, a government publication at http://www.sec.noaa.gov/spacewx/Solar_Spectrum.html.  

[v][v][vii]  From http://nfrc.thecornerstonepros.com/label.aspx



 

 

 

 

 


 

 

 

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