Common Terminology
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Emittance
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The ratio of the radiant flux emitted by a specimen to that emitted by a blackbody under the same
conditions. This is expressed as a number between zero and one.
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A lower emittance value indicates reduced radiative heat flow.
Winter U-Factor or U-Value
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A measure of the amount of non-solar heat that is transferred through a window. This is usually expressed
in units of Btu/hr-sq ft- °F.
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A lower U-Factor indicates greater resistance to heat flow and better insulating performance.
R-Value

A measure of the resistance of a window to non-solar heat transfer. It is the inverse of a U-Value (R= 1/U).
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A higher R-Value indicates greater resistance to heat flow and better insulating performance.
Solar Heat Gain Coefficient
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The amount of the solar heat that is passed through a window. This is expressed as a number between
zero and one.
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A lower SHGC indicates that less solar heat is transmitted through the window.
Visible Light Transmission
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The amount of day light that is passed through a window. This is usually expressed as a percent.
Ultraviolet (UV) Transmission
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The amount of the UV portion of the solar spectrum that is passed through a window. This is expressed as
a percent.
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A lower UV transmission percent indicates greater protection of home furnishings from fading.
Room-Side Glass Temperature
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This is the temperature of the glass surface that you can touch from the inside of your home.
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A higher glass temperature when it’s cold outside, and a lower glass temperature when it’s hot outside will
contribute to comfort and energy efficiency.
1
Heat Moves From Where It’s Hot, To where It’s
Not
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Radiation losses occur through the window glass and
represent about 66% of total heat loss in a standard
window
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Low-E glass dramatically reduces radiation heat loss
Conduction losses in windows occur primarily through the
edges of the glazing and through the sash and frames
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Changes in sash and frame construction, use of “warm
edge” spacer material, and the use of inert gasses have
reduced conductive heat loss
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Convection losses occur due to air movement caused by
temperature fluctuation
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Air Leakage is a significant contributor to heat losses

Window design and installation technique must be carefully
managed to minimize air leakage
2
Thermal Imaging Demonstrates Advances In Window
Glass Technology
Double Glaze
Std. Clear Glass
Metal Spacer
Double Glaze
Std. Clear Glass
Foam Spacer
Double Glaze
Low-E Glass
Foam Spacer
↑
↑
↑
↑
↑
↑
↑
↑
21
28
35
41
48
55
62
68 °F
3
Low-E Coatings Are Comprised Of Carefully Tuned Layers,
Providing Thermal, Solar and Optical Properties.
Ozone Layer (20 – 25 KM Above Ground Level)
Gamma
Rays
X-Rays
UVC
100 – 380nm
380 – 760nm
760 – 1,000nm
Ultraviolet Rays
Visible Rays
Infrared Rays
280 – 320nm 320 – 380nm
UVB
UVA
Si3N4
Scratch Resistance
Sno2
Optical Filter
NiCrOx
Adhesive Barrier
Ag
ZnO
Thermal & Solar Performance
Performance Enhancer
Sno2
Optical Filter
NiCrOx
Ag
Adhesive Barrier
Thermal & Solar Performance
ZnO
Performance Enhancer
TiO2
Si3N4
Optical Enhancer
Barrier Layer
Glass
Glass
4
Low-E Coatings Have Changed Glass From An
Energy Liability To An Energy Asset
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Standard glass has a high emissivity
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Furnace-generated heat moves freely from in to out.
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Solar heat moves freely from out to in.
Low-E coated glass has a low emissivity
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Furnace-generated heat is retained.
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Solar heat is rejected.
Low-E coatings act like a filter to manage heat
transfer
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Some energy wavelengths pass through, while
others are rejected
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Furnace energy (long wave radiation)
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Solar energy (short wave radiation)
5
Low-E Glass Keeps The Heat Where You Want It, Regardless
Of The Climate Or Season
Retains Furnace Heat, Keeping
You Warm When It’s Cold Outside
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Escaping long wave energy (furnace energy)
leads to discomfort associated with heat loss
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Better insulating performance (lower U-factor)
minimizes home heat loss when outdoor
temperatures are lower than desired indoor
temperatures
U-Factor
6
Low-E Glass Keeps The Heat Where You Want It, Regardless
Of The Climate Or Season
Rejects Excessive Solar Heat, Keeping
You Cool When It’s Hot Outside
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Unwanted short wave energy (solar energy)
leads to discomfort associated with
excessive heat gain
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Low solar gain (lower SHGC) low-E eliminates
overheating when outdoor temperatures are
higher than desired indoor temperatures
SHGC
7
Low-E Glass Transmits Natural Daylight And Reduces
Damaging UV Radiation
Transmits Natural Daylight, Brightening
Your Home And Your Mood
Visible
Light
Reduces Damaging UV Rays,
Protecting Home Furnishings
UV
8
The Evolution of Window Glass as Positively Impacted
Comfort, Energy Efficiency, and the Life-Span of Typical
Home Furnishings
Glass Package
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Visible
Light
Ultraviolet
Light
SHGC
Room-Side Glass
Temperature
U-Factor
(Daylight)
(Fading)
(Solar
Management)
(Summer / Winter)
(Thermal
Management)
air
argon
Single Pane, Clear Glass
90%
72%
0.87
90˚ F
15˚ F
1.11
n/a
Double Pane,Clear Glass
82%
57%
0.77
90˚ F
44˚ F
0.48
0.46
Double Pane, Low-E Glass
70%
29%
0.36
83˚ F
56˚ F
0.29
0.24
Heat loss has been reduced
Heat gain has been reduced
Day light has been maintained
Fading and deterioration have been reduced
9