Daylight and Solar Transmittance: the percentage of either visible light or total
solar energy that a glazing transmits through the window. The number ranges from 0 to
100%. Standard clear insulating glass has a daylight transmission of 82% and a solar
transmittance of 71%.
Efficacy factor (ke): the ratio of light transmission and shading coefficient, Tvis/SC.
The higher the number, the better the glass filters heat from the sun's daylight.
Superior products have a ke greater than 1.0.
Exterior Daylight and Solar Reflectance: the percentage of either visible light or
total solar energy that a glazing reflects to the outside of the building (i.e., a measure
of the reflection looking into a window from the outside). The number ranges from 0 to
100%. Standard clear insulating glass has an exterior daylight reflectance of 16% and
an exterior solar reflectance of 14%.
R-value: measures the insulation effectiveness of the window. The R-value equals
one divided by the U- value (R=I/U). The higher the R-value, the better the insulating
performance.
Relative Heat Gain (RHG): the total heat flow through the glass for a specific set of
conditions. This value considers indoor/outdoor temperature differences and the effect
of solar radiation (English units = Btu/hr•ft2; Metric units = W/m2).
Shading Coefficient (SC): measures the total solar heat gain through the glazing
compared to 1/8" clear glass under the same design conditions. It includes both the
solar energy transmitted directly plus any absorbed solar radiation that is re-radiated
as heat into the interior. The number ranges from 0.0 to 1.0. Standard insulating glass
has an SC of 0.81. The lower the shading coefficient, the lower the solar heat gain.
U-value: measures the heat loss or gain due to differences between indoor and
outdoor air temperatures (English units = Btu/hr•ft2-°F; Metric units = W/m2•°C). The
U-value equals one divided by the R-value (U= I/R). The lower the U-value, the better
the insulating performance. Unless noted, calculations are for the center of insulating
glass units; edge effects and window system frame effects have not be been
considered and will influence actual overall insulating performance of the window
svstem.
|
