Page 24 - GRIHA Manual Volume III - Introduction to National Rating System
P. 24
Building and system design optimization 17
Window Area = 6035 m 2
Wall Area = 4885 m 2
Figure 13 Building facade showing windows and wall area
6. Solar heat gain coeficient
The solar heat gain coefficient (SHGC) is the fraction of incident solar radiation admitted through a
window. It measures how well a fenestration blocks heat from the sun. The lower the SHGC, better
are the products in blocking unwanted heat gain. SHGC is expressed as a number between 0 and
1. For example, if a glazing allows 40% of incident heat to pass through it, then SHGC of glass is
represented as 0.4.
SHGC accounts for incident solar radiation. It means, the heat gain read by the SHGC value is
due to all the direct components of the sun, which is easily controlled by the provision of external
shading devices or any other external obstructions. Therefore, SHGC shall be determined for the
entire fenestration product (including the sash and frame), and not just for the glass. Therefore,
besides glazing properties, the type of window and shading devices can affect the SHGC calculation
of fenestration. In other words, effective SHGC is the cumulative solar heat gain coefficient of the
fenestration with both the glass and shading devices (overhang and/or vertical fin).
Calculation of effective SHGC
Since the total incident radiation on a fenestration can be reduced by providing shading devices,
their impacts are considered while calculating the effective SHGC for windows. The effective SHGC
is calculated as prescribed by ECBC-2007. There are two important components that need to be
determined to calculate the effective SHGC for a given window. These are projection factor (PF) and
corresponding M-factors (M).
a. Projection factor
Projection factor is the ratio of the depth of the shading device and the height or width of the
window. PF is calculated to determine how well an overhang or vertical fin shades the building
glazing.
PF = H / V
Window Area = 6035 m 2
Wall Area = 4885 m 2
Figure 13 Building facade showing windows and wall area
6. Solar heat gain coeficient
The solar heat gain coefficient (SHGC) is the fraction of incident solar radiation admitted through a
window. It measures how well a fenestration blocks heat from the sun. The lower the SHGC, better
are the products in blocking unwanted heat gain. SHGC is expressed as a number between 0 and
1. For example, if a glazing allows 40% of incident heat to pass through it, then SHGC of glass is
represented as 0.4.
SHGC accounts for incident solar radiation. It means, the heat gain read by the SHGC value is
due to all the direct components of the sun, which is easily controlled by the provision of external
shading devices or any other external obstructions. Therefore, SHGC shall be determined for the
entire fenestration product (including the sash and frame), and not just for the glass. Therefore,
besides glazing properties, the type of window and shading devices can affect the SHGC calculation
of fenestration. In other words, effective SHGC is the cumulative solar heat gain coefficient of the
fenestration with both the glass and shading devices (overhang and/or vertical fin).
Calculation of effective SHGC
Since the total incident radiation on a fenestration can be reduced by providing shading devices,
their impacts are considered while calculating the effective SHGC for windows. The effective SHGC
is calculated as prescribed by ECBC-2007. There are two important components that need to be
determined to calculate the effective SHGC for a given window. These are projection factor (PF) and
corresponding M-factors (M).
a. Projection factor
Projection factor is the ratio of the depth of the shading device and the height or width of the
window. PF is calculated to determine how well an overhang or vertical fin shades the building
glazing.
PF = H / V
