Page 25 - GRIHA Manual Volume III - Introduction to National Rating System
P. 25
18 GRIHA Manual: Volume 3
Where
H = Depth of shading (the distance from the window to the
farthest edge of the overhang or vertical fin)
V = Height of window in case of overhang, and width of window
in case of vertical fin
Note Height is the distance from the bottom of the window to the lowest point of the overhang. Width
expands between the internal points of the vertical in.
b. Multiplication factor
The multiplication factor is identified for various ranges of projection
factors as shown in table 2 below. Whenever the fenestration is
provided with overhangs and/or vertical fins, a separate ‘M’ factor shall
be determined for each orientation and unique shading condition. Figure 14 Diagram showing
The multiplication factor is derived from the projection factor, and is projection factor
calculated based on the four ranges of PF (from 0.25–0.49, 0.50–0.74,
0.75–0.99, and 1.00 or more).
The effective SHGC for the complete fenestration (with shading device) is calculated by dividing
the maximum SHGC recommended by table 13.1 in GRIHA Criterion 13 by the determined M factor
for the given fenestration from table 2. For further details on ‘Effective SHGC Calculation, refer
example 4.2 in Energy Conservation Building Code user guide.
Maximum SHGC
Effective SHGC = ________________________
Multiplication factor (M)
Table 2 Multiplication factor (M) for various range of PF
Overhang ‘M’ factors for four Vertical in ‘M’ factors for four Overhang + in ‘M’ factors for four
projection factors projection factors projection factors
Project Orientation 0.25 – 0.50 – 0.75 – 1.00 + 0.25 – 0.50 – 0.75 – 1.00 + 0.25 – 0.50 – 0.75 – 1.00 +
Location 0.49 0.74 0.99 0.49 0.74 0.99 0.49 0.74 0.99
North N .88 .80 .76 .73 .74 .67 .58 .52 .64 .51 .39 .31
latitude E/W .79 .65 .56 .50 .80 .72 .65 .60 .60 .39 .24 .16
>15º
S .79 .64 .52 .43 .79 .69 .60 .56 .60 .33 .10 .02
North N .83 .74 .69 .66 .73 .65 .57 .50 .59 .44 .32 .23
latitude E/W .80 .67 .59 .53 .80 .72 .63 .58 .61 .41 .26 .16
<15’º
S .78 .62 .55 .50 .74 .65 .57 .50 .53 .30 .12 .04
Source Table 4.4, ECBC-2007
The overall thermal performance of windows can be evaluated on the basis of the following two
energy performance parameters of windows.
(a) Maximum U factor (thermal transmittance)
(b) Maximum SHGC (heat gain through direct solar radiation)
It has been observed that SHGC of glass has greater impact on heat gain as compared to U-value of
glass. It can be proved from the following example that equivalent properties of SHGC and U-factor
have drastic difference in terms of heat gain/loss.
Where
H = Depth of shading (the distance from the window to the
farthest edge of the overhang or vertical fin)
V = Height of window in case of overhang, and width of window
in case of vertical fin
Note Height is the distance from the bottom of the window to the lowest point of the overhang. Width
expands between the internal points of the vertical in.
b. Multiplication factor
The multiplication factor is identified for various ranges of projection
factors as shown in table 2 below. Whenever the fenestration is
provided with overhangs and/or vertical fins, a separate ‘M’ factor shall
be determined for each orientation and unique shading condition. Figure 14 Diagram showing
The multiplication factor is derived from the projection factor, and is projection factor
calculated based on the four ranges of PF (from 0.25–0.49, 0.50–0.74,
0.75–0.99, and 1.00 or more).
The effective SHGC for the complete fenestration (with shading device) is calculated by dividing
the maximum SHGC recommended by table 13.1 in GRIHA Criterion 13 by the determined M factor
for the given fenestration from table 2. For further details on ‘Effective SHGC Calculation, refer
example 4.2 in Energy Conservation Building Code user guide.
Maximum SHGC
Effective SHGC = ________________________
Multiplication factor (M)
Table 2 Multiplication factor (M) for various range of PF
Overhang ‘M’ factors for four Vertical in ‘M’ factors for four Overhang + in ‘M’ factors for four
projection factors projection factors projection factors
Project Orientation 0.25 – 0.50 – 0.75 – 1.00 + 0.25 – 0.50 – 0.75 – 1.00 + 0.25 – 0.50 – 0.75 – 1.00 +
Location 0.49 0.74 0.99 0.49 0.74 0.99 0.49 0.74 0.99
North N .88 .80 .76 .73 .74 .67 .58 .52 .64 .51 .39 .31
latitude E/W .79 .65 .56 .50 .80 .72 .65 .60 .60 .39 .24 .16
>15º
S .79 .64 .52 .43 .79 .69 .60 .56 .60 .33 .10 .02
North N .83 .74 .69 .66 .73 .65 .57 .50 .59 .44 .32 .23
latitude E/W .80 .67 .59 .53 .80 .72 .63 .58 .61 .41 .26 .16
<15’º
S .78 .62 .55 .50 .74 .65 .57 .50 .53 .30 .12 .04
Source Table 4.4, ECBC-2007
The overall thermal performance of windows can be evaluated on the basis of the following two
energy performance parameters of windows.
(a) Maximum U factor (thermal transmittance)
(b) Maximum SHGC (heat gain through direct solar radiation)
It has been observed that SHGC of glass has greater impact on heat gain as compared to U-value of
glass. It can be proved from the following example that equivalent properties of SHGC and U-factor
have drastic difference in terms of heat gain/loss.
