representing (h) = 80%. From the point of intersection, draw
a vertical construction line downward until the ratio
(W/A) second graph % 50% is found on the horizontal axis. (see
Fig. 4).
To calculate the window area (W) for the city of Prague,
Czech Republic:
W ¼
W
A
second graph
ÂA ¼ 0:50 Â 20 ¼ 10 m
2
Example Using the first design chart, it is required to
analyze the daylighting, i.e., find the daylight factor (DF), in
the city of Cairo, Egypt ((LAT) = 30° N), given that the
transmittance of glazing (s) = 45%, the sky exposure angle
(h) = 60°, the window area (W) = 6 m
2 and the floor area
(A) of the room = 20 m
2 .
Solution. To find the (DF), in the city of Cairo, Egypt, all
the previous steps are made in reverse as follows. Enter the
second graph (at the left) at first by specifying the ratio
(W/A) second graph = 6/20 = 30% on the horizontal axis. Draw
a vertical construction line until it intersects the diagonal line
representing (h) = 60%. From the point of intersection, draw
a horizontal construction line until it intersects the diagonal
line representing (s) = 45%. From the point of intersection,
draw a vertical construction line downward until the (DF)
2% is found on the horizontal axis (see Fig. 4).
3.2 Description of the Second Design Chart
The second design chart shown in Fig. 5 is a combination of
three graphs; the first graph at the top right, the second graph
at the top left, and the third graph at the bottom left.
Depending on the sequence of use of these three graphs,
the second design chart could be used for both design and
analysis. If it is used in an anticlockwise direction, i.e., the
first graph, then the second graph and then the third graph,
it is used for design by finding the window area (W) as a
percentage of the total area of internal surfaces (RA) of the
room, i.e., finding the ratio (W/RA). While, if it used in a
clockwise direction, i.e., the third graph, then the second
graph and then the first graph, it is used for analysis by
finding the daylight factor (DF) resulting from the existence of a window or more with given surface areas (see
Fig. 6).
The first graph (at the top right) determines the ratio
(W/RA) between the required unobstructed window area
(W) and the total area of internal surfaces (RA) of the room.
The obtained ratio (W/RA) is a function of the design daylight factor (DF) and the transmittance of glazing (s), while
maintaining the average internal surface reflectance
(R) = 50% and the sky exposure angle (h) = 90°, and it is
drawn using the formula:
W
RA
¼
2 Â 1 À R
ð
ÞÂDF%
s  h
¼
2 Â 1 À 0:50
ð
ÞÂDF%
s  90
¼
DF%
s  90
It consists of two axes and a number of diagonal lines; the
horizontal axis represents the design daylight factor (DF)
needed for most basic applications ranging from (DF) = 1%
(for lower latitudes) to (DF) = 6% (for higher latitudes),
while the vertical axis represents the ratio (W/RA) between
the required unobstructed window area (W) and the total
area of internal surfaces (RA) of the room. That ratio
(W/RA) on the vertical axis is obtained after intersecting a
number of diagonal lines that represent the transmittance of
Fig. 4 Examples for using the
first design chart in design and
analysis (Source The author)
A Design Chart to Determine the Sizing …
17
a vertical construction line downward until the ratio
(W/A) second graph % 50% is found on the horizontal axis. (see
Fig. 4).
To calculate the window area (W) for the city of Prague,
Czech Republic:
W ¼
W
A
second graph
ÂA ¼ 0:50 Â 20 ¼ 10 m
2
Example Using the first design chart, it is required to
analyze the daylighting, i.e., find the daylight factor (DF), in
the city of Cairo, Egypt ((LAT) = 30° N), given that the
transmittance of glazing (s) = 45%, the sky exposure angle
(h) = 60°, the window area (W) = 6 m
2 and the floor area
(A) of the room = 20 m
2 .
Solution. To find the (DF), in the city of Cairo, Egypt, all
the previous steps are made in reverse as follows. Enter the
second graph (at the left) at first by specifying the ratio
(W/A) second graph = 6/20 = 30% on the horizontal axis. Draw
a vertical construction line until it intersects the diagonal line
representing (h) = 60%. From the point of intersection, draw
a horizontal construction line until it intersects the diagonal
line representing (s) = 45%. From the point of intersection,
draw a vertical construction line downward until the (DF)
2% is found on the horizontal axis (see Fig. 4).
3.2 Description of the Second Design Chart
The second design chart shown in Fig. 5 is a combination of
three graphs; the first graph at the top right, the second graph
at the top left, and the third graph at the bottom left.
Depending on the sequence of use of these three graphs,
the second design chart could be used for both design and
analysis. If it is used in an anticlockwise direction, i.e., the
first graph, then the second graph and then the third graph,
it is used for design by finding the window area (W) as a
percentage of the total area of internal surfaces (RA) of the
room, i.e., finding the ratio (W/RA). While, if it used in a
clockwise direction, i.e., the third graph, then the second
graph and then the first graph, it is used for analysis by
finding the daylight factor (DF) resulting from the existence of a window or more with given surface areas (see
Fig. 6).
The first graph (at the top right) determines the ratio
(W/RA) between the required unobstructed window area
(W) and the total area of internal surfaces (RA) of the room.
The obtained ratio (W/RA) is a function of the design daylight factor (DF) and the transmittance of glazing (s), while
maintaining the average internal surface reflectance
(R) = 50% and the sky exposure angle (h) = 90°, and it is
drawn using the formula:
W
RA
¼
2 Â 1 À R
ð
ÞÂDF%
s  h
¼
2 Â 1 À 0:50
ð
ÞÂDF%
s  90
¼
DF%
s  90
It consists of two axes and a number of diagonal lines; the
horizontal axis represents the design daylight factor (DF)
needed for most basic applications ranging from (DF) = 1%
(for lower latitudes) to (DF) = 6% (for higher latitudes),
while the vertical axis represents the ratio (W/RA) between
the required unobstructed window area (W) and the total
area of internal surfaces (RA) of the room. That ratio
(W/RA) on the vertical axis is obtained after intersecting a
number of diagonal lines that represent the transmittance of
Fig. 4 Examples for using the
first design chart in design and
analysis (Source The author)
A Design Chart to Determine the Sizing …
17