be applied to allow for deterioration of the reflectance of the decorations. dust or dirt on the glazing, types of glazing material other than clear glass, and obstructions caused by the window framing. With the exception of deterioration of the decorations it is usually more convenient to apply these corrections i.e. the daylight fac- tor. In all cases the product of the correction factors is taken.
TABLE 6
Use of Nomogram III for rooflights sloping at 60 ̊ to the horizontal Variation of function K with angles of obstruction from centre of sloping roof-light (degrees above horizontal)
Obstruction facing slope of glazing
0°
Obstruction opposed to slope of glazing
0°-60° 70° 80°
70
70
68
10°
65
65
64
20°
59
59
58
30°
52
52
51
40°
44
44
43
50°
36
36
34
50°
28
28
26
70°
21
21
20
80°
16
16
15
of
cities may
be rapid and there may be infrequent cleaning. The following correction factors are sug- gested according to the location and slope of the glazing:
Type of Vertical Sloping Horizontal location glazing glacing glazing
Clean 0.9 Dirty 0.7 (e.g. industrial)
0.8
0.7
0.6
0.5
Very dirty 0.6
0.5
0.4
Allowances for alternative types of glazing materials
This can be made by multiplying the total daylight factor by the ratio: actual glass area of typical window area of window aperture Where the net area of glass is diffi cult to estimate it is often sufficient to allow a correction factor of 0.75.
Allowance for window framing and window bars
The methods described in this Digest and the previous one, with the exception of B.R.S. Daylight
TABLE 7
Use of Nomogram III for vertically glazed rooflights
Variation of function K with angle of obstruction
(overcast sky conditions)
Angle of obstruction from centre of window (degrees above horizontal)
0°
(no obstruction)
K
37
10°
34
20°
30
30°
26
40°
21
50°
16
60°
12
70°
10
80°
9
Factor Protractors Nos. 9-10, have incorporated an allowance for the transmission losses when ordinary clean clear sheet glass or plate glass is used. Further corrections to the
TABLE 8
Transmission factors and correction factors for various glazing materials
Material
Transparent glasses Flat drawn sheet
in. Polished plate
in. Polished wired
Recommended correction factor to apply to daylight factor
Diffuse transmission
factor (Average)
0.85
1.0
0.85
1.0
0.80
0.95
0.80-0.85
0.95
0.80 0.85
0.95
0.75
0.90
0.85
1.00
0.85
1.00
0.80
0.95
0.85
0.95
0.75
0.90
0.74
0.90
0.48 0.65
0.50-0.60
0.60-0.75 0.60--0.70
Deterioration of the decorations
of
Except for domestic buildings, an allowance for deterioration of the decorations may need to be made. An average correction factor 0.75 is usually assuumed and is ap- plied to the value for the internally reflected component only. Alterna- tively, where the location and type of work done in the building are known the following maintenance correction factors may be used: Type of Type of Depreciation
correction factor
location
work
Clean
Clean
Dirty
Clean
Clean
Dirty
Dirty
Dirty
Allowance for dirt on glass
0.9
0.8
0.7
0.6
The allowance for dirt on glass is applied to the total daylight factor. In practice it will vary depending on the degree of air pollution, the slope of glazing, and how often the windows are cleaned. In rural areas and for domestic buildings little or no allowance is usually necessary but the rate of dirt deposition for factories and offices in the centres
Patterned and diffusing glasses
in. Rolled in. Rough cast in. Wired cast Cathedral
Hammered
Arctic
Reeded
Small Morocco
Special glasses
Heat-absorbing tinted plate Heat-absorbing tinted cast Laminated insulating glass Plastic sheets
Corrugated resin-bonded glass- fibre reinforced roofing sheets
Moderately diffusing Heavily diffusing Very heavily diffusing
*0.75 0.80 *0.66 *0.55-0.70
0.75
0.55-0.78
(depending on grade)
**
Typical measured values
Diffusing opalin. acrylic plastic sheets
THE HONG KỌNG & FAR EAST BUILDER-VOLUME 19, NUMBER 4
0.90 0.75-0.90 0.65-0.80
0.65 0.90
107
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