a method to determine a single point percentage daylight factor (%df) value from field work data

Advances in Environmental Biology, 9(4) March 2015, Pages: 39-42

AENSI Journals

Advances in Environmental Biology

ISSN-1995-0756 EISSN-1998-1066

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Corresponding Author: Sharifah Fairuz Syed Fadzil, School of Housing Building and Planning, Universiti Sains Malaysia,

11800, Penang, Malaysia.

E-mail: [email protected]

A Method to Determine a Single Point Percentage Daylight Factor (%DF) Value from

Field Work Data

1Sharifah Fairuz Syed Fadzil,

1Wan Mariah Wan Harun and

1Adel Abdullah

1School of Housing Building and Planning, Universiti Sains Malaysia, 11800, Penang, Malaysia

ARTICLE INFO ABSTRACT

Article history:

Received 12 October 2014 Received in revised form 26 December

2014

Accepted 1 January 2015 Available online 17 February 2015

Key words: Daylight factor %DF, Window to wall

ratio (WWR) window to floor ratio

(WFR)

The daylight factor is an established way to measure daylight level in an indoor space,

whereby natural illumination level indoors (Ei) is compared to the simultaneous illumination outdoors (Eo). The relative values Ei/Eo is considered constant under

varying sky conditions. The %DF however does not consider direct sunlight intensity as

the occasional occurrence of direct sunlight entry will not make the ratio constant as %DF assumes a uniform and overcast sky. In Malaysia, skies are in the cloudy to near

overcast. Direct sunlight do enter the interiors occasionally especially in the mornings

and late afternoons depending on the orientation of the window openings. This paper suggests a way to measure %DF using field work data taking samples of 2 room with

the worst case scenario of facing directly East and West for comparison. Two varied

window to wall (WWR) and window to floor (WFR) ratios were tested for this experiment. The %DF values calculated from this research were in the range of 0.8 to

2.3% and natural illumination levels were found to be more than adequate for normal

bedroom activities.

© 2015 AENSI Publisher All rights reserved.

To Cite This Article: Sharifah Fairuz Syed Fadzil, Wan Mariah Wan Harun1 and Adel Abdullah., A Method to Determine a Single Point

Percentage Daylight Factor (%DF) Value from Field Work Data. Adv. Environ. Biol., 9(4), 39-42, 2015

INTRODUCTION

The most popular and most used formula to measure and analyze day lighting performance in buildings is

the percentage daylight factor [1],[2]&[3]. The daylight factor quantifies daylight levels and distribution pattern

in the interiors of a building. The formula assumes overcast skies condition and it does not consider the

excessive illumination caused by direct sunlight penetration.

The formula for %DF is derived as follows:

%DF = Ei/Eo X 100% whereby

Ei is the illuminance due to daylight at a point on the indoors working plane

Eo is the simultaneous outdoor illuminance on a horizontal plane from an unobstructed hemisphere of an

obstructed sky [4]. By using relative values which compare indoor to outdoor illuminance, this factor is constant

under widely varying outdoor sky and day lighting conditions [5].

The real sky conditions vary throughout the world depending on location, climate and sunpath but generally

skies can be divided into 3 categories [6]and they are:

Clear skies

Cloudy and partly cloudy skies

Overcast skies

Realistically, there is no uniform sky that is uniformly illuminated so that values of the percentage daylight

factor in building spaces can be directly measured. The real skies are a dynamic phenomena with the movement

of the sun via its sunpath and also the availability of clouds along with the conditions of dust, water vapour and

other gaseous molecules in the air that affects the level and quality of illumination.

The Malaysian skies has been catergorized as being very cloudy to near overcast. From meteorological

stations like Alor Setar Kedah and Bayan Lepas Penang, cloud cover data was given at 6.4 octa and 6.9 octa

respectively [7]. The value “0” octa indicate a totally clear sky and “8”octa a completely overcast sky thus the

very cloudy description suits the sky condition well indeed.

40 Sharifah Fairuz Syed Fadzil et al, 2015


Sunshine hours, on the other hand, is an Indication of the amount of time during daylight hours that direct

light from the sun is received. It is measured with a sunshine recorder and data from meteorological stations has

Alor Setar Kedah at 6.9 sunshine hours’ daily average, and Bayan Lepas Penang 7.0 hours [3]. From the 12

hours daylight duration experienced in Malaysia, about half is sunshine hours.

Due to the very cloudy skies and long duration of sunshine hours experienced, direct sunlight entry in

buildings are common especially when the buildings have a lot of windows and glazing areas. This is made even

more critical when the orientation of the glazing is toward the East and West directions when direct sunlight

penetration in buildings can occur extensively in the hours after sunrise and the late hours before sunset.

The problem is how then can %DF be measured in East and West facing spaces in Malaysia considering the

dynamic real sky conditions and the extensive direct sunlight entry?

Methodology:

A case study building was selected from one of the residential blocks of the Universiti Sains Malaysia

campus called Fajar Harapan, which is located in Penang Malaysia. The rooms selected faces the East and West

direction directly (Figure 1). The rooms measure 2.9m by 4.43m with floor area of 12.85meters square. The

window area is 4.43 meter square which makes up 50% of window wall ratio (WWR) and 35% of window floor

ratio (WFR).

To get more %DF readings, the windows were also half covered using polystyrene boards (Figure 2)

making 25% WWR and 17% of window floor ratio (WFR).

Fig. 1: Fajar Harapan : Plan and section of investigated rooms and probes locations

Indoor illumination were taken at mid points of east and west rooms (Le and Lw) using luxmeter probes

located 1m above floor level. Readings were logged every 10 minutes using a data logger simultaneously with

an outdoor weather proof light probe (Lo). Similar techniques were carried out with the window areas covered

by half (Figure 3) All measurements were carried out from in March. March days were chosen as the sun path

for the locality of Penang is more directly perpendicular to the room’s orientation.



Fig. 2: East and West room position and windows shape.

Fig. 3: Diagram to show the varied WWR and WFR tested

RESULTS AND DISCUSSION

Indoor illumination at the East room usually peaked at around 9am while in the West room at around 5pm

and this is an indication of some direct sunlight penetration according to the sun’s location and altitude at the

particular times. In the field work daylight factor calculations ie Le/Lo X 100 and Lw/Lo X 100 the readings

which were seen to have the influence of direct sunlight were disregarded . This is because the daylight factor

calculations do not consider the component from direct sunlight and assumes completely uniform and overcast

skies. Readings, which were too early in the morning and too late in the evening, were also disregarded due to

the skies not being uniformly illuminated. Only readings from 10.30am to 3.30 pm (Figure 4) are considered as

those are the approximate times that direct sunlight penetration do not occur in both rooms. This duration is

derived through observation and pattern of data from field work.

Fig. 4: Diagram showing the times data were accepted for %DF calculation



Summary:

Table 1 below summarizes the data collected and described above. It could be seen that the west room

experiences slightly lower average %DF values compared to east room. From the average %DF values, how

bright they translate into illuminance level in the rooms under the real Malaysian skies could be seen in the

average, max and min Li data collected. Data on Lo, on the other hand describe the high illumination of the

Malaysian skies. Here in Malaysia, 1%DF and even 0.5% DF may be sufficient for activities in a residential

room like resting, reading or just taking a nap.

Table 1: Natural Illumination Level and daylighting factor

East Room 6 Days (10:30am - 3:30pm)

WWR Li (lux) Lo (klux) DF (%)

min max ave min max Ave min max ave

50% (10 - 15 Mar) 586.67 2104.33 952.37 28.587 52.359 42.108 1.25 5.26 2.28

25% (16 - 21 Mar) 276.33 965.33 515.39 13.699 41.714 29.112 0.63 2.41 1.2

3

Difference 0.62 2.85

1.0

5

West room 6 Days (10:30am - 3:30pm)

WWR Li (lux) Lo (klux) DF (%)

min max ave min max Ave min max ave

50% (10 - 15 Mar) 508.17 742.17 612.72 28.587 52.359 42.108 1.27 4.24 2.26

25% (16 - 21 Mar) 106.17 353.83 224.85 13.699 41.714 29.112 0.28 1.62 0.82

Difference 0.99 2.62 1.44

REFERENCES

[1] Fadzil, S.F.S., A. Abdullah and W.M.W. Harun, 2009. “The Impact of Varied Orientation and Wall

Window Ratio ( WWR ) To Daylight Distribution in Residental Rooms,” International Symposium on

Construction in Developing Economies.

[2] Chung, T.M., 2003. “Daylighting in Hong Kong: potential and problems,” Light. Res. Technol., 35(1): 39-

41.

[3] Abdullah, A., S. Fairuz S. Fadzil, 2009. “Daylight Illumination levels in varied room configurations” Third

International Conf on Built Environment in Developing Countries, 1: 1–14.

[4] Hopkinson, R.G., P. Petherbridge and J. Longmore, 1966. Daylighting. First edit. London: William

Heinemann Ltd., p: 606.

[5] Ahmad, M.H.A., D.R. Ossen and K.A.M. Khaidzir, 2008. Methodology in architectural research, First Edit.

Johor Darul Ta’zim: UTM, pp: 272.

[6] Ibrahim, N., 2007. “Daylight availability in an office interior due to various fenestration options,” vol. 1,

no. September, pp: 436-440.

[7] Ministry of Science Technology and Innovation (MOSTI), “Malaysian Meteorological Department,” 2014.

[http://www.met.gov.my/index.php?option=com_frontpage&Itemid=1.

a method to determine a single point percentage daylight factor (%df) value from field work data

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