LIGHT 3 May 2009, Yeoh OonSoon Architect/Lecturer Daylight Factor Prediction/Calculation using the Split Flux Method The sky model used will influence prediction of the DF. The assumption made as a basis for daylighting analysis in temperate climates is for a heavily overcast sky giving a total unobstructed illumination on a horizontal plane at ground level of 5000 lux. (Standard Overcast Sky – between 0800 to 1700 hrs, illumination measured is 5000 lux for 85% of time) (pp.39, BURBERRY, Peter. (1979). “Mitchell’s Building Series: Environment & Services”, B.T.Batsford, London.) The CIE sky assumes that for an overcast sky, the sky hemisphere is about three times as bright at the zenith as at the horizon. This will reduce oversizing of rooflights and undersizing of low windows, in temperate climes. For convenience, the BRE has created a graphical method using a protractor to compute the sky component and the external reflected component. There are 5 protractors available for and overcast sky and 5 for uniform sky brightness viz,

1. Vertical glazing protractor 2 / protractor 1 2. Horizontal Glazing protractor 4 / protractor 3 3. 30deg glazing protractor 6 4. 60deg glazing protractor 8 5. Unglazed Apertures protractor 10

In this part of the tropics with abundant sunshine, clear skies are more common. Calculation of DF for clear sky is developed in the US and is rather complicated. It takes account of the sun’s position in the sky. Further reading pp 182-186, ROBBINS, Claude L. (1986). “Daylighting : Design & Analysis”, Van Nostrand Reinhold Company, New York. An extract of pp 183 is as follows;

Being close to the equator here, the sun’s path across the sky is almost equal to the north and south. Thus, windows in almost any orientation will receive sunlight over some period of time throughout the year. It is my opinion that sun shading design to reduce solar radiation and glare will in most cases negate whatever window sizing or configuration done using any daylight factor prediction. If DF has to be calculated (perhaps for academic reasons), then the graphical method using the BRE Protractor for Uniform Sky (also overcast like CIE sky, but with uniform luminance) is the next best thing and is recommended to avoid oversizing windows as heat gain and glare is an issue here, all year round. The information about daylighting (DF) will either be;

1. Value of DF for one or more critical spots in room 2. the general distribution of daylighting across the working plane in a room.

Scaled Plans & sections of rooms concerned are needed. The behavior of light is consistent irrespective of scale, thus any conveniently sized drawing can be used. Plan and Section need not be to same scale.

DF = {SC + ERC + IRC (MF)} x {GF x FF} Where MF = Maintenance factor GF = Glazing Factor FF = Framing Factor

Protractor 2 : Vertical Glazing, Overcast Sky

SZOKOLAY, S.V. (1980). “Environmental Science Handbook for Architects and Builders”, The Construction

Press, UK

Daylight Factor Protractor use – example window;

SECTION through room at window

iSC (i.e. initial SC) = 5.75

CF = 0.61 PLAN to establish correction factor to initial reading from section.

Thus, for a single window, the SKY COMPONENT becomes;

SC =iSC x CF

5.75 x 0.61 = 3.51

Should there be multiple windows contribute light, eg;

BURBERRY, Peter. (1979). “Mitchell’s Building Series: Environment & Services”, B.T.Batsford, London. Apply correction factors as follows;

1. In same wall. Add plan correction factors for each window, multiply section value by this sum to get the direct sky component.

2. In other wall (i.e. on different plane) or roof, compute separate direct sky components for each plane and total.

The protractor can be used in a similar manner to calculate the ERC (EXTERNAL REFLECTED COMPONENT)

ERC obstruct = SC obstruct x R obstruct

Where R obstruct = average reflectivity (in decimal) Use 0.2 if CIE overcast sky protractor is used Use 0.1 if Uniform sky protractor is used

Further reading, pp 190-193, ROBBINS, Claude L. (1986). “Daylighting : Design & Analysis”, Van Nostrand Reinhold Company, New York. Also pp108, SZOKOLAY, S.V. (1980). “Environmental Science Handbook for Architects and Builders”, The Construction Press, UK

SZOKOLAY, S.V. (1980). “Environmental Science Handbook for Architects and Builders”, The Construction Press, UK

To determine the IRC (INTERNAL REFLECTED COMPONENT);

1. Calculation. 2. Graphical Method

1. Calculation

pp108, SZOKOLAY, S.V. (1980). “Environmental Science Handbook for Architects and Builders”, The Construction Press, UK

3. Graphical Method, BRE nomogram below based on the following assumptions;

Floor reflectivity = 15 % Ceiling reflectivity = 70 % Exterior ground reflectivity = 10 % Room size no greater than 500 sq ft Glazing transmission = 0.85

pp.109, SZOKOLAY, S.V. (1980). “Environmental Science Handbook for Architects and Builders”, The Construction Press, UK

SZOKOLAY, S.V. (1980). “Environmental Science Handbook for Architects and Builders”, The Construction Press, UK

The sum of the SC, ERC and the IRC can then be multiplied by the Glazing Factor and the Framing Factor to get the DF. Method 7 : Model Studies

1. Artificial Skies

SZOKOLAY, S.V. (1980). “Environmental Science Handbook for Architects and Builders”, The Construction Press, UK In Malaysia, there is a no statute/law specifically pertaining to Daylight Factor, Sun Shading or Site Overshadowing. However there is a statute requiring natural lighting in buildings i.e. The Uniform Building By-Laws, Part III “Space, Light & Ventilation” : By-laws 39 and 40.

Daylight Factor : Ongoing Research / Debate / Forums on Standard Clear Sky, Partially Overcast Sky, etc.

http://lrt.sagepub.com/cgi/content/refs/38/2/137 http://www.schorsch.com/kbase/glossary/skies.html http://www.agi32.com/forums/view_topic.php?id=96&forum_id=11

Formula on pp5, http://lrt.sagepub.com/cgi/content/refs/38/2/137.pdf

http://lrt.sagepub.com/cgi/content/refs/38/2/137 - Web abstract below;

Lighting Research and Technology, Vol. 38, No. 2, 137-149 (2006) DOI: 10.1191/1365782806li165oa © 2006 SAGE Publications

Average daylight factor for the 15 CIE standard skies DHW Li, BSc PhD CEng MCIBSE MHKIE MIEAust

Building Energy Research Group, Department of Building and Construction, City University of Hong Kong, Hong, Kong SAR, China, bcdanny@cityu.edu.hk

GHW Cheung, BEng

Building Energy Research Group, Department of Building and Construction, City University of Hong Kong, Hong, Kong SAR, China

For daylighting calculations, the design criteria are often expressed in terms of average daylight factors (DFave) with the computations being based on the CIE (International Commission on Illumination) overcast sky, which is generally considered to provide the worst daylight condition. The daylight illuminance of a room is mainly influenced by the luminances and patterns of the sky in the direction of view of the window at any given time. Overcast skies may not always be the appropriate reference sky types for daylighting analysis. Recently, Kittler et al. have proposed a new range of 15 standard sky luminance distributions including five clear, five partly cloudy and five overcast sky types. These 15 sky luminance models have been adopted as the CIE General Standard Skies. This paper presents an approach to computing the DFave for the 15 standard skies. The techniques for calculating the two configuration parameters for the DFave

determination, namely; (a) light received directly from the sky above the horizon on a vertical surface, and (b) light received directly from the ground below the horizon on a vertical surface are established and described. The performance of the proposed method was assessed against the results obtained by other independent calculation methods and computer simulations. It is shown that the results predicted by the proposed approach are in reasonably good agreement with those produced from the other two calculation tools. The findings provide architects and building designers

with a reliable and simple method for estimating the interior daylight illuminance under various standard overcast and nonovercast sky conditions.

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