Design Consideration
Home Up Solar Radiation Sky Conditions for Daylighting Luminance Distribution in Overcast Sky Daylight Calculation Daylight Factor Methods for Daylight Factor Estimation Daylight Factor Contours Models of Artificial Sky Design Consideration


9. Design Consideration

9.1 Environmental factors

Daylight design is closely related to a number of environmental factors because the use of glass windows to let in light also allows the penetration of solar heat and noise and increases the rate of heat gain or loss of the building fabric. Therefore daylight design can never be considered alone. Fortunately by the proper choice of window orientation and suitable sun shading device or constructions, the solar heat gain can be greatly reduced. The use of double glazed window units is able to cope with the noise problem and reduce significantly the heat gain or loss through the window. The remaining problem would be the proper design of windows or rooflights to provide a visually comfortable and efficient environment for the type of tasks and activities to be performed.


9.2 Fenestration

The lighting conditions in a room depend primarily on the fenestration. As Figure 14 shows, the DF and consequently the illumination near the window decreases as the height of the window above the floor is increased. However, the illumination increases away from the window, giving greater uniformity to the lighting .

Figure 15 shows how the DF varies across a room using two types of windows, both having the some total area:

(a) is for three tall windows, and

(b) is for one long, high-level window.

Window (a) will give a good open view, but (b) will provide more even daylight illumination.

The following points should be considered in the design of fenestration:

(a) The window head should be as high as possible, say at least 2 m above floor level, to enable one can see out when standing.

(b) The window sill should not be higher than 1 m from floor level to enable one can see out when sitting.

(c) The window surface area should be evenly distributed over the outside wall and the window heights and widths should not be too small in relating to the window wall because this reduces the uniformity of lighting and produces undesirable shadows.

Figure 14 The Effect of Window Height on DF


Figure 15 The Effect of Window Shape on DF

(d) Better lighting can be achieved with windows on opposite walls since the illumination produced by the individual windows are superimposed. The region with the minimum daylight factor (see Figure 16) is then displaced towards the centre of the room. The usable depth of the room thus increase.

For rooflights, as shown in Figure 17, the daylighting is evenly spread over the working area and much higher DF can be obtained. Yet DF are often drastically reduced by overhead obstructions and poor window maintenance.

Figure 16 The Effect of Windows on Two Sides of a Room on DF

Figure 17 The Distribution of Daylight Across the Floor for a Roof-Lit Interior


9.3 Obstructions

Any obstruction (buildings opposite the window, trees, etc.) decreases the illuminance in rooms. Obstructions which subtend elevation angles of more than 25-30o significantly reduce the daylight penetration. This can be improved by increasing the height and width of the window. If possible, the angle subtended by obstructions should not be greater than 30o. The minimum distances between adjacent building and the window as laid down in the building regulations should be complied with.

9.4 Room Dimensions

With room heights of 2.5-3 m which are usual in residential building, room depths, from the lighting point of view, should ideally be limited to 4-6 m, depending on the width of the room and the nature of obstructions if any. To achieve good illumination, greater room depths are only possible with windows on two sides, with rooflights or with the installation of supplementary artificial lighting.

9.5 Uniformity of lighting

Uniformity is essentially dependent on:

(a) the dimensions of the room, the windows and all the obstructions to light,

(b) the reflectances of the surfaces enclosing the room, the obstructions to light and the room furnishings, as well as the type of glazing.

The uniformity of illumination is expressed in terms of the ratio of the minimum illumunance in the room to the average illuminance measured on a horizontal reference plane. Uniform distribution of the luminance in the room should be the objective.

9.6 Glare

One will experience glare if the difference in the luminances of adjacent surfaces become excessive. This refers to impairment of the visual efficiency of the eye by excessive relative and absolute luminances. In addition to direct glare from the sun, glare may also arise in interior rooms through excessive contrast between the high luminance of the patch of sky visible through the windows and the substantially lower luminances of the adjacent surfaces enclosing the room. The degree of glare thus depends on :


(a) the luminance and the size of the light-emitting surface seen by the eye,

(b) the ratio of this luminance to the luminance of this environment or background,

(c) the distance of the glare-producing surface from the eye and its position in the field of vision.

All types of glare, both direct glare from the sun and glare from reflections, e.g. from polished floors, are to be avoided.

9.7 Shadow intensity and direction of incident light

For the perception of the solidity and surface texture of objects a reasonable shadow intensity is required. Adequate shadow intensity is generally provided by the lateral incidence of daylight into rooms with side windows. Working positions should be arranged so that hand and body shadows do not fall on the working surface.

9.8 Permanent supplementary artificial lighting of the interior (PSALI )

By considering windows solely as admitters of daylight it may seem that the achievement of a certain minimum DF is the main design criterion. Electric lighting is today often used during daylight hours and it may be the case that in trying to achieve a minimum DF, say 2 per cent, the window becomes too large causing more serious glare and heating /cooling problems. It may be preferable to design to a lower DF and integrate the daylight with the electric light.

The method known as 'permanent supplementary artificial lighting of the interior' (PSALI) (See Figure 18) is based on three principles:

(a) Utilisation of daylight as far as practicable,

(b) Use of electric lighting to supplement the daylight in the interior parts of the room,

(c) Installation of the electric lighting in such a way that the daylight character of the room is retained.

Figure 18 PSALI Scheme

The required quantity of artificial light input is to achieve an illumance at the rear of the room comparable to, but slightly less than, the daylight illuminance near the window. For example:


Daylight factor

Artificial light (lux)









  skylight05.jpg (54456 bytes)


skylight06.jpg (30175 bytes)


skylight07.jpg (41946 bytes)