Carbon Trust Report - Daylighting by Syntegra

Daylighting

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Contents

Daylighting

Outline Proposals - RIBA stage C

Scheme Design - RIBA stage D

Detail Design RIBA stage E

Queens Building De Montford University, Engineering Laboratories, Leicester.

Questions

Further reading

Skylight Sunlight View Skylight and optimum window Size

Skylight and room appearance â&#x20AC;&#x201C; the average daylight factor Sunlight control Lighting for task performance Daytime electric lighting

2 2 3 3

4 4 5 5

Carbon Trust Report – Daylighting

Daylighting

Windows and rooﬂights are important elements of building design since most people prefer to work in daylit spaces. The reason for this is not certain. It might be the colour performance of daylight or the visual modelling of three dimensional objects from side windows or just the visual contact with the outside. But more likely it is a combination of all of them.

Nevertheless good daylighting has the potential to save energy used for electric lighting. This module gives a summary of daylighting practice in climates such as Britain’s. It describes the aims of good design, lists some useful rules of thumb, referring to the stages of the RIBA Plan of Work: • • •

Outline Proposals Scheme Design Detail Design

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Outline Proposals - RIBA stage C

Skyline obstruction

25o Centre of window (outside)

Figure 1: Angle of obstruction above the horizon

Skylight

Sunlight

In cloudy climates, diffuse light from the sky is the main source of daylighting. At the site planning stage the aim is to ensure that there is a sufﬁcient area of sky visible to give good interior lighting with windows of reasonable size.

People’s needs and preferences for sunlight depend on the type of building – incoming sunlight can give warmth and brightness but can also cause glare and thermal discomfort. It is the responsibility of the architect in the analysis of clients’ requirements to determine which parts of a building would beneﬁt from direct sunlight – for visual reasons, thermal comfort or solar energy gains – and from which areas direct sunlight should be excluded.

The availability of skylight at a window is determined primarily by the block form of the building and its surroundings, so wrong decisions at an early design stage are difﬁcult to correct later. The reference point is taken to be the centre of the window opening and the angle of obstruction is measured perpendicular to the window, as in Figure 1. A useful rule of thumb is when a window is to be used as a main source of light, external obstructions should not be higher than 25° above the horizon.

Another rule of thumb is: when a window is to be used to provide sunlight in a room, it should face within 90° of south and external obstructions should not be higher than 25° above the horizon. The rule is based on achieving 25% probable sunlight hours (including 5% in winter) on the face of the window. The difference between the two rules is the need for orientation of windows for sunlight.

Carbon Trust Report – Daylighting

Scheme Design - RIBA stage D

View In relation to the issue of views to the exterior, this is at least as important to the occupants of a building as the level of interior lighting and almost any view is better than none. While it may be perceived that the main visual beneﬁt of windows is the introduction of daylight into a room, studies have shown that occupants can perceive a room as being too dark even when plenty of natural light is available, if a pleasant view is restricted.

Energy cost

Skylight and optimum window Size Achieving very high levels of daylight usually means large windows and tall rooms; these are associated with excessive heat gain and loss, and a high building cost. Using large window areas to increase daylighting and reduce the use of artiﬁcial lighting can backﬁre as glare problems result in blinds being dropped

Full electric lighting Full daylighting

Small windows

Figure 2: Window size and total energy

Large windows

and all of the lights being turned on. Conversely, with very small windows, little use is made of daylight as a source of energy; in addition, occupant satisfaction tends to be lower. For most buildings the graph of lifetime energy costs against window size is U-shaped, as in ﬁgure 2: the optimum is found where daylight and electric light complement each other during daytime hours.

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Skylight and room appearance – the average daylight factor An interior can often appear daylit even if the task lighting is predominantly electric. This occurs when the main room surfaces receive enough daylight for the room to appear ‘light’. A third rule of thumb is that: a room can have a daylit appearance if the area of glazing is at least 1/25th of the total room surface area (ﬂoor, ceiling and walls including the windows). This rule of thumb works on the assumption that the room is approximately rectangular in plan and that there are no factors that signiﬁcantly reduce the amount of light in the space, such as dark room surfaces, low transmittance glazing or high external obstructions (more than 25° – see Figure 1). The rule of thumb is based on achieving an average daylight factor of at least 2% at table-top level in the room. The ‘daylight factor’ is the amount of daylight at a point expressed as a percentage of the daylight falling on an unobstructed horizontal surface outside, excluding direct sunlight. The average daylight factor can be used at an early design stage to estimate the total glazed area required.

Sunlight control

It is easily calculated: Average Daylight Factor = Ag x — x T A (1 —

R 2)

Where the meaning of the symbols in the equation is as follows: Ag glazed area of windows (not including glazing bars or other obstructions) — angle of visible sky, measured in section from the centre of the window opening in the plane of the inside window wall see figure X. T transmittance of glazing to diffuse light, including the effect of dirt. A total surface area of room (floor, ceiling and walls including windows). R average reflectance of room surfaces. Even if the average daylight factor is high, supplementary electric lighting may be necessary if a room is to be very deep or L-shaped. It is necessary to distribute the calculated area of glazing so that daylight can reach all parts of the room; it is especially valuable to have windows in more than one wall. Unless the sky is visible, a point is lit only by interreflection. This rarely provides sufficient light for visual tasks, and the room appears gloomy unless surface reflectances are high.

Even in rooms where sunlight is desirable it is always necessary to provide a means of controlling this, to prevent overheating, discomfort or disability glare or damage to light sensitive materials e.g. fabrics etc. The obstruction caused by shading devices must be estimated and taken into account in daylight factor calculations. Where sunlight penetration needs to be restricted, this is best done by window orientation and by exterior obstructions, such as trees. Controllable sun screening (louvre or roller blinds) can be used to prevent solar transmission in summer but should also be capable of allowing transmission of daylight to minimise the use of electric light under overcast conditions. Care must be exercised in the design and control of such systems to ensure that, where possible, the user has an override facility. Case studies have shown that removing user control causes irritation and a signiﬁcant reduction in occupant tolerance to the internal environment.

window head obstruction

Centre of window (inside)

0°

skyline obstruction

section through side window

Figure 3: Section through side window

Carbon Trust Report – Daylighting

Detail Design RIBA stage E

Lighting for task performance

Daytime electric lighting

The daylight factor indicates the balance between the daylight outside and the amount of daylight within a room. It can be used as a measure of the brightness appearance because the eye adapts to the ambient level of lighting. For good working conditions it is necessary also to ensure that the task illuminance is above a minimum level. (‘Illuminance’ is the amount of light falling on a surface and measured in ‘lux’.) The required illuminance depends on the purpose of the room. To achieve high levels of daylight it is necessary to plan the room so that workplaces are related to window positions.

With some room shapes and layouts, full daylighting leads to the lowest lifetime costs of money and energy. But with many buildings, particularly those above domestic scale, it does not – the best value is given where daylight is supplemented by controlled electric lighting.

A further rule of thumb is that surfaces that are closer to a window than twice the height (2h) of the window head above desktop level, receive adequate daylight for tasks for most of the working year (see figure 4). The rule of thumb assumes that there is no significant loss of light due to external obstructions, tinted glazing or interior screening. It then approximately defines a zone where the daylight factor is always above 2% on desktops. The rule illustrates the importance of window head height (and therefore ceiling height) to daylight distribution. The higher the window, the deeper the zone of strong daylight.

Daytime electric lighting is usually required for a bright room appearance when the average daylight factor is less than 5%. It should supplement the daylight and not swamp its natural variation. The amount needed depends on the type of task and hence the illuminance required. No energy saving is made from daylight availability if the use of electric lighting does not respond. In a medium-size rectangular room this means, typically, that lights distant from the window wall are always on during the day, those close to the windows are switched on only after dusk, and intermediate lights react to changing conditions. This means

The electric lighting must then serve two distinct purposes during daylight hours: • to enhance the general brightness of the room, reducing glare and gloominess • to increase the illuminance on visual tasks. that the electric lighting circuits need to be zoned relative to the daylight distribution. The following example demonstrates some of the principles of daylighting outlined above.

Window head

Desktops

Approximate limit of working daylight 2h

Figure 4: Zone of strong daylight

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Queens Building De Montford University, Engineering Laboratories, Leicester.

The laboratories are housed in two, four storey wings running east – west. The laboratories have high level windows on both sides. Below the windows are horizontal light shelves. A combination of the windows, the light shelves and the high reflectance internal surfaces provides a good level and even distribution of daylight. There are also small low level windows mainly for view purposes. The south side of the building faces onto a courtyard which provides some sun shading from the buildings across the courtyard. This is assisted by the internal light shelves which protects the area underneath from direct sunlight. The vertical exterior surfaces of the building are clad in a white material. This aids the reflection of daylight into the building opposite. The main element of the electric lighting is ceiling suspended fluorescent lamp luminaires running in rows parallel to the windows. Each row can be switched to complement the daylight. In addition there are luminaires positioned over the laboratory benches to act as task lighting which can switched individually as users require.

Carbon Trust Report – Daylighting

Questions

Please tick ONE box only in each of the following questions. 1 Which of the following factors is a prime determinant of the availability of skylight The block form of the building and its surroundings The existence of lightshelves Daylighting control by building occupants 2 When a window is to be used to provide a daylit appearance in a room: It should have a glazing area not less than 1/25th of the total room area External obstructions should not be higher than 25° above the horizon Both of the above 3 True or False: if building occupants do not have a view out they can perceive a room as being under lit even when plenty of light is available. True False 4 Which of the following should be included to provide a basic energy efﬁcient electric lighting strategy? Full automatic control of electric lighting Full automatic control of daylighting Complete control of daylighting and electric lighting by occupants Zoning the electric lighting circuits for daylight distribution (correct) 5 True or False: In the northern hemisphere windows facing within 90º of south will receive some sunlight? True False 6 In order to minimise lighting energy costs: Windows should be as large as are permitted The daylight factor should be not less than 2% on working surfaces Electric lighting should be able to be controlled relative to the daylighting All of the above

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