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TitleGPG245-Desktop-Guide-to-Daylighting-for-Architects.pdf
TagsInterior Design Applied And Interdisciplinary Physics Lighting Window
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Document Text Contents
Page 1

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GOOD PRACTICE GUIDE 245

Desktop guide to daylighting
– for architects

BEST PRACTICE
P R O G R A M M EARCHIVED DOCUMENT

Page 2

RIBA PLAN OF WORK

The process of creating a building can be divided into a number of definable stages

which take the project from the early discussions and rough sketches through to more

detailed design drawings and finally to the building operations on site.

The RIBA Plan of Work does just this and divides the whole process into stages A to M.

It provides a model outline procedure for methodically working through the whole

design and construction process by all the different members of the design team.

At each of the stages, a different level of daylighting design input is needed. This Guide is

written to reflect this need. It is written to assist the designer at Stage C (Outline Proposals),

Stage D (Scheme Design) and Stage E (Detail Design).

The stages of the RIBA Plan of Work that lead up to going out to tender are:

STAGES Inception and Feasibility for appraisal in order to decide whether and

A AND B how to proceed.

STAGE C Outline Proposals to develop the brief and determine the general

approach to layout, design and construction.

STAGE D Scheme Design to finalise brief, decide final direction using input from

other consultants, complete a full design and submit for approvals.

STAGE E Detail Design to obtain the final decisions on all matters relating to

design specification, construction, and cost.

STAGE F Production Information to produce the details of the design in order to

complete the drawings, schedules and specifications in readiness for

preparing the Bill of Quantities for tendering.

STAGE G Bill of Quantities to complete the documentation for tendering.

STAGE H Tender action to invite tenders from contractors.

The work described in this report was carried out by Professor Peter Tregenza of the University of

Sheffield under contract as part of the New and Renewable Energy Programme, managed by the

Energy Technology Support Unit (ETSU) on behalf of the Department of Trade and Industry.

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SCHEME DESIGN

DESKTOP GUIDE TO DAYLIGHTING

Using the block layout of a scheme, rule of thumb 2

checks that adequate sunlight is available at the

positions of critical windows.

The rule is based on achieving 25% probable

sunlight hours (including 5% in winter) on the

face of the window. The angle of 25° is measured

in the same way as for skylight (figure 1). The

difference between the two rules is the need for

orientation of windows for sunlight.

Complex skylines

It is possible for adequate skylight and sunlight to

fall on a window where part of the opposite

skyline lies above 25° and other sections are lower.

If there is only moderate variation, drawing a

horizontal line at the average height provides a

reasonable estimate, as in figure 2.

Where the skyline varies greatly in height – such as

where a window looks into a courtyard – a more

precise calculation is needed in critical cases.

Techniques are described in BR209, which gives

graphical methods for checking sunlight

availability by estimating probable sunlight hours.

For calculating skylight availability it gives a

routine for finding the vertical sky component

(this is the ratio of light received on a vertical

surface from an overcast sky to that received by

unobstructed ground).

The techniques enable the effects of complex

obstructions to be compared with simple

horizontal skylines.

View

Unless the function of the building requires the

exclusion of daylight, views to the exterior should

be provided. Windows give information about the

outside – the weather and the time of day – and

this is at least as important to the occupants of a

building as the level of interior lighting. Almost

any view is better than none, but the following

points are found in research on users’ preferences.

■ Views of natural objects are preferred to those

of buildings and man-made objects.

■ Preferred views include the horizon but not a

high expanse of sky.

■ Views of people and activities are needed by

those confined in buildings such as residential

homes and hospitals.

■ Some views are considered essential for

security and supervision – such as the

approach to the front door of a dwelling, or

clear supervision of children playing.

■ If, in workplaces such as offices, there are

windows in only one side of a room, a

minimum glazed area of 20% of the inside

6

estimated
height

horizon

Figure 2 Estimating the skyline

High external obstructions

block the view of the sky and

make the room gloomy

Scheme Design
RIBA stage D

©
P

eter T
regen

za

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Page 7

Rule of thumb
A room can have a daylit

appearance if the area of

glazing is at least 1/25th

of the total room area.

SCHEME DESIGN

DESKTOP GUIDE TO DAYLIGHTING

window wall area is needed when the depth of

the room is less than 8 m; the minimum glazed

area increases to about 35% for rooms deeper

than 14 m. This is needed to give people at the

back of the room an adequate view.

■ Attitudes to privacy (views into the building)

vary greatly with culture and building type.

In some cases occupants will minimise a view

out to ensure that privacy is maintained.

It is important to note that windows designed to give

preferred views must usually look out at the horizon

and below; therefore they may not alone give sufficient

light in the room to achieve a bright daylight

appearance or illuminate workplaces adequately.

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. 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 3:

the optimum is found where daylight and electric

light complement each other during daytime hours.

Skylight and room appearance – the average

daylight factor

An interior can look daylit even if the task lighting

is predominantly electric. What is required is that

the main room surfaces receive enough natural

light. Daylight – direct and inter-reflected – on the

walls and ceiling is important to users, not only

because it enhances the overall brightness of the

room but because it carries information. It changes

continuously, and people can sense the outside

even without a direct view.

The ‘total room area’ is the sum of the floor,

ceiling and wall areas, including the window.

Rule of thumb 3 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

surfaces, low transmittance glazing or high

external obstructions (more than 25°).

The rule of thumb is based on achieving an

average daylight factor of 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.)

In cloudy climates, the average daylight factor

indicates the likely daylit appearance of a room:

■ less than 2%

– room looks gloomy under daylight alone

– full electric lighting often needed during

daytime

– electric lighting dominates daytime

appearance

■ 2%-5%

– windows give a predominantly daylit

appearance but supplementary electric

lighting needed

– usually the optimum range of daylighting

for overall energy use

■ 5% or more

– the room is strongly daylit

– daytime electric lighting rarely needed

– major thermal problems from large windows.

7

Figure 3 Window size and total energy

energy
cost

small
windows

large
windows

full electric
lighting

full
daylighting

3

1/25

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Page 11

DETAIL DESIGN

DESKTOP GUIDE TO DAYLIGHTING

Where activities permit, good value is given by

local task lighting instead of overall workplace

illumination.

Local manual switching is an alternative to

automatic control but full energy savings occur

only when electric lights are continuously

dimmable in response to changing daylight. The

high-frequency electronic ballasts required to dim

fluorescent lamps smoothly have at present a

higher initial cost than conventional controls but

can give lower long-term costs. They offer other

efficiency savings too and are likely to become the

normal solution.

With all types of environmental control in

buildings, user acceptability is essential to energy-

saving success.

11

off during
daytime

sometimes on
during daytime

on during
daytime

daylight
illuminance

Figure 7 Control of electric lighting. (A lighting layout such as this must be checked in plan to ensure that

desktop reflections are avoided.)

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Page 12

References

The British Standards Institution, BS 8206

‘Lighting for buildings: Part 2: 1992: Code of

practice for daylighting’. BSI, London, 1992.

Building Research Establishment. ‘Site layout

planning for daylight and sunlight. A guide to

good practice’. BR 209. BRE, Garston, 1991.

The Chartered Institution of Building Services

Engineers. ‘CIBSE Code for interior lighting’.

CIBSE, London, 1994.

Introductory guides for architects

Building Research Establishment. ‘Designing

buildings for daylight. Professional studies in British

architectural practice’. BR 288. BRE, Garston, 1995.

European Commission Directorate-General for

Energy. ‘Daylighting in buildings’. University

College, Dublin, 1994.

Lawrence Berkeley National Laboratory. ‘Tips for

daylighting with windows. The integrated approach’.

LBNL-39945. LBNL Berkeley, California, 1997.

Building Research Establishment. ‘Daylighting

design in architecture’. BRE, Garston, 1998.

Available from RIBA bookshops.

Technical references

The Chartered Institution of Building Services

Engineers. CIBSE applications manual AM1.

‘Window design’. CIBSE, London, 1987. (New

edition due to be published 1998.)

Building Research Establishment. ‘Estimating

daylight in buildings: part 1’ and ‘Estimating

daylight in buildings: part 2’. BRE digests 309 and

310. BRE, Garston, 1996.

Building Research Establishment. ‘Availability of

sunshine’. CP75/75. BRE, Garston, 1975.

Building Research Establishment. ‘People and

lighting controls’. IP 6/96. BRE, Garston, 1996.

The Stationery Office. ‘The UK Environment’;

page 5, figure 1.6. TSO, London, 1992.

(ISBN 0 11 752 420 4)

The following publications are available from

BRECSU Enquiries Bureau. Contact details are

given below.

Thermie maxibrochure: ‘Energy Efficient Lighting

in Buildings’

General Information Report

27 Passive solar estate layout

35 Daylighting for sports halls

Good Practice Guide

160 Electric lighting controls – a guide for

designers, installers and users

Energy Consumption Guides: compare energy use in
specific processes, operations, plant and building types.

Good Practice: promotes proven energy efficient techniques
through Guides and Case Studies.

New Practice: monitors first commercial applications of new
energy efficiency measures.

Future Practice: reports on joint R&D ventures into new
energy efficiency measures.

General Information: describes concepts and approaches
yet to be fully established as good practice.

Fuel Efficiency Booklets: give detailed information on
specific technologies and techniques.

Introduction to Energy Efficiency: helps new energy managers
understand the use and costs of heating, lighting etc.

© CROWN COPYRIGHT FIRST PRINTED MARCH 1998

Industrial projects contact:
Energy Efficiency Enquiries Bureau

ETSU
Harwell, Oxfordshire
OX11 0RA
Tel 01235 436747
Fax 01235 433066
E-mail [email protected]

Buildings-related projects contact:
Enquiries Bureau

BRECSU
BRE
Garston, Watford, WD2 7JR
Tel 01923 664258
Fax 01923 664787
E-mail [email protected]

Internet BRECSU – http://www.bre.co.uk/brecsu
Internet ETSU – http://www.etsu.com/eebpp/home.htm

The Department of the Environment, Transport and the Regions’ Energy Efficiency
Best Practice programme provides impartial, authoritative information on energy efficiency
techniques and technologies in industry and buildings. This information is disseminated
through publications, videos and software, together with seminars, workshops and other
events. Publications within the Best Practice programme are shown opposite.

For further information on:

FURTHER INFORMATION

DESKTOP GUIDE TO DAYLIGHTING

DETR ENERGY EFFICIENCY BEST
PRACTICE PROGRAMME PUBLICATIONS

NOTES
[1] Transmittance

The diffuse transmittance of

clear single glazing is

approximately 0.8; clear

double glazing

approximately 0.7. Low-

emissivity double glazing has

a diffuse transmittance of

about 0.65 but the value for

tinted glazing can be even

lower (eg 0.4 for 6 mm grey

glass). Other types of glazing

are listed in BS 8206 and in

manufacturers’ literature but

care must be taken to use

values of diffuse

transmittance, which is

lower than the transmittance

of a beam perpendicular to

the glazing. Dirt typically

gives a further 5% reduction

with normal cleaning in an

urban setting but this can

become 20% or more with

horizontal rooflights in

industrial areas.

[2] Reflectance

During design, this can

often be only an educated

guess; typical values are 0.5

for a room with white

ceiling and light-coloured

walls, 0.3 for a normal

office or living room.

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