Comfort, CIBSE

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Comfort CIBSE KNOWLEDGE SERIES Direct and accessible guidance from key subject overviews to implementing practical solutions

Transcript of Comfort, CIBSE

Page 1: Comfort, CIBSE

Comfort

CIBSE KNOWLEDGE SERIES

Direct and accessible guidance from key subjectoverviews to implementing practical solutions

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The rights of publication or translation are reserved.

No part of this publication may be reproduced, stored in a retrieval system or transmittedin any form or by any means without the prior permission of the Institution.

© January 2006 The Chartered Institution of Building Services Engineers London

Registered charity number 278104

ISBN-10: 1-903287-67-7ISBN-13: 978-1-903287-67-5

This document is based on the best knowledge available at the time of publication.However no responsibility of any kind for any injury, death, loss, damage or delay howevercaused resulting from the use of these recommendations can be accepted by theChartered Institution of Building Services Engineers, the authors or others involved in itspublication. In adopting these recommendations for use each adopter by doing so agrees toaccept full responsibility for any personal injury, death, loss, damage or delay arising out ofor in connection with their use by or on behalf of such adopter irrespective of the cause orreason therefore and agrees to defend, indemnify and hold harmless the CharteredInstitution of Building Services Engineers, the authors and others involved in theirpublication from any and all liability arising out of or in connection with such use asaforesaid and irrespective of any negligence on the part of those indemnified.

Typeset by CIBSE Publications

Printed in Great Britain by Latimer Trend & Co. Ltd., Plymouth PL6 7PY

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CIBSE Knowledge Series – Comfort 1

ComfortCIBSE Knowledge Series: KS6

Principal author Gay Lawrence Race

EditorsJustin RoebuckKen Butcher

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Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11.1 Use of this guidance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2

2 Thermal comfort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42.1 What is thermal comfort? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42.2 What determines thermal comfort? . . . . . . . . . . . . . . . . . . . . . . .42.3 Key environmental factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .82.4 Ventilation and air quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .132.5 The adaptive approach to thermal comfort . . . . . . . . . . . . . . . .142.6 How hot is hot? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .162.7 Design criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .192.8 Practical issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21

3 Visual comfort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .243.1 Key environmental factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . .243.2 Design criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27

4 Acoustic comfort (aural comfort) . . . . . . . . . . . . . . . . . . . . . . . . .294.1 Key environmental factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . .304.2 Design criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32

5 Key questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

Appendix A: Measuring operative temperature . . . . . . . . . . . . . . . . .37

Appendix B: Thermal comfort studies . . . . . . . . . . . . . . . . . . . . . . . . .38

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43

Further reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44

CIBSE Knowledge Series – Comfort

This publication is intended to provide information and guidance on thesubject of comfort for those responsible for the operation of buildings and forthe design, installation, commissioning, operation and maintenance of buildingservices, but is not primarily intended for use in design. It is not intended tobe exhaustive or definitive and it will be necessary for users of the guidancegiven to exercise their own professional judgment when deciding whether toabide by or depart from it. Detailed design guidance is provided in otherCIBSE publications such as CIBSE Guide A: Environmental design (2006).

Note from the publisher

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1 Introduction

Buildings are designed to meet our basic need for:

— shelter: protection from the elements

— security: safety

— comfort: warmth and light.

Many of these basic needs were originally met by a cave with a fire at theentrance to provide both security and warmth and light. Nowadays, althoughwe might expect more sophistication in delivery, and more facilities, thefundamental needs remain the same.

Once the needs for shelter and security are met, the remaining mainrequirement is for a ‘comfortable’ internal environment. Whilst this may seema simple task to achieve, in practice there are many factors to be considered inthe aim to provide comfortable conditions for the building occupants.

Thus one of the primary functions of buildings and building services systems isto create and maintain a comfortable environment. Achieving the ‘right’environment is the main goal of good building services design — whether acomfortable work or leisure environment for people or the correct operatingconditions for machinery or equipment. Electronic and process equipmentoften requires far more stringent conditions than people.

The main factors that influence comfort for people relate broadly to oursenses i.e. touch, vision, smell, hearing. Thus the design of the buildingservices systems must provide a good thermal, aural and visual environment -i.e. fresh air and warmth or cooling, no unwanted noise or odours and goodlighting. Design criteria exist for all these factors but the choice depends onmany variables including use of the space, activity level, clothing level and ageof occupants, etc. Decisions on design conditions are made harder by the factthat comfort is a very subjective response with different people havingdifferent comfort levels; so the main aim is literally to ‘keep most of thepeople happy most of the time.’

In surveys of user satisfaction within buildings* comfort issues, particularlytemperature and air freshness, are among those rated as the most importantaspects. The same studies also show that dissatisfaction with the internalenvironment, particularly the thermal environment, is widespread withcomplaints of overheating in winter and coldness in air conditioned buildingsin summer commonplace.

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Definition

Comfortable: at ease; free from want,trouble, hardship or pain; quietly happy.

Cassel Concise English Dictionary

Aim

The primary aim of building servicessystems is to create, and maintain acomfortable environment.

* For example the series of PROBE studies in Building Services journal

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All those involved in the design, specification and delivery of the internalenvironment therefore need a good appreciation of comfort requirements. It isparticularly important for building owners and users to be able to explain theirinternal environmental comfort requirements and to be aware of the constraintson what can be achieved or delivered with building services systems.

1.1 Use of this guidance

This guidance is intended to enable and assist the non-expert client, facilitiesmanager and building user to:

— understand comfort requirements

— communicate their needs and requirements to theirengineers/advisors.

It can also be used by building services engineers involved in design,installation and commissioning to facilitate discussion with their clients, andprovides students with an accessible introduction to the subject of comfort.Detailed guidance on the environmental criteria for design can be found inCIBSE Guide A, chapter 1(1).

This publication provides an introduction to the subject of comfort:

— Sections 2–4 explain the basic principles governing thermal, visual andacoustic comfort, covering key factors and the main design criteria.

— Section 5 provides guidance on the information that may be neededwhen deciding on comfort requirements.

The publication answers the following questions, which can be used to helpyou find the most relevant sections to you:

— What is thermal comfort? (section 2.1)

— What determines thermal comfort? (section 2.2)

— How does the environment affect thermal comfort? (section 2.3)

— How do ventilation and air quality affect thermal comfort? (section 2.4)

— What is the adaptive approach? (section 2.5)

— How hot is too hot? (section 2.6)

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— What are the design criteria for thermal comfort? (section 2.7)

— What can systems deliver? (section 2.8)

— What determines visual comfort? (section 3)

— What are the design criteria for thermal comfort? (section 3.2)

— What determines acoustic comfort? (section 4)

— What are the design criteria for acoustic comfort? (section 4.2)

— What questions do I need to ask? (section 5)

— What information do I need to provide? (section 5)

Finally, a selected bibliography is provided for those who want further readingon the subject.

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2 Thermal comfort

2.1 What is thermal comfort?

Thermal environments can be divided loosely into three broad categories:

— thermal comfort

— thermal discomfort

— thermal stress

Thermal comfort is where there is broad satisfaction with the thermalenvironment i.e. most people are neither too hot nor too cold. Another wayto regard this is as an absence of discomfort!

Thermal discomfort is where people start to feel uncomfortable i.e. they aretoo hot or too cold, but are not made unwell by the conditions, i.e. they donot suffer medical symptoms due to the discomfort, beyond irritability andtiredness or chills and shivering.

Thermal stress, heat stress or cold stress, is where the thermal environmentwill cause clearly defined potentially harmful medical conditions, such asdehydration or heat exhaustion in hot environments or frost bite in coldones. Respiratory problems can occur and there can also be the risk ofhypothermia or hyperthermia, where there is a fall or rise in body coretemperature which can be harmful, and could potentially prove fatal.

Other than in some extreme industrial applications, conditions withinbuildings in the UK are unlikely to cause thermal stress and therefore furtherdiscussion of this is outside the scope of this publication. However thermaldiscomfort can occur, and although this will not directly harm people it cancause other problems, such as fatigue and irritability. Work productivity canfall and there is also an increased risk of error in task activities which couldpotentially cause an accident. Thermal discomfort is therefore undesirablefrom a health and safety viewpoint.

2.2 What determines thermal comfort?

Although there are many factors to take into account, thermal comfort isfundamentally all about how people interact with their thermal environment.When people talk about feeling hot or cold, draughty or stuffy, what they arereally doing is responding to the transfer of heat from their body to thesurroundings, and to the quality of the air within the space.

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Legislation

The Workplace (Health, Safety andWelfare) Regulations 1992 state that‘During working hours, the temperaturein all workplaces inside buildings shall bereasonable’. The Approved Code ofPractice (ACOP) to these regulationsdefines a ‘reasonable temperature’ as thatwhich secures the thermal comfort ofpeople at work, without the need forspecial clothing. This is further defined asbeing met by ‘maintaining a ‘reasonable’temperature of at least 16 °C (or at least13 °C if the work involves physicaleffort)’.

In practice the CIBSE guidelines oncomfort, given in CIBSE Guide A(2), areoften taken as a good practice indicationof thermal comfort and used for designpurposes.

Definition of thermal comfort

That condition of mind which expressessatisfaction with the thermal environmentand is assessed by subjective evaluation.

ASHRAE Standard 55-2004

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The key factors are:

— Temperature: a comfortable temperature level depends on activity andclothing level. For example sitting and reading requires a highertemperature than playing an active sport such as squash; or shoppersin winter coats can require lower temperatures than the shop staffmanning the tills.

— Humidity: if there is too much moisture in the air it can feel humid anduncomfortable, whereas if there is too little the eyes, throat and skincan all feel uncomfortably dry and static electricity can build up.

— Air movement: completely still air can get very stuffy and stale, but airmoving too fast can also cause discomfort – a pleasant cooling breezein the summer can be an annoying cold draught in the winter.

— Air quality: a feeling of freshness, rather than stuffiness and a build upof odours, depends on how much fresh air is supplied and whatcontaminants are present or are produced in the space.

Our bodies produce energy by using oxygen to metabolise food, and convertit to useful forms of energy. This rate of energy production is known as themetabolic rate. Whilst some is used for maintaining body function(respiration, digestion etc) and activity, most of the energy produced is in theform of heat. Heat is therefore is produced by the body all the time, theamount depending on activity, with a base production rate of around 60 Wfor an average person i.e. the amount of heat produced when we sleep. Themore active we are the more heat is produced. For example when doingnormal office work we generate around 140 W, with this increasing to around250 W for physical activity such as dancing or gym work.

Therefore, in order to be comfortable we need to balance this heatproduction by an equal amount of heat loss from the body. If the two are notevenly balanced then we can start to feel uncomfortable, or become ill. If theloss exceeds generation we feel cold; conversely if we cannot lose heat fastenough we feel hot. If the imbalance is severe then body temperature,normally at a core temperature of around 37 °C, can rise or fall to dangerouslevels. For example too much strenuous activity in conditions where heat cannot be lost can lead to a rise in body temperature and heat stress, whereasinsufficient heat production in the body to balance heat loss can lead to adrop in body temperature, i.e. hypothermia.

Heat is lost from the body in four ways:

— by evaporation — by radiation — by convection — by conduction

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Key factors

The key factors in thermal comfort are:

� temperature

� humidity

� air movement

� air quality

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In most situations, the heat loss by conduction tends to be negligible, leavingevaporation, radiation and convection to provide the main heat loss routes.Evaporation heat loss takes place via respiration, insensible perspiration(continuous evaporation at the skin surface and from the lungs) and, ifnecessary, by the emergency route of sweating. Radiation and convectionlosses and gains take place at the skin surface, with some further convectiveheat exchange via respiration.

So, for thermal comfort we need to be in thermal balance with oursurroundings; that is, the loss of heat from our body must be equal to therate at which we generate heat, see Figure 1. We can control activity leveland clothing to some extent — increasing activity and/or putting on an extrajumper or jacket if too cold for example, or dressing lightly and sitting still inhot conditions.

We are also affected by the surrounding environment, for example if it is sunnyor if the air temperature is relatively hot or cold. In buildings the main internalenvironmental factors of temperature, humidity, air movement and air qualitydepend on the design of the building together with the design and operation ofthe building services, the use of the space and the external weather conditions.Two different temperatures are important, air temperature and radianttemperature, as these affect the different ways we lose heat.

The amount of heat the body loses by each of the different heat transferroutes varies with the conditions. For example in moderate thermalenvironments the body might typically lose around 25% of the heat loss byevaporation, 45% by radiation and 30% by convection. In well insulatedbuildings where the air and radiant temperatures are similar values then therelative heat loss could typically be around 24% by evaporation, 38% byradiation and 38% by convection.

Each of these modes of heat transfer depends on different environmentalfactors:

— air temperature affects evaporation and convection

— relative humidity affects evaporation only

— mean radiant temperature affects radiation

— air velocity affects evaporation and convection.

Although evaporative heat loss is always a loss, the body can gain as well aslose heat by radiation and convection, for example the radiant heat gain fromsitting in the sun or near an open fire or the convective gain from conditions

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Body heat production(metabolic rate – rate of work)

Heat loss or gain by evaporation,radiation and convection

=

Figure 1: Body heat balance

Thermal comfort summary

The four main environmental factors thataffect thermal comfort are:

� air temperature (ta )

� relative humidity

� mean radiant temperature (tr )

� air movement and specifically airvelocity (v)

with two further personal factorsaffecting comfort being:

� clothing level

� activity level and thereforemetabolic heat production

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where the air temperature is higher than the skin surface temperature, suchas can occur in equatorial regions (see Figure 2).

In cases where there are both convective and radiant gains rather than losses,the evaporative loss remains the only way for the body to lose heat. This iswhy, when unaccustomed to the conditions, we can find very hot and humidconditions so uncomfortable as all mechanisms of heat transfer are reduced.Further detail on the human physiology and heat transfer mechanisms isoutside the scope of this publication but can be found in a number of texts onthermal comfort (see bibliography).

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Figure 2: Body heat exchange withthe thermal environment

(a) Cool evening

(b) Sunny day

There can be radiant heatgains from warm surfaces

Body heat is lost byradiation to cool surfaces and spaces

Body heat is lost by convection and evaporation to the surrounding air

There can be convectiveheat gains if the surrounding air is warmer than skin temperature

Body heat is lost by convection and evaporation to the surrounding air

There can be directradiant heat gains

Heat gains or losses by conduction are negligible

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2.3 Key environmental factors

The four main environmental factors that affect thermal comfort are therefore:

— air temperature (ta)

— relative humidity

— mean radiant temperature (tr)

— air velocity (v)

As discussed, air quality is also relevant and this is further discussed in section2.4, but tends to be considered as a separate issue when specifying designrequirements.

Over the years there have been many efforts to come up with a comfortindex (e.g. ‘the comfy-meter reads 7 therefore everyone is comfortable’) thataccurately reflects human perceptions of comfort, including scales such globetemperature, effective temperature, corrected effective temperature,equivalent temperature etc. However these have all either omitted one orother of the key factors or have since proved flawed. Partly because everyoneis different it has proved very difficult, if not impossible, to find one singleindex that exactly matches human comfort under all possible conditions.There are some measures that do relate to predictions of comfort levels that amajority might usually find acceptable, such as predicted mean vote (PMV)(see Appendix B on thermal comfort studies for further discussion).

Therefore, for design it is necessary to specify measurable limits or ranges foreach of the environmental factors, making allowance, where possible, for anyinteractions that might occur.

2.3.1 Temperature

Air temperature is defined as the dry bulb temperature of the air in the spaceand is measured by a thermometer that is protected from any radiant heatexchanges, or not affected by them. An ordinary, fixed location, mercury-in-glass thermometer will not usually sense air temperature accurately as it canbe affected by, for example, sunshine falling on the bulb or by the heat from anearby radiator or computer etc.

Mean radiant temperature at any point in a space is a measure of the effect ofthe radiant interchanges at that point i.e. the relative effect of all the radiantheat transfers from the various solid surfaces and objects in the space, such asthe walls, ceiling, windows etc and any other radiant sources in the space such

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as heaters, lights, equipment etc. One way to envisage this radiant interchangeis to think of the relative reflections of the various objects and surfaces in asmall shiny globe, such as a Christmas tree ornament (see Figure 3). The closerit is to the radiant object, such as a hot fire, the larger the reflection (andtherefore the radiant effect) and the higher the mean radiant temperature atthat point.

Mean radiant temperature can be predicted mathematically from knowledgeof the surface temperatures in the space. It cannot be measured directly butcan be found by using a globe thermometer to determine globe temperatureand using measurements of the air temperature and air velocity at the samepoint to then determine the radiant temperature.

CIBSE suggests that the room air temperature and mean radiant temperaturecan be combined as the operative temperature, which is also used in bothInternational Standards and ASHRAE Standards.

Operative temperature (to) is commonly used as a design parameter, as itcombines the effects of air temperature, radiant temperature and, to someextent, air velocity. A full discussion and definition of operative temperature(to) is given in CIBSE Guide A, but for practical purposes it can be taken to beequivalent to the average of the air and radiant temperatures at air speeds ofaround 0.1 m/s i.e.

to = 1/2 ta + 1/2 tr

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Close to fire the relative effectof the hot radiation is large and the mean radiant temperature will be higher

Radiant source, e.g. fire

Further away the relative effectof the hot radiation is much lessand the mean radiant temperaturewill be lower

Figure 3: Mean radiant temperature

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Operative temperature approximates closely to the temperature at thecentre of a painted globe of some 40 mm diameter. A table-tennis ball is asuitable size, and may be used to construct a thermometer appropriate forindoor spaces. Appendix A describes how to make and use a suitablethermometer to assess operative temperature.

In well insulated buildings that are predominantly heated by convectivemeans, the difference between the air and the mean radiant temperatures(and hence between the air and operative temperatures) is usually small.

2.3.2 Humidity

Humidity is the term used for the amount of moisture in the air i.e. theconcentration of water vapour in the atmosphere. It is usually expressed interms of a percentage ratio of the amount of moisture in the air at aparticular condition compared to the maximum amount of moisture the air atthat same temperature and pressure can hold, so a value of 0% means thatthe air would be completely dry whereas at 100% it would be fully saturatedand any more moisture would condense out. The amount of moisture the aircan hold is temperature and pressure dependent, for example warm air canhold much more moisture than cold air, (see Figure 4). So if warm air iscooled enough you can get moisture precipitating out as condensation ordamp; for example the condensation that often occurs on the cold surface ofsingle glazed windows in winter. Equally, if air containing a certain amount ofmoisture is warmed then the humidity level will gradually fall.

Two different ratios are commonly used in building services engineering:

— relative humidity (RH)

— percentage saturation.

For comfort and design the term relative humidity is more commonly usedwhich is a ratio of water vapour pressures.

Within building services design, the other expression commonly used ispercentage saturation which is a ratio of moisture content masses. This isparticularly useful for air conditioning design as it is then easy to work outhow many grammes of water to add or remove in the air conditioning unit, by humidification or by dehumidification, to achieve the required room condition.

For most practical purposes the values of both are interchangeable fornormal occupied environments although the values can differ by as much as5% at extreme conditions, such as might be used in industrial drying.

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Figure 4: How humidity varies withtemperature

At 0 °C the air can only hold this smallamount of water vapour before it is completely saturated i.e. 100% RH

0 °C

At 21 °C the air can hold four times as much water vapour before it is completely saturated i.e. 100% RH

21 °C

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Humidity has little effect on feelings of warmth, at the moderatetemperatures found in most UK buildings, although it does affect theperceived air quality. As long as conditions are neither too dry nor too humidwe are relatively unaffected by changes in humidity level.

Relative humidities below 30% can result in shocks due to static electricity,and below about 25% can cause eyes and skin to feel dry. Levels above 80%feel very sticky and uncomfortable, and can lead to condensation and mouldgrowth on building surfaces. The air can feel very stale and stuffy at highrelative humidities.

CIBSE Guide A(1) recommends that relative humidities in the range 40–70%RH are generally acceptable.

2.3.3 Air movement

Air movement in the occupied zone, i.e. where the people are (see Figure 5),is important to comfort as too high a speed can gives rise to complaints ofdraught whereas too low a speed can reduce the air quality to a point whereit becomes stale and stuffy. Both the speed and the direction (i.e. velocity) ofthe moving air are important for comfort.

Moving air will cause a cooling effect as heat is removed from the body byconvection and evaporation. Acceptable air speeds do depend on thetemperature and direction of the moving air. If the air is warm then a higherspeed may be acceptable whereas if the air is cool then even a low speed canfeel draughty. Also people are more tolerant of air movement if the directionof the air movement varies. Generally the range of comfortable air velocitiesin the occupied zone is 0.1 to 0.3 m/s.

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Mixing zone: air supplied at high level can mix with room air and reduce in air speed before entering the occupied zone

Occupied zone: air velocities need tobe low to avoid a feeling of draughtand discomfort

Figure 5: Occupied zone

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To put this in context a typical supply velocity from a high level outlet wouldbe in the region of 3 m/s, depending on room height, which means the air hasto mix and slow down a lot before it enters the occupied zone. The design ofthe supply outlet and the direction and temperature of the supply air needscareful consideration to ensure comfort at all operating conditions. Thetemperature of the moving air will generally be somewhere between that ofthe room air and the supply air, which means that the cooling situation needsto be particularly carefully considered. Analysis of the patterns of airmovement in a space is known as room air diffusion (RAD).

The two parts of the body most susceptible to draughts are the back of theneck and the ankles. This therefore again emphasises the need to carefullyconsider the room air diffusion patterns in a space, as high level supply canpotentially cause draughts on the back of the neck for people working atdesks. Equally, low level supply can cause ankle level draughts therefore thesupply velocity needs to be very low as the air is directly entering theoccupied zone, and again careful consideration of supply temperature andRAD pattern is needed.

2.3.4 Other factors

Other environmental factors affecting thermal comfort include:

Temperature variations in the spaceThe ideal for comfort is to have ‘warm feet and cool head’ i.e. thetemperature should be warmer at foot level than at head level, literally warmfeet for comfort and cool head for clear thinking. In practice the opposite isoften the case (see section 2.8) as warm air rises, leading to stratification in aspace. If this is too great then it can feel uncomfortable, with cold feet and afeeling of stuffiness at head level. To avoid discomfort it is recommended thatthe air temperature rise between ankles and head should not exceed 3 °C.

Air and radiant temperature differencesIf the radiant temperature is above the air temperature, it will tend to give afeeling of freshness. This can occur with heating systems that have more of aradiant component such as radiant panels or radiator systems or withsunshine entering an air cooled space in summer. If the air temperature isabove the radiant temperature it can tend to feel stuffy. This can occur withheating systems that are more convective, such as warm air heating. In orderto avoid discomfort the two temperatures should not be too far apart with,ideally, the radiant temperature slightly above the air temperature.

Localised radiationExcessive radiation, particularly if it is on one side of the body only, can causediscomfort, for example if sat next to a cold window surface or next to a

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roaring fire in winter. To consider the case of the roaring fire in a cold room,one side of the body is excessively hot and the other is cold, and, although theaverage temperature may well be theoretically acceptable, in practice theimbalance causes discomfort. The same imbalance can be caused, to a lesserextent, by heated or cooled surfaces in a room such as overhead radiantheaters, overhead lighting, solar radiation through glass, cold window surfacesetc. In order to avoid discomfort large imbalances in radiant temperaturesshould be avoided. Further guidance in given in CIBSE Guide A(1), section 1.5.9.

Warm or cold floorsLocalised discomfort can be caused if the floor surface temperatures are too cold or too hot, for example if there is underfloor heating. To avoiddiscomfort it is recommended that floor surface temperatures should be inthe range 19–29 °C.

2.4 Ventilation and air quality

Fresh air for ventilation is required to both provide air for respiration and toachieve acceptable air quality. People tend to assess air quality in two ways,firstly by smell and secondly by sensitivity to irritants, such as pollen, tobaccosmoke or other pollution, by the eyes, nose and throat. As yet there are nogenerally accepted measurement criteria for air quality assessment such aswe have for warmth or humidity. However good air quality within the workplace can be achieved by ensuring that there are no significant sources ofpollutants within the space and that there is an adequate supply of clean,fresh air. As discussed in section 2.3 the room air diffusion in the space i.e.the degree of good mixing or temperature stratification with consequentstagnant areas, will directly affect the air quality.

Fresh air is required for comfort to:

— provide oxygen for respiration

— dilute carbon dioxide, produced as a by-product of respiration

— dilute contaminants produced as part of occupation, such as odours

— give a feeling of freshness.

Interestingly, the amount of fresh air required for these is approximately:

— to provide oxygen: 0.2 litre/s per person

— to dilute carbon dioxide: 1.0 litre/s per person

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— to dilute occupation contaminants: 5 litre/s per person

— to give a feeling of freshness: 10 litre/s per person

Therefore we require around 50 times more fresh air to both dilute odoursand create an acceptable fresh feeling than we do to provide oxygen.

For general occupation, where the main potential contaminants areoccupation odours, CIBSE recommends outdoor air supply rates for differenttypes of space given as an outdoor air supply rate in litre/s per person. See Table 2 in section 2.7 for examples. Detailed guidance for a wider range ofbuilding and room types is given in CIBSE Guide A(1) Table 1.5 and CIBSEGuide A section 1.7.

Spaces in which smoking is permitted should be regarded as ‘smoking rooms’,and a minimum outdoor air supply of 45 litre/s per person is suggested byCIBSE for such rooms. However it should be noted that this recommendationaims only to reduce discomfort and does not ensure health protection.

If there are other contaminants in the space, such as odours from new paintor a glued floor covering, or if there are pollutants produced as part of anindustrial process, then other requirements apply based on the need to limitthe concentration limits for pollutants to safe levels. In these cases theventilation strategy should be based on a risk assessment under the Controlof Substances Hazardous to Health Regulations 1994(2). Design guidance isgiven in CIBSE Guide B chapter 3(3).

Further discussion of air quality and health issues is given in CIBSE Guide A(1)

chapter 8.

2.5 The adaptive approach to thermal comfort

As is evident from the preceding sections the thermal interaction betweenpeople and their environment is a complex area, and has been the subject ofmuch research over the last hundred years or so. What this research hasshown is that our feelings of comfort do not just depend on humanphysiology and mechanisms of heat transfer but also on social factors and onour psychological responses to the environment.

Interestingly, some major work on thermal comfort and measurement ofenvironmental conditions was born of necessity when, during the SecondWorld War, submarine crews had to stay underwater for long periods of timeand thus literally became guinea pigs for immediate studies of the effect of heatbuild up, thermal stress, stale air etc. The majority of subsequent research onthermal comfort in buildings has taken one of two main approaches:

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— Laboratory based studies: based on experimental work carried out in aspecial laboratory or climate chamber, with the aim of relating givenspace conditions (such as temperature, humidity, and air speed), andgiven clothing and activity levels, to the likely level of occupantcomfort. PO Fanger(4) carried out much research using this approachand used the terms PMV (predicted mean vote) and PPD (predictedpercentage dissatisfied) to predict acceptable comfort conditions. (SeeAppendix B for further information.)

— Field studies: based on surveys asking building occupants about theirfeelings of comfort, with the aim of establishing how comfortexpectations vary with different climates and internal conditions. (SeeAppendix B for further information.) This has led to the adaptiveapproach to thermal comfort.

The level of thermal comfort or discomfort for both approaches is oftenexpressed in terms of the percentage of people who are happy or not happywith the conditions. However it is often impossible to achieve 100%satisfaction i.e. literally ‘you cannot please all of the people all of the time’.

The adaptive approach to comfort The adaptive approach(5) to comfort has been developed from field studies ofpeople in their daily life and aims to provide guidance that is relevant toordinary living conditions. It is based on the observation that people, givenboth the time and the opportunity, do take various actions in order to adaptto their environment and achieve thermal comfort. See CIBSE Guide A(1)

section 1.6 for further discussion on the adaptive approach and field studiesof thermal comfort.

People adapt to changed conditions in various ways, from involuntarymechanisms such as shivering or sweating to voluntary ones such as changingtheir activity or their clothing or closing a window blind. These include:

— being more active if cold to raise the metabolic rate, or converselyresting in hot conditions

— changing to warmer or cooler clothing, or adding a blanket.

— taking warm or cool drinks

— modifying the local environment e.g. by opening a window, closing ablind or switching the heating on

— changing the environment e.g. by moving out of the sunshine or movingto a different room, or by going outside or to a different building.

15

PMV and PPD

The predicted mean vote (PMV) is themean value of the votes on a seven pointcomfort scale (e.g. hot, warm, slightlywarm, neutral, slightly cool, cool andcold) of a large group of people who areall exposed to the same environment andhave the same clothing level and activity.

The term percentage persons dissatisfied(PPD) is intended to represent the way alarge number of people would judge theirfeeling of comfort within the space socould be thought of as the predictedpercentage of persons who would bedissatisfied with a particular condition.

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Obviously, in some situations such as at work, it is not always possible to takeall potential actions to improve comfort due for example to constraints ofwork dress code or lack of control such as non-openable windows.

Discomfort will occur where temperatures:

— change too fast for adaptation to take place

— are outside normally accepted limits

— are unexpected

— are outside individual control.

The concept of adaptability, whilst very obvious to many, has only recently beenincluded in comfort standards such as ASHRAE(6) and CIBSE(1). This is largelybecause the current need to reduce carbon emissions and the drive towardsmore holistic approaches has led to increased interest in naturally ventilatedbuildings rather than closely controlled air conditioned ones. For these buildingsintrinsically conditions will vary more, and ways of moderating the environmentto achieve comfort for the occupants, without resorting to complex solutionssuch as air conditioning, have become necessary. Adaptation strategies form partof this new approach, as discussed in the next section.

2.6 How hot is too hot?

Temperatures in summer in buildings that are not air conditioned will varywith the weather; however the occupants also make changes to adapt to thechanges in temperature. Certainly experience shows that people do adapt tochanged conditions over time and a temperature that may feel uncomfortablywarm in a sudden short hot spell in April may be quite acceptable duringwarm weather in July. As a result the temperature people find comfortableindoors also changes with the outdoor temperature. (See Appendix B andCIBSE Guide A section 1.6 for further discussion.)

Much of the available design guidance on comfort temperatures, includingprevious CIBSE guidance, has assumed that cooling is available. As such, thedesign guidance has not been applicable to buildings without cooling or airconditioning systems under summertime operation. For naturally ventilatedbuildings or free-running modes*, such as non-air conditioned buildingsoperating in summer, the adaptive approach to comfort indicates that higher

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What is hot?

“I call it hot, when a man sweats at rest,and excessively on moderate motion. I callit very hot, when a man, with thin or littleclothing, sweats much, though at rest. I call it excessive hot, when a man in hisshirt, at rest, sweats excessively, when allmotion is painful, and the knees feel feebleas if after a fever. I call it extreme hot,when the strength fails, a disposition tofaint comes on, a straightness is found intemples, as if a small cord was drawnround the head, the voice impaired, theskin dry, and the head seems more thanordinary large and light. This, I apprehend,denotes death at hand…”

James Bruce — Travels to Discover theSources of the Nile, in the years, 1768,1769, 1770, 1771, 1772 and 1773 (London 1804)

* Free-running can be defined as a mode of operation of a building rather than a specificbuilding type. A building is free-running when it is not using energy for heating orcooling. Thus, typically, non-air conditioned UK buildings are in the free-running mode insummer, but not in winter. When the heating or air conditioning is operational then abuilding is not free-running

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internal temperatures may be generally acceptable, as discussed above, butthere had been little to say at what point this becomes uncomfortably hot insummer for buildings in the UK.

Research shows that, during warm summer weather, 25 ºC is generally anacceptable summer indoor operative temperature in non-air conditionedoffices, with few people feeling uncomfortable. Between 25 ºC and 28 ºC anincreasing number of people may feel hot and uncomfortable. Indooroperative temperatures that stay at or over 28 ºC for long periods of the daywill result in increasing dissatisfaction for the majority of occupants.

After consultation and research, CIBSE has produced design guidance inGuide A(1) section 1.4 on peak indoor temperatures and overheating criteriafor some non-air conditioned building and room types, under normal UKsummer time temperature and humidity conditions. Further guidance on theapplication of the adaptive approach to naturally ventilated offices is given inGuide A section 1.6.

2.6.1 Summer overheating criteria

It is not only the value of the peak temperature but also the length of timethat temperatures remain high that can lead to discomfort, therefore designshould include an assessment of the risk of overheating, which may requirethermal modelling. Summer thermal performance is usually measured againsta benchmark temperature, related to the likelihood of discomfort, whichshould not be exceeded for more than a certain length of time, usuallyexpressed as a designated numbers of hours or a percentage of the annualoccupied period. When the benchmark temperature is exceeded the buildingis said to have ‘overheated’ and if this occurs for more than the designatedamount of time the building is said to suffer from ‘overheating’.

Table 1 gives guideline benchmark summer peak temperatures andoverheating criteria for three non-air conditioned building types — offices,schools and dwellings — for use in design. Further discussion and guidance isgiven in Guide A(1) section 1.4.2.

2.6.2 Good practice ways to reduce summer discomfort

During hot summers internal temperatures in non-air conditioned buildingsmay rise above the design temperature and could also rise above thebenchmark summer peak temperatures for periods of time. It then becomesthe responsibility of the building owner/operator to recognise this situationand to act to minimise the length and severity of any discomfort.

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Good design practice for non-air conditioned office buildings would normallylimit the expected occurrence of operative temperatures above 28 ºC to anagreed percentage of the annual occupied period (such as 1%, or around 25–30 hours).

Good practice ways to reduce discomfort for occupants of office buildings inhot summer conditions when indoor operative temperatures rise above 25ºCinclude:

— relaxation of formal office dress to encourage individual adaptation toconditions

— individual control over the thermal environment, where practicable,such as opening windows, the use of blinds, or moving out of sunnyareas

— flexible working so people can work at more comfortable times.

— availability of hot or cool drinks

— increased air movement; for example, the cooling effect of local fanscan be equivalent to reducing the temperature by around 2 ºC.

Indoor operative temperatures of 30 ºC or more are rarely acceptable tooffice building occupants in the UK.

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Building Benchmark summer Overheating Notes type peak temp / °C criterion

Dwellings: — living areas 28 °C 1% annual occupied

hours over 28 °C operative temp.

— bedrooms 26 °C 1% annual occupied hours over 26 °C operative temp.

Offices 28 °C 1% annual occupied hours over 28 °C operative temp.

Schools 28 °C 1% annual occupied The DfES BB87 hours over 28 °C recommends an operative temp. allowable overheating

criterion of 80 occupied hours in a year over an air temperature of 28 °C,using the TRY (Test Reference Year)

Note 1: It is reasonable to calculate thepercentage of occupied hours over a yearto reflect true hours of occupation, e.g.8 am to 6 pm, and allow for 5-, 6- or 7-dayworking as appropriate.

Note 2: It is recommended that theoverheating criteria be assessed against theCIBSE Design Summer Years (DSYs) usingthe calculation methods recommended inCIBSE Guide A chapter 5, which mayinclude thermal modelling. It is incumbentupon the designer to ensure that anysoftware used for the purpose ofpredicting overheating risk is validated forthat purpose and operated in accordancewith the QA procedures described inGuide A chapter 5.

Table 1: Benchmark summer peaktemperatures and overheatingcriteria(Taken from Table 1.8 in CIBSE Guide A.(1) Referto Guide A section 1.4 for further informationnecessary for design)

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2.7 Design criteria

Building designers should aim to provide comfortable conditions for the greatestpossible number of occupants and to minimise discomfort. This is achieved byconsidering comfort requirements and setting appropriate design criteria.

Design criteria to achieve comfort conditions in spaces are discussed and setout at the briefing stages of a project and are usually expressed in terms ofacceptable values or ranges for the key comfort criteria. For the thermalenvironment these would usually be the operative temperature and humidity,together with a fresh air supply rate. A typical initial design condition mighttherefore be written as 21 °C and 50% RH for operative temperature andrelative humidity respectively, with 10 litre/s per person of fresh air required.More often some variation is allowed i.e. 21 °C ±1 °C and 50% RH ±10%.

However, as can be seen from the preceding discussion there is much more toconsider for comfort than just these values alone. Some factors to consider are:

— acceptable comfort temperatures will differ between winter andsummer operation

— acceptable comfort temperatures will be different in naturallyventilated or non-air conditioned buildings to those with airconditioning

— relative humidities in the range 40–70 % RH are generally acceptable

— the range of comfortable air velocities in the occupied zone is generally0.1 to 0.3 m/s

— conditions will vary within a space.

It is therefore vital to discuss comfort requirements and priorities at an earlystage, and to be aware that the primary purpose of the whole of the rest ofthe system design is to achieve these requirements efficiently and effectively.However complex, innovative or expensive the system it cannot be classifieda success if it fails to achieve and maintain the conditions required by theclient or building users. However it is also important for everyone to beaware of the constraints on what can be achieved or delivered with buildingservices systems, and this is discussed in section 2.8.

Table 2 gives example winter and summer design conditions for thermalcomfort for a range of common building types. More detailed guidance for awider range of building and room types is given in CIBSE Guide A(1) Table 1.5,which also relates the design guidance to the expected clothing and metabolic

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rates of occupants to achieve a predicted percentage persons dissatisfied(PPD) of around 5%. For design purposes reference should be made to thefull table together with the associated footnotes as given in CIBSE Guide A.

The summer comfort temperatures given in Table 2 below and in Guide ATable 1.5 apply to air conditioned buildings. The adaptive approach to comfortindicates that higher temperatures may be acceptable if full air conditioning isnot present, as discussed in section 2.6. For the free-running mode (e.g. non-air conditioned buildings), Table 3 indicates acceptable values for generalsummer indoor comfort temperatures for a range of buildings. However it isessential to realise that, in normal operation, it may not be possible to achievethese values under all conditions without the provision of mechanical cooling, asin hot weather conditions internal temperatures are likely to rise above thesevalues. It is therefore necessary to analyse the risk of overheating and aim tominimise the length and severity of any discomfort, as discussed in section 2.6,with further guidance given in CIBSE Guide A(1) sections 1.4 and 1.6.

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Table 2: Recommended thermalcomfort criteria for someselected building types

(Taken from Table 1.5 in CIBSE Guide A(1)

Refer to this table for guidance for a fullerrange of building and room types, andadditional information necessary fordesign.)

Building/ Winter operative Summer operative Suggested air supplyroom type temp range °C temp range for rate l/s per person

air conditioned (unless stated buildings °C otherwise)

Dwellings

bathrooms 20–22 23–25 15 litre/s

bedrooms 17–19 23–25 0.4–1 ACH

halls, stairs 19–24 21–25 —

kitchen 17–19 21–23 60 litre/s

living rooms 22–23 23–25 0.4–1 ACH

Offices

conference/ 22–23 23–25 10board rooms

computer rooms 19–21 21–23 10

corridors 19–21 21–23 10

drawing office 19–21 21–23 10

entrance halls/lobbies 19–21 21–23 10

general office space 21–23 22–24 10

open plan 21–23 22–24 10

toilets 19–21 21–23 >5 ACH

Retail

department stores 19–21 21–23 10

small shops 19–21 21–23 10

supermarkets 19–21 21–23 10

shopping malls 12–19 21–25 10

Schools

teaching spaces 19–21 21–23 10

Note 1: ACH stands for air changes perhour

Note 2: For design purposes please referto the full table given in CIBSE Guide A(1),together with the associated footnotes.

Note 3: The summer comforttemperatures above and in CIBSE Guide ATable 1.5 apply to air conditioned buildings.Higher temperatures may be acceptable iffull air conditioning is not present, and thisis discussed further in section 2.6, withfurther guidance in CIBSE Guide A(1)

section 1.4.

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2.8 Practical issues

Establishing the required system performance criteria at the briefing stage isone of the most critical tasks in the design and it is vital that clients and theirdesigners have a thorough understanding of what conditions are required andwhat can practically be achieved. For example the difference betweenspecifying an internal condition of 21 °C±1 °C or a condition of 21 °C±2 °Ccan have a considerable impact on energy consumption, control choice andsystem performance. The closer the control the more expensive the system.If conditions can be relaxed a little and allowed to vary (within reasonablelimits) the system can be simpler and cheaper to install and to operate.

Although the design brief might give the required internal conditions asspecific values, it is important to realise that conditions, particularlytemperature and air speed, will fluctuate within a space in practice. This iscaused by a number of factors, including:

— Temperature gradient: warm air rises and cool air sinks which can leadto temperature stratification. Air at floor level can be up to 3 °C coolerthan at head level. The floor to ceiling temperature gradient with somesystems can be much greater, as shown in Figure 6.

— Localised conditions: controls are usually placed to reflect a goodindication of space temperature but there can be features such as largeareas of glazing or heat producing equipment such as a photocopierwhich can create localised cold radiation, downdraughts, solarradiation or excessive warmth.

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Building Indoor summer comfort Notestype temperature / °C

Dwellings:

— living areas 25 °C operative temperature Assuming warm summer conditions in UK

— bedrooms 23 °C operative temperature Sleep may be impaired above 24 °C

Offices 25 °C operative temperature Assuming warm summer conditions in UK

Retail 25 °C operative temperature Assuming warm summer conditions in UK

Schools 25 °C operative temperature Assuming warm summer conditions in UK

Table 3: General summer indoor comfort temperatures for non-air conditioned buildings

(Taken from Table 1.7 in CIBSE Guide A(1).Refer to this table and to CIBSE Guide Asection 1.4.2 for additional informationnecessary for design.)

Note: In normal operation it may not be possible to achieve these summercomfort temperatures under all conditionswithout the provision of mechanicalcooling, and internal temperatures may riseabove these values. It is thereforenecessary to analyse the risk of overheatingand aim to minimise the length andseverity of any discomfort. See Guide A(1)

section 1.4 for further guidance.

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— Horizontal temperature variations: as well as vertical temperaturegradients in a space there can also be horizontal temperaturevariations due for example to the localised conditions described above,or if there is furniture or partitioning for example that affects the roomair diffusion from outlets so that there is inadequate mixing or‘dumping’ of cold air.

— Time lag: many building services heating and cooling systems havesome inertia and can take a little while to respond to a control signalcalling for more heat or more cooling. During this time thetemperature can drop a little further below the set point value on thethermostat. Equally there can be an overshoot when the space is up totemperature. This will mean that the temperatures in the main part ofthe occupied zone will vary around the set point value — often by±2 °C. With the temperature gradient effect the impact over thewhole space can be even greater.

— Equipment limitations: many of the thermostats used to measure roomtemperature and control the output of heating/cooling systems senseair temperature not operative temperature. This may not be a problem in well-insulated buildings where the air and radianttemperatures are fairly close in value but can create problems in some situations.

— Building thermal response: heavyweight materials and finishes will takelonger to respond to a system input of heating or cooling thanlightweight ones. Thus the room surfaces can be relatively hot or coolwhich will affect the radiant temperature in the space and could causelocalised conditions. A slow thermal response can also exacerbate theeffect of any system time lags.

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15 20 25 15 20 25 15 20

Radiator Underfloor heating Warm air heater at high level

25

3·0

2·0

1·0

0

Room

hei

ght

/ m

Air temperatures / °C

Figure 6: Vertical air temperaturegradients for different heating types(Source CIBSE Guide A Figure 5.6)

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— System type: some systems can increase temperature stratification, asshown in Figure 6, or cause local radiant effects which can increaselocal temperature variations, for example the temperature gradientwithin a space heated by radiators can vary considerably as shownbelow in Figure 7.

Good design will of course minimise variations occurring within the space,and will carefully consider the use and layout of each room, but it isimpossible to guarantee a fixed and finite value for the internal roomconditions. As discussed earlier, people are adaptable and often do not noticeminor variations in temperature, air movement and humidity.

In the design brief it is therefore usual for a room condition to be specifiedwith some variation, such as an operative temperature of 21 °C ± 1 °C, anda humidity of 50% RH ± 10% RH. However some equipment or processescan be far more sensitive to fluctuating conditions than the occupants, andmay require closer control, which can be both more complex and moreexpensive.

23

26 °C

35 °C

1 °C

21 °C

18 °C

Occupiedzone

Vertical and horizontal temperature gradientscan vary considerably within a space

Figure 7:Typical temperature variation inspace heated by radiators(Based on diagram from BSRIA AG15/02, itselfbased on Guide A, Section 5 data)

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3 Visual comfort

In any environment it is essential that people can see well to carry out anytasks safely and comfortably, from simply moving around safely to carryingout some visually demanding activity such as museum restoration workwhere contrast and colour accuracy are essential. In order to ‘see well’ thereneeds to be sufficient light, with adequate, but not too much, brightness. Verybright sources in the field of view cause glare which can cause visualdiscomfort or disability.

Sufficient light is usually described in terms of the illuminance or the amountof light on the task, measured in lumens/m2 or lux. For example brightmoonlight has an illuminance of 0.5 lux, a typical brightly lit shop may have500 lux and sunlight outside has an illuminance of 100,000 lux. Different tasksrequire different illuminances depending on the degree of task difficulty.

The eye can adapt to a wide range of lighting conditions. For example,headlines in a newspaper can be read both under moonlight, at around 0.5lux, and in bright sunshine at around 100,000 lux. However, the eye cannotadapt to the whole of this range at one time. At night the headlights of an on-coming car will dazzle someone who has adapted to the night-time darkness,whereas on a sunny day these lights would be barely noticeable. Inside aroom daylit by large windows, conditions might allow all objects and surfacesto be viewed comfortably, but looking into the room from the outside, whenadapted to the bright daylight conditions, the windows will appear black andno internal objects or surfaces will be visible.

The ability to see degrees of detail is mostly determined by size, contrast andhow good a person’s eyesight is. For example reading newspaper textdepends on the contrast of the letters against the white background, theirsharpness and the size of the text (see Figure 8), as well as on the illuminance– small print may be readable under a bright desk light but may be illegible ina poorly lit corridor.

Figure 9 shows the general relationship between performance, illuminanceand task difficulty.

3.1 Key environmental factors

Interior lighting therefore has to provide several functions: it allows worktasks to be performed, allows people to move around in safety and can alsobe used for dramatic effect or to create a certain ambiance. However,subjective response to a space depends on more factors than task illuminancealone, as shown by the way we describe lit spaces as variously ‘bright’, ‘dull’,‘gloomy’, ‘under-lit’ and ‘well lit’.

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Lighting in a building

Lighting in a building has three purposes:

— to enable the occupant to work andmove about in safety

— to enable tasks to be performedcorrectly and at an appropriate pace

— to create a pleasing appearance

Brightness

Brightness is generally used to mean thevisual sensation associated withluminance (previously called luminosity).For example the moon has a certainluminance, but by night its brightness ishigh whereas by day its brightness is low.

Light flux

This is the rate of flow of luminousenergy and is measured in lumens (lm).Lamp performance is usually quoted interms of the lumens it emits and itsefficacy in terms of the lumens producedper watt of electrical input energy. A typical domestic 60 W incandescentlamp (light bulb) emits around 700 lm,and a 36 W fluorescent tubular lampemits around 3000 lm.

This is the amount of light reaching asurface and is measured in lumens /m2 orlux. Task illuminance is the amount oflight that people need to see well for aparticular task. For example, offices withtasks such as reading, writing andcomputer use require a task illuminanceof between 300 and 500 lux, with theappropriate illuminance depending on thetask difficulty.

Luminance is a measure of what the eyeactually sees and is related to the amountof light reflected from the surface,depending on both the surface reflectivityand the illuminance i.e. the incident lightlevel on the surface. It is the physicalmeasurement of the stimulus whichproduces the sensation of brightness. The unit is the candela/m2

Illuminance

Luminance

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Good lighting design needs to consider both the quantity and quality of light,and improvements to these can make an important contribution to improvedvisual performance.

Factors relating to quantity include:

— illuminance: the amount of light reaching a surface, i.e. the light level.

— distribution of light: luminaire type, spacing and layout; whether all athigh level or a combination of background and task lighting.

Factors relating to quality include:

— colour: both the colour of the light itself, whether warm or cool, andthe colour rendering i.e. how colours appear in that light.

— contrast: to allow task detail to be clearly seen – such as reading printor information from a computer screen.

— modelling: whether objects are perceived as three-dimensional i.e.some variation in shadow.

— glare: good lighting design should reduce or eliminate glare (see below) which can be caused by very bright light or by excessivedazzle or reflection.

Other factors that affect visual comfort are:

— non-uniformity

— veiling reflections and highlights

— shadows

— flicker.

25

Figure 8: The effect of lighting, contrast andtask size on visual performance(Source SLL Code for lighting)

Figure 9:

Visual performance with respectto task difficulty and taskilluminance(Source: GPG 272: Lighting for people, energyefficiency and architecture)

Illuminance (lux)10 100 1000 10,000

Task

per

form

ance

Difficult task

Easy task

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Glare

Bright light sources in the field of view, such as a sunlit window or a brightlamp, can cause glare both as a direct light source or by reflection in, forexample, a computer screen (see Figure 10).

Glare can have two effects:

— disability glare, where vision is impaired by excessive dazzle from abright light source or reflection such as light reflecting from a glossysurface or from water (see Figure 11)

— discomfort glare, where visual discomfort is caused by very bright lightsuch as direct sunlight or bright lamps (see Figure 12).

These two types of glare can occur simultaneously or separately.

Non-uniformity

This is where there is excessive difference between the maximum andminimum light levels so the eye has problems in adapting to the change inlight levels, for example moving indoors after being out in bright sunshine.

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Direct

Reflected

Direct

Reflected

Figure 10:

Examples of direct and reflectedglare(Source GPG 272: Lighting for people, energyefficiency and architecture)

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Veiling reflections and highlights

Veiling reflections occur when there is reflection of a light source in a shinysurface which reduces visibility by reducing luminance contrast, for examplethe reflection of a light on the glossy surface of some printed pages or on acomputer or television screen (see Figure 13). Highlights are areas ofincreased luminance in a space, sometimes used as lighting accents for effect.They may well improve the visual conditions, although excessively brighthighlights could potentially cause a glare or veiling reflection problem.

Shadows

Larger area shadows are simply a reduction in illuminance and are caused byinadequate light distribution and/or by large objects obstructing the light.Localised shadows can reduce visibility and be confusing. However someshadows can help to reveal form and show objects as three-dimensional i.e.modelling.

Flicker

Flicker is sustained instability in light output, caused by the control gear ofsome lamp types, and can cause eyestrain, headaches and fatigue.

3.2 Design criteria

Lighting design criteria are usually given in terms of a maintained illuminancefor various different building and room types (see Table 4 below forexamples). However, as discussed above, many other factors need to beconsidered as part of design in order to create a comfortable visualenvironment. These include the need to provide adequate illuminance with good colour rendering and glare control, whilst avoiding sharpshadows, sudden large changes in luminance and excessively bright andfrequent highlights.

Required lighting illuminances should always be related to the task, with thehighest levels only for the immediate task area and lower levels asappropriate for the surrounding areas and lower still for circulation areas.Consideration must also be given to the occupancy profile, for example age isrelevant to lighting requirements, with the elderly requiring higher light levels.More detailed guidance on lighting design criteria for a wider range ofbuilding and room types is given in CIBSE Guide A(1) Table 1.5, and in CIBSEGuide A section 1.8. For design purposes reference should be made to thefull table together with the associated footnotes as given in Guide A. Furtherdesign guidance is given in the Society of Light and Lighting Code for lighting(2004) and Lighting Guide LG7: Office Lighting.

27

Figure 11:

Disability glare from bright sky infront of a VDT makes the screendifficult to read.(Source SLL Code for lighting)

Figure 12: Discomfort glare from brightlights(Source SLL Code for lighting)

Figure 13:Effect of veiling reflections fromelectric lighting on a VDT screen(Source SLL Code for lighting)

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Table 4: Recommended lighting designcriteria(Taken from Table 1.5 in CIBSE Guide A. Referto this table for guidance for a fuller range ofbuilding and room types, and additionalinformation necessary for design.)

Building/room Maintained Illuminance (lux) Notestype at the appropriate

working plane or height

Dwellings

— bathrooms 150

— bedrooms 100 Study bedrooms require 150 lux at desk

— halls, stairs 100

— kitchen 150–300

— living rooms 50–300

Offices

— conference/board rooms 300–500

— computer rooms 500

— corridors 100

— drawing office 750

— entrance halls/lobbies 200

— general office space 300–500

— open plan 300–500

— toilets 200

Retail

— department stores 300 for circulation areas Note: higher lighting levels will be required at checkouts and tills and for display lighting

— small shops 300 for circulation areas

— supermarkets 400 for circulation areas

— shopping malls 50–300

Schools

— teaching spaces 300

Note: For design purposes please refer tothe full table given in CIBSE Guide A(1)

together with the associated footnotes.

Note 2: Lighting levels should beappropriate to the immediate task area

Maintained illuminance

Maintained illuminance is the averageilluminance over the reference surface atthe time maintenance has to be carriedout by replacing lamps and/or cleaningthe equipment and room surfaces

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4 Acoustic comfort (aural comfort)

The main requirement for acoustic comfort is for a sufficiently ‘quiet’environment to enable the task to be carried out comfortably and withoutdistraction, i.e. with no unwanted sounds (noise) or vibration.

Noise can affect people in different ways depending on its level, varying fromsimple annoyance to actual hearing damage. There are three main potentialproblems:

— annoyance: where the noise is noticeable and can affect concentration

— masking: where the noise effectively covers or masks another wantedsound, for example speech can become masked by road traffic ormachinery noise causing interference to speech intelligibility

— hearing damage: where the noise is loud enough to cause temporary oreven permanent hearing damage.

However an excessively quiet environment can also cause problems as somebackground noise is useful to ensure a degree of privacy.

Sound is an aural sensation caused by pressure variations in the air, producedby some source of vibration, which we ‘hear’ when these are sensed by theear. The sensitivity of the ear varies with both frequency and sound pressurelevel (see Figure 14).

— Frequency: the human hearing system responds to frequencies in therange 20 Hz to 20,000 Hz, with the precise range differing fromperson to person. We are less sensitive to low and high frequenciesthan to mid-range frequencies, and hearing ability at high frequenciestends to diminish with age.

— Sound pressure level: sound pressures detectable by the hearing systemvary from 2 × 10–5 N/m2, which is the quietest sound it is normallypossible to hear (hearing threshold), up to 200 N/m2, which can causeinstant hearing damage.

Because the sound pressure level hearing range gives a very inconvenientscale, and because the ear responds in a way that is not directly proportionalto the value of pressure, a different scale is used to measure sound level thatcan be related to our response to sounds, called the decibel (dB). Thesensitivity of the ear can be represented by the curves of equal loudnessshown in Figure 14, which have been derived by subjective experiments.As hearing response is non-linear and we are more sensitive to certain

29

Noise

Noise can be defined simply as‘unwanted sound’

Sound

Sound is a vibration or pressure wavethat moves through a suitable mediumsuch as air or structure at a frequencyand intensity that can be detected by thehuman ear.

The frequency of a sound is the numberof vibrations or pressure fluctuations persecond and is measured in hertz (Hz)

Sound pressure levels are the pressurescaused by a sound vibration and aremeasured in N/m2. Slow pressurefluctuations cause a very low sound, and rapid pressure fluctuations a highpitched sound.

Frequency

Sound pressure

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frequencies, usually a frequency weighted decibel scale is used to measuresound levels, with the most common being the A-weighting – dBA. The dBAmeasure is often used as an indicator of human subjective reactions to noiseacross the full audible frequency range. Sound levels in dBA can be measuredusing a sound level meter incorporating an A-weighting network. Where asingle figure value is quoted in dBA the behaviour of the sound at variousfrequencies has been considered using the A weighting to produce a singlefigure. This therefore contains less information about the original sound than if the values at various frequencies had been quoted. For example the A-weighting reduces the impact of low frequency sound significantly as theear is less sensitive to these frequencies.

4.1 Key environmental factors

There are two ways sound can reach us (see Figure 15):

— Airborne sound: where the sound travels mainly, but not exclusively,through the air and is heard by the ear. Sound from an external noisesource can therefore enter a building not only through open windowsbut also through any cracks and gaps in the structure. Internal noisecan carry through a space and can also be transferred through falseceiling voids and through ventilation ductwork. The amount of noisetransmitted is not directly proportional to the size of opening. Evenvery small gaps and cracks can have a large detrimental effect on theability of an element to reduce sound transmission.

— Structure-borne sound: where vibration travels through solid structure andis ‘felt’ (although we still usually interpret this as a ‘sound’), or re-radiatedon the other side into air borne sound. Causes include machinery, oranything that can cause an impact such as footsteps on hard floors.

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Figure 14: Equal loudness contours

(Source CIBSE Guide A(1) Figure 1.15)

20 50 100 500 1000

102030405060708090100

110120 Loudness

level (phon)

500010000

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Decibels (dB) are a measure of soundpressure level, using a logarithmic scaleto relate the sound pressure level to abase sound pressure level at the hearingthreshold

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CIBSE Knowledge Series – Comfort

The most effective and the most obvious way to reduce noise is to stop thenoise at source; however this is often not feasible. Other ways to reducenoise depend on the method of sound transmission, as follows.

Airborne noise reduction can be achieved by the use of mass to insulate fromthe noise, by stopping noise transmission routes, and by absorbing soundalong a transfer route:

— Mass: the greater the mass the larger the insulation provided as thiseffectively dampens the sound and stops it being transmitted. Thus asingle leaf brick wall will give substantially more insulation than a singleleaf lightweight partition. Double leaf partitions can provide enhancedsound insulation if the two leaves are sufficiently isolated.

— Completeness: air paths through any structure will allow soundtransmission, so for example even small air gaps around a window ordoor will allow external noise to enter; or a false ceiling can allownoise transfer between rooms. For good sound insulation constructionmust therefore be complete and avoid cracks and gaps

— Absorption: absorbing sound en route by the use of sound absorbentmaterials. For example a reduction in the transmission of fan noisealong a ductwork system is achieved by the use of a silencer(attenuator) in the air handling unit which absorbs some of the noisegenerated by the fan. Similarly noise transfer between rooms can bereduced by the use of acoustic baffles, acoustic linings in ducts etc.

Structure borne noise reduction is achieved by isolating the source of vibrationso that the sound cannot be transmitted. For example noisy machinery canbe mounted on anti-vibration mountings – which work in the same way as

31

Figure 15: Airborne and structure-bornesound

Sound travels throughthe structure as vibration

Airborne sound can also enter the room via a ventilation duct

Airborne sound travels through the air via an open window or other route

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car shock absorbers to dampen the transmission of vibration. Another way isby ‘fire’ breaks i.e. gaps that the vibration cannot bridge or by the use ofdifferent materials with better attenuation characteristics.

For further guidance on the reduction of noise see CIBSE Guide A(1) sections1.9 and 1.10 and CIBSE Guide B(3) chapter 5.

Within a space sound is affected by the room acoustics. The shape and formof the room and the surface finishes, hard or soft, as well as the furniture, allaffect whether sounds reflect and give a reverberant effect or are deadened.For further guidance on room acoustics see CIBSE Guide A section A1.9 andCIBSE Guide B(3) chapter 5.

4.2 Design criteria

Various criteria are used to specify acceptable sound levels for the acousticenvironment giving sound level in decibels (dB) against sound frequencies,with the two most common for building services being:

— noise rating (NR) curves

— noise criteria (NC) curves.

Noise rating (NR) curves (see Figure 16) are used by CIBSE to indicateacceptable building services noise levels for varying building and room types,as shown in Table 5. Measured values of the noise spectrum in dBA can becompared with these reference curves to check that appropriate conditionsare met, using the rule of thumb that:

NR ≈ dBA – 6. NR curves are commonly used in Europe for specifying noise levels frommechanical services in order to control the character of the noise. However,it should be noted that NR is not recognised by the International StandardsOrganisation or similar standardisation bodies.

Noise criteria (NC) curves are similar to NR but less stringent at high frequenciesand more stringent at low frequencies. NR and NC curves are very close atmiddle frequencies and, as long as there are no spectrum irregularities at lowand high frequencies, they may be regarded as reasonably interchangeable.

Table 5 below gives some typical design NR values to indicate acceptablenoise levels for varying building and room types. More detailed guidance onacoustic design criteria for a wider range of building and room types is givenin CIBSE Guide A(1) Table 1.5, and in CIBSE Guide A section 1.9. For designpurposes reference should be made to the full table together with theassociated footnotes as given in CIBSE Guide A(1).

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8 k0 16 31·5 63 125 250 500 1 k 2 k 4 k

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ave-

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sou

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NR65

NR55

NR45

NR35

NR25

NR15

Figure 16: Noise rating (NR) curves(source: CIBSE Guide A(1) Figure 1.17)

Table 5: Recommended acoustic designcriteria(Taken from Table 1.5 in CIBSE Guide A(1). Referto this table for guidance for a fuller range ofbuilding and room types, and additionalinformation necessary for design.)

Building/room type Noise rating (NR)

Dwellings:

— bathrooms —— bedrooms 25— halls, stairs —— kitchen 40–45— living rooms 30

Offices:

— conference/board rooms 25–30— computer rooms 35–45— corridors 40— drawing office 35–45— entrance halls/lobbies 35–40— general office space 35— open plan 35— toilets 35–45

Retail:

— department stores 35–40— small shops 35–40— supermarkets 40–45— shopping malls 40–50

Schools:

— teaching spaces 25–35Note: For design purposes please refer tothe full table given in CIBSE Guide A,together with the associated footnotes.

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5 Key questions

Key questions to consider, in advance, in order to provide information toyour consultants, or to discuss with them, are:

Comfort criteria

Q. What factors are important to you and to the building users in order toachieve the required comfortable and productive environment?

Given that this is the fundamental outcome required from the design processthen it is important to get it as ‘right’ as possible. User satisfaction surveys ofthe current environment related to the tasks required can provide muchuseful data to inform the design brief for a new building. Involving the finalusers in the consultation process also provides useful information on designpriorities and key issues.

Building users

Q. Who will be the main users of the building?

Some user populations such as the young, elderly or infirm may requiredifferent comfort conditions – such as higher lighting levels, or warmerinternal temperatures.

Type of building

Q. Is the intent to design for a naturally ventilated rather than a highly servicedbuilding?

If the initial approach is to consider a building that will require less in the wayof complex services such as air conditioning then this has fundamentalimplications for both the building design and for the internal comfortconditions that will be achievable in the building. The decision to let internalconditions vary within wider limits, and leave some uncontrolled, is afundamental one and will need to be considered as part of the main decisionprocess (see below).

The acceptable variation in internal space conditions versus the levelof control required

Q. How happy are you to let the internal environmental conditions vary?

Are conditions that will vary within a space during the course of a day andover the year acceptable, and to what degree? The tighter the level of

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control the more expensive the system, for example ±0.5 °C is a lot moreexpensive than ±1 °C or even ±1.5 °C for control of internal spaces. It isnormally acceptable to allow space conditions to float, commonly by 2-3 °C,although a greater variation is often acceptable. Allowing seasonal variations,for example higher temperatures in summer, can also provide acceptableconditions and energy savings.

Is humidity control required at all? If it is required for occupation, quite largevariations in humidity are often acceptable, as very few buildings requirecontrol to within ±5% relative humidity. Occupants will normally tolerate arelative humidity range between 40–70%., although in winter somehumidification may be required to achieve this when the outside air is verycold to ensure spaces do not get too dry and cause discomfort such as dryeyes and throat.

Consider the use of the space; whether the primary users are people orequipment and the consequences of temperature or humidity variation —some electronic equipment or industrial processes can be more sensitive thanpeople. Consider whether you want to link the operation of the lighting tothe availability of daylight and/or the pattern of occupancy.

Adaptation to conditions

Q. Are you willing to make provision to allow adaptation to changingenvironmental conditions, particularly in summer?

Allowing some flexibility for occupants to adapt to hotter conditions canimprove individual levels of comfort and increase satisfaction with internalenvironmental conditions, particularly in naturally ventilated or non-airconditioned buildings. This could include flexible working hours, somerelaxation in formal office dress, provision of hot and cool drinks, localisedfans etc.

Occupant control

Q. How much do you want the occupants to be able to vary their local conditions?

Increased occupant control can give improved occupant satisfaction with theinternal environmental conditions, particularly in naturally ventilated or non-air conditioned buildings. Localised control can be as simple as openablewindows, task lighting, local desk fans, adjustable window blinds and/orthermostatic radiator valves or it can be far more complex with individuallyswitched lights and dimmers and localised sensing and control of some typesof air conditioning and heating systems. This will require adequate plantzoning and can mean more controls are required. Where local control is

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provided for any system, such as manual switching or overrides etc,accessibility and understanding of function are both important and need to beconsidered.

Use of the building

Q. Are there any areas in the building that require different conditions? Are theresome areas with different hours of occupancy, or with different requirements, suchas a computer room?

The fact that some areas may require closer control of conditions andnecessitate more complex systems does not necessarily mean that thisapproach is needed for the whole building. It is important to consider thediffering needs of different areas and different occupants rather than go for a‘one size fits all’ approach. For example the comfort needs of staff manningthe information desk in an out-of-town retail ‘shed’ who have to stay in afixed location are very different from those of the transient customers whocan move location and thus move away from hot spots or draughty areas.These issues impact on the zoning strategy for the building which needs to beconsidered at a very early stage of the design process. (Further informationon controls issues can be found in CIBSE Knowledge Series KS4:Understanding controls.)

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CIBSE Knowledge Series – Comfort

Appendix A: Measuring operative temperature

An ordinary thermometer, liquid-in-glass or digital, is not suitable formeasuring operative temperature if the radiant temperature differs greatlyfrom the air temperature.

The 40mm globe thermometer (see Figure 17) is an instrument thatcombines the effects of air and radiant temperature in a similar way to theresponse of a human subject. It is essentially an integrating sphere (made ofmetal or plastic) whose temperature will approximate the operativetemperature.

Spheres of various diameters have been used for globe thermometers(7) inthe past, but it has been estimated that the optimum diameter for the sphereof such a thermometer to sense operative temperature to be about 40 mm(similar to that of a table tennis ball). The surface of the sphere should bepainted grey or black to approximate the reflectivity of the clothed humanbody to any diffuse solar radiation reflected from the room surfaces.

A suitable thermometer can be made by inserting a temperature sensor(electronic or liquid-in-glass) into a suitable 40 mm sphere (such as a tabletennis ball), with a grey or black-painted surface. The sensor should be at thecentre of the sphere. The thermometer should fit closely through the sphere,to prevent the exchange of air between its interior and the room. Thetemperature measured at the centre will approximate the mean temperatureof the enclosing sphere. Depending on the thermal capacity of the sphere andof the sensor itself, the instrument will take some time to settle. This meansthat from 5 to 20 minutes may need to elapse before taking the final reading.

To assess the operative temperature of a space several readings of thethermometer should be taken, in places representative of the occupied area— such as on the working plane — but out of direct sunlight. Thethermometer should be suspended or clamped, and not held in the hand.Each time the thermometer is moved it needs time to stabilise, so it may beuseful to have two or more identical thermometers, allowing multiplereadings to be taken in different locations in the space over a period of 30minutes or so. This will be particularly important if the temperature ischanging. The operative temperature for the space can be taken as theaverage of the readings.

37

Temperaturesensor shouldbe at centre

of sphere

40 mm

Figure 17: 40mm globe thermometer

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Appendix B: Thermal comfort studies

The thermal interaction between people and their environment is highlycomplex and has been the subject of a great deal of study, involving not only astudy of human physiology and mechanisms of heat transfer but also a study ofour psychological responses to the environment and consideration of the socialfactors which can also determine the way we react to the environment(8,9).

Some initial work on thermal indices was carried out during the 1920s, withsome major studies by Bedford(10) in 1936 and continuing over the next 20years. The majority of subsequent research on thermal comfort in buildingshas taken one of two main approaches:

— laboratory based studies: based on experimental work carried out in aspecial laboratory or climate chamber

— field studies: based on surveys in the field asking people about theirfeelings of comfort.

B1 Laboratory studies

In analytical laboratory-based studies the conditions are controlled, forexample by using a climate chamber. A climate chamber is in effect alaboratory room where the environmental conditions such as temperature,humidity and air velocity can be accurately controlled and set to specificcombinations.

People in the chamber are monitored to measure factors such as skintemperature, metabolic rate and sweat rate at different combinations ofenvironmental conditions, and with different specific clothing levels, with theinsulation value of the clothing known. The aim is to find a specificrelationship for thermal comfort that relates metabolic rate, clothing level andenvironmental conditions.

B2 Field studies

In empirical field studies the conditions are left to vary as they normallywould and the people carry out their normal activities, dressed as theychoose. People are asked to rate their subjective feelings of thermal comforton a seven-point descriptive scale such as the ASHRAE or the Bedford scales,see Table 6. The researcher then measures the environmental conditions atthe time of the survey, such as temperature, humidity etc. and relates theseto the subjects’ feeling of warmth to find any relationship. Over a number ofsurveys the aim is to find a link between certain combinations of theenvironmental variables and the responses gathered.

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ASHRAE Bedford scalethermal sensation scale(6)

+3 Hot Much too warm

+2 Warm Too warm

+1 Slightly warm Comfortably warm

0 Neutral Comfortable neither warm nor cool

–1 Slightly cool Comfortably cool

–2 Cool Too cool

–3 Cold Much too cool

Table 6 Comfort scales

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CIBSE Knowledge Series – Comfort

These two research approaches have led to two different approaches tospecifying comfort conditions:

— deterministic methods: which relate given space conditions, e.g. interms of temperature, humidity, and air speed, and given clothing andactivity levels, to the likely level of occupant comfort

— adaptive methods: which are based on the outcome of occupancysurveys and aim to capture the variations in comfort expectations withdifferent climates.

The level of thermal comfort or discomfort in both types of model is oftenexpressed in terms of the percentage of people who are happy or not happywith the conditions. However it is often impossible to achieve 100%satisfaction i.e. literally ‘you cannot please all of the people all of the time’.

B3 Deterministic methods

Fanger(4) used deterministic methods to develop comfort temperaturethresholds, and these form the basis of the International Standard for comfortin office spaces(11). Fanger uses two terms to predict acceptable comfortconditions: PMV (predicted mean vote) and PPD (predicted percentagedissatisfied). The PMV is the mean value of the votes on a comfort scale, suchas that given in Table 6, of a large group of people who are all exposed to thesame environment and have the same clothing level and activity. The termPPD is intended to represent the way a large number of people would judgetheir feeling of comfort within the space so could be thought of as thepredicted percentage of persons who would be dissatisfied with a particularcondition. PMV and PPD can be related such that a PMV of ±0.5 (where +1is slightly warm and –1 is slightly cool) relates to a PPD of 10% i.e. around10% will be dissatisfied. (See CIBSE Guide A(1) section 1.3 for furtherdiscussion of the application of this in practice.)

The graph, Figure 18, overleaf relates the predicted percentage of personsdissatisfied against indoor temperatures for different clothing levels, at a fixedhumidity level and with low air movement.

Only at the extremes of the graph in Figure 18 would people, on average, saythey are ‘hot’ or ‘cold’, i.e. at the extremes of the ASHRAE or Bedford scales.Furthermore at the level of ‘9 out of 10 satisfied’, people would be saying thingslike ‘I am pretty comfortable’ while when ‘8 out of 10 satisfied’ the feeling wouldbe one of ‘slightly cool’ or ‘slightly warm’, but broadly comfortable.

This graph also illustrates that where people are able to adjust their clothingto adapt to conditions then they can be reasonably comfortable over a wide

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range of temperatures. As such this provides a link between the deterministicresearch done by Fanger and the adaptive approach of other researchers.

B4 Adaptive methods

The adaptive approach(13) to comfort has been developed from field studiesof people in their daily life and aims to provide guidance that is relevant toordinary living conditions.

Unlike the deterministic approach the adaptive approach does not requireknowledge of the clothing level and the metabolic rate of occupants in orderto establish the temperature required for thermal comfort, but takes a morebehavioural approach. It is based on the observation that people, given boththe time and the opportunity, do take various actions in order to adapt totheir environment and achieve thermal comfort. See CIBSE Guide A(1) section1.6 for further discussion on the adaptive approach and field studies ofthermal comfort.

People adapt to changed conditions in various ways, from involuntarymechanisms such as shivering or sweating to voluntary ones such as changingtheir activity or their clothing or closing a window blind. Obviously, in somesituations such as at work, it is not always possible to take all potential actionsto improve comfort due for example to constraints of work dress code orlack of control such as non-openable windows.

The concept of adaptability, whilst very obvious to many, has only recentlybeen included in comfort standards such as ASHRAE(6) and CIBSE(1). This islargely because the current need to reduce carbon emissions and the drivetowards more holistic approaches has led to increased interest in naturallyventilated buildings rather than closely controlled air conditioned ones. Forthese buildings intrinsically conditions will vary more, and ways of moderatingthe environment to achieve comfort for the occupants, without resorting tocomplex solutions such as air conditioning, have become necessary.Adaptation strategies form part of this new approach.

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Figure 18: Deterministic comfort model(after Fanger(11)) — Effect ofclothing level on comforttemperatures(source: CIBSE TM36 Figure 3.3(15))

20 3028262422

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No jacket

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8 out of 10satisfied

9 out of 10 satisfied

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Temperatures in summer in buildings that are not air conditioned will varywith the weather; however the occupants also make changes to adapt to thechanges in temperature. Certainly experience shows that people do adapt tochanged conditions over time and as a result the temperature people findcomfortable indoors also changes with the outdoor temperature.

Guidance on comfortable indoor temperatures for naturally ventilatedbuildings may therefore be related to the outdoor temperature. Two slightlydifferent approaches have been used for this. In the USA the relationshipbetween indoor comfort and outdoor temperature has usually beenexpressed in terms of the average monthly outdoor temperature(6,12,13). In the UK a running mean of outdoor temperature is used as research(14)

shows that UK weather can give considerable variations of outdoortemperature at much shorter than monthly intervals. As adaptive theorysuggests that people respond and adapt on the basis of their thermalexperience, with more recent experience being more important, a runningmean of outdoor temperatures, weighted according to their distance in thepast, is recommended as more appropriate than a monthly mean. Figures 19and 20 below show both approaches.

Figure 19 shows the relationship between indoor comfort temperature andaverage monthly external temperature as given in ASHRAE Standard 55-2004(6) Comfort thresholds for both too warm and too cool are shown forlevels of 10% and 20% PPD i.e. 90% and 80% satisfied.

Figure 20 shows the relationship between indoor comfort temperatures foroffices and the outdoor running mean temperature as given for the UK inCIBSE Guide A(1) section 1.6. Bands of comfort temperatures are shown withthe lines giving the upper and lower limits for the indoor temperature toavoid a rise in discomfort.

41

0 4035302520

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9 out of 10 satisfied

15105

Indo

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omfo

rt t

empe

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re, T

/ °C

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3432302826242220181614

Figure 19:Adaptive comfort model (afterASHRAE(6))(source: CIBSE TM36 Figure 3.1(15))

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The outdoor weighted running mean temperature basically considers dailymean temperatures over the past week or two but gives more emphasis tothe recent temperatures over the past few days. So if there has been arecent hot spell that will have more effect than the cooler temperaturesearlier in the month giving a higher value than a straight average would do. Inthe UK the running mean outdoor temperature rarely exceeds 20 °C, givingan upper band limit of around 27.5 °C. For further discussion and relevantcalculation approaches for this see CIBSE Guide A(1) section 1.6.4.1.

Looking at both the deterministic approach and the adaptive approach it canbe seen that there is broad agreement that, provided they are dressedappropriately, a working environment with temperatures in the range ofaround 20 °C to 28 °C will be broadly acceptable to most people. Thedegree of comfort within these bands is affected by other factors such as theamount of radiant heat from the sun and surrounding surfaces, air speed, airquality and humidity, as discussed in sections 2.2 and 2.3.

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Indo

or li

mit

ing

tem

pera

ture

/ °C

2520151050Outdoor running mean temperature / °C

30

28

26

24

22

20

18

Free-running upper limitFree-running lower limitHeated or cooled upper limitHeated or cooled lower limit

Figure 20:Bands of comfort temperatures in officesrelated to the running mean temperature(source: CIBSE Guide A(1) Figure 1.9)

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References

1 Environmental design CIBSE Guide A (London: Chartered Institution of BuildingServices Engineers) (2006)

2 Control of Substances Hazardous to Health Regulations 1994 (COSHH) (London:The Stationary Office) (1994)

3 Heating, ventilating, air conditioning and refrigeration CIBSE Guide B (London:Chartered Institution of Building Services Engineers) (2005)

4 Fanger PO Thermal comfort: Analysis and applications in environmental engineering(McGraw Hill) (1970)

5 Humphreys M A and Nicol J F Understanding the adaptive approach to thermalcomfort ASHRAE Trans. 104(1) 991–1004)

6 Thermal environmental conditions for human occupancy ASHRAE Standard 55-2004(Atlanta, USA: American Society of Heating, Refrigerating and air conditioningEngineers) (2004)

7 Humphreys M A The Optimum diameter for a globe thermometer for use indoorsAnn. Occupational Hygiene 20 (2) 135–140)

8 Leaman A and Bordass B Comfort and Complexity: Unmanageable Bedfellows? Proc.Workplace Comfort Forum, 18-19 May 1995, London (1995)

9 Nicol F Thermal comfort: a handbook of field studies toward an adaptive model(London: University of East London) (1993)

10 Bedford T The warmth factor in comfort at work (London: HMSO) (1936)

11 ISO 7730 Moderate thermal environments. Determination of the PMV and PPD indicesand specification of the conditions for thermal comfort (Geneva: International StandardsOrganisation) (1994)

12 de Dear R and Brager G Thermal Comfort in Naturally Ventilated BuildingsRevisions to ASHRAE Standard 55 Energy and Buildings 34 (6) 549–561) (2002)

13 Brager and De Dear Thermal adaptation in the built environment Energy andBuildings (1998)

14 McCartney K J and Nicol J F Developing an Adaptive Control Algorithm for Europe:Results of the SCATs Project Energy and Buildings 34 (6) 623–635) (2002)

15 Climate change and the internal environment CIBSE TM36 (London: CharteredInstitution of Building Services Engineers) (2005)

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Further reading

Environmental design CIBSE Guide A (London: Chartered Institution of Building ServicesEngineers) (2006)

Fanger PO Thermal comfort: Analysis and applications in environmental engineering (McGrawHill) (1970)

Humphreys M Thermal comfort temperatures and the habits of hobbits in Nicol F,Humphreys M, Sykes O and Roaf S Standards for thermal comfort (London: Spon) (1995)

Oseland N and Humphreys M Thermal comfort: Past, present and future (Watford: BuildingResearch Establishment) (1993)

Jones WP Air Conditioning Engineering ch. 4 (Butterworth Heinemann) (2001)

Mulcom A teaching package about buildings and comfort can be downloaded from:www.learn.londonmet.ac.uk/packages/mulcom/index.html

Brager G and de Dear R Thermal adaptation in the built environment Energy and Buildings(1998)

The illustrated guide to mechanical building services BSRIA AG 15/2002 (Bracknell: BuildingServices Research and Information Association) (2002)

Bedford T Basic principles of ventilation and heating (London: HK Lewis) (1964)

Lawrence Race G Understanding Controls CIBSE Knowledge Series KS3 (London:Chartered Institution of Building Services Engineers) (2005)

Lighting for people, energy efficiency and architecture GPG 272 (The Carbon Trust)(www.thecarbontrust.co.uk)

Code for lighting (London: Society for Light and Lighting) (2004)

Office Lighting SLL Lighting Guide LG7 (London: Society for Light and Lighting) (2005)

CIBSE Knowledge Series – Comfort44

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CONTACT US AT:

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Membership Enquiries: 020 8772 3650Events: 020 8772 3660General Enquiries: 020 8675 5211

General Info Email: [email protected]: www.cibse.org

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