Standards for Thermal Comfort

Indoor air temperature standards for the 21st century

M. Humphreys, F. Nicol, S. Roaf, O. Sykes

• Publisher: Routledge
• Year: 2015
• eBook ISBN: 9781135273507
• Topic: Architecture
• Subtopic: Architecture Methods & Materials

Current Standards for Indoor Air Temperature are inappropriate in many regions of the world. This forces designers to use highly serviced buildings to achieve air temperatures that accord with the standards to the detriment of the local and global environment. Standards for Thermal Comfort brings together contributions from around the world, reflecting new approaches to the setting of standards which can apply to all climates and cultures.

Session 1

1 Thermal Comfort Temperatures and the Habits of Hobbits

Comfort temperatures and climate

M.A. HUMPHREYS
Dunstable, UK

Abstract

Thermal comfort temperatures can be established from field study data without using thermal physiology or the theory of heat exchange. The shape of the ASHRAE standard and the ISO 7730 thermal comfort standard are discussed with reference to human behaviour, building design and energy consumption. A new international thermal comfort guideline should be developed, based on the Adaptive principle of thermal comfort and related to the climate. Research needed fully to implement such a guideline is suggested.

Climate, field-studies, temperature- standards, thermal comfort.

1 Introduction

Suppose we were visiting Middle Earth and encountered Hobbits [1]. Being interested in thermal comfort, our curiosity would certainly lead us to enquire what thermal environments were best for Hobbits. Perhaps we would first enquire into the climate in those parts of Middle Earth where Hobbits lived and thrived. Next we might notice that although the Hobbits spent some time outdoors in contented contemplation, they spent much more time in their holes. So if the opportunity arose, we would measure the thermal conditions inside a sample of occupied Hobbit holes. We would observe which rooms they most frequented, and thereby obtain a more accurate knowledge of their preferences. We might notice in what circumstances Hobbits opened or closed doors and windows or stoked up the fire to control the hole-temperature. We would notice what the Hobbits chose to wear from their enormous stock of clothing - a choice related to, among other things, their desire for comfort. Finally, if we had learned a smattering of the Hobbit-tongue, we would ask whether theirs was a good hole, or whether really they might prefer one which was warmer or cooler.

In the course of our enquiries we would have learned to know a good Hobbit hole when we saw one, and we would also have learned a good deal about the preferences and adaptive strategies of Hobbits in their quest for comfort. So, if some time later, we were told to expect a visit from a party of Hobbits, we would know how to provide accommodation comfortable for them. Various strategies suggest themselves:

1. we could construct a good traditional Hobbit hole for them, appropriate to the climate, having all the usual facilities for thermal control, and well-stocked with Hobbit-clothes. Then we could trust the Hobbits to make themselves comfortable;
2. we might (unless the Hobbits were on a holiday-visit) provide them with individually adjustable environment-control work-stations, capable of providing the range of the thermal environments we knew Hobbits liked best. Then we could trust them to adjust the work stations to suit themselves. Whether they would aim for maximum comfort or for maximum task-efficiency is a question we could ask Dr Wyon [2];
3. alternatively, we could provide a thermal environment for the visiting party, regulated according to an algorithm which estimated the best conditions for Hobbits, according to time of day and season of year, based on our observations when visiting Middle Earth.

So, either we give control to the Hobbits, knowing that the hole or the hardware can provide anything in the range Hobbits like best, or (perhaps less satisfactorily since Hobbits prefer to control their own thermal environment), we give the Hobbits the precise temperatures which our experience has shown they like best.

Notice that our observations from Middle Earth were in principle more than sufficient to enable us to specify and provide comfort conditions for our guests. Observation of the habits and preferences of the Hobbits and the measurement of their environments was sufficient to enable us to understand how Hobbits got comfortable, and to:

1. produce a 'deemed to satisfy' design for building thermally acceptable Hobbit holes;
2. identify the range of environmental adjustment which a Hobbit might need;
3. suggest the best daily and seasonal temperature profile for spaces occupied by Hobbits.

No invasive measurements or tiresome experimental routines were necessary. An enquiry about comfort and the concurrent measurement of the thermal environment yielded all that we needed to know. This simplicity makes field study work a practical method for investigating conditions for thermal comfort.

It is interesting to notice what we did not need to know:

1. We did not need to know anything at all about the thermal physiology of Hobbits, such as the diurnal cycle of their body-temperature, the metabolic heat production of their various activities, whether they could sweat or shiver or pant, or whether the Dubois relation between height, weight and skin surface-area held good for Hobbits;
2. we did not need to know anything about the heat exchange between Hobbit-skin and the hole, such as the surface heat-transfer coefficients by convection or by radiation, the mean skin temperature and at what sites it is best measured, the thermal insulation of their colourful clothing-ensembles, or the vapour permeability of their clothing materials.

It would be fascinating to know about these things, and thermal comfort researchers whose original education was, like mine, in the physical sciences would only with great difficulty be able to restrain their curiosity. Such knowledge would help to explain quantitatively the thermal balance of Hobbits, and would give us a theoretical explanation of their comfort conditions, and might be useful in identifying potentially dangerous environments, but it would not be needed to enable us to provide comfortable apartments for our Hobbits. This is not surprising if we recall that achieving thermal comfort pre-dates by thousands of years the development of the theory of heat exchange.

2 What is the Adaptive approach to thermal comfort?

This imaginary project illustrates what has become known as the Adaptive approach to thermal comfort. The Adaptive approach notices that people use numerous strategies to achieve thermal comfort. They are not inert recipients of the environment, but interact with it to optimise their conditions. Among the means used are:

1. the choice of areas of the globe suitable for habitation;
2. the choice of the building site (for example, shelter from wind, shade from trees);
3. the choice of design and construction of the building (for example, shape, orientation, thermal capacity, glazed area, thermal insulation);
4. the choice of heating or cooling systems, simple or sophisticated;
5. the use of controls (thermostats, switches, valves, openable windows, blinds, ceiling fans);
6. the choice of clothing suitable to the climate, season, indoor temperature and social needs;
7. the operation of sometimes unconscious changes of posture and activity, and of any physiological acclimatisation there may be to the season of the year.

The Adaptive model notices that, provided there are adequate possibilities for selection and adjustment, people will make themselves comfortable if they wish. Discomfort is caused by excessive constraints being placed on these processes of choice and adjustment, rather than by the temperature itself, except in extreme conditions. Thermal comfort, then, is not to be seen primarily as a matter of the physiology of heat regulation and the science of clothing, but rather as a wide-ranging and intelligent behavioural response to climate. It follows that comfort-temperatures are flexible rather than fixed, and may be more conveniently specified by climate and culture than by physics and physiology.

3 Some current standards and their effects

3.1 The ASHRAE Standard

Some current Guidelines give a recommended winter indoor temperature and a recommended summer indoor temperature, with a 'comfort' zone around each of these temperatures. An example of this type is the ASHRAE Standard [3,41.

This formulation gives the impression that comfort-zones are stable in time and space. However, it is well known that comfort-temperatures have changed markedly during the last hundred years. Also, the distinction between winter and summer is not straightforward. 'Winter' and 'summer' mean different things in different places. Winter in New York is different from winter in New Delhi or New Zealand, so why should the same comfort zone apply? Also, the climatic difference between winter and summer depends upon the distance from the equator and the distance from an ocean. Should not a Standard allow for this?

3.2 The ISO Standard

The ISO Standard [5] offers greater flexibility, since it is a standard method rather than a standard environment. It consists of a heat-exchange calculation, Fanger's 'comfort equation' [6]. To obtain comfort-temperatures from the equation it is necessary to supply, from tabulated information, estimates of the clothing-insulation and the rate of metabolic heat-production of the people. In practice an unchanging metabolic rate is assumed for a particular activity, independent of the climate or the season, while different levels of clothing insulation are assumed - perhaps one 'Clo' in winter and half a 'Clo' in summer.

The suitability of the resulting comfort temperatures depends on the correctness of Fanger's equation, and on the accuracy of the values for the clothing insulation and the metabolic rate which were entered into the equation. The Fanger comfort equation incorporates values for skin temperature and for sweat production which were ascertained from steady-state experiments in climate chambers, a procedure which by no means ensures validity in everyday circumstances. The equation has not been consistently successful in estimating comfort temperatures for normal living, particularly if conditions differ sharply from those from which the comfort equation was derived [7,8], Some of these discrepancies appear to be systematic, and it seems that the straightforward application of the Fanger equation underestimates human adaptability to indoor climates world-wide by about 50% [8,9].

3.3 Thermal comfort Standards and building design

Present guidelines hamper good thermal design of buildings in a number of ways:

3.3.1 Rigid limits lead to needless cooling

A rigid maximum indoor temperature for summer tends to force the provision of cooling plant when it may in practice not be necessary for thermal comfort. The designer sees that provision of cooling plant is the only way to guarantee that the temperature will not exceed the level given in the Standard, so cooling equipment is specified.

Field-study thermal comfort research in every-day conditions has demonstrated that the upper limit is not rigid but flexible. There exists a large body of data from field-study research which demonstrates that this is so. It was correlated some years ago in the UK at the Building Research Establishment [9,10,11], and has since been supplemented by other data [8], These studies demonstrate an empirical link between the outdoor temperature and the desired indoor temperature [10]. When the outdoor temperature rises, then the indoor temperature for comfort also rises.

3.3.2 Rigid limits inhibit traditional design

Present Guidelines may prevent the construction of buildings to designs which have been found to be very comfortable by previous generations. Sometimes Standards originating in Europe or the USA are assumed to be valid for all climates and cultures. When this assumption is made, traditional designs which have performed well for centuries may be discarded because one cannot guarantee that they will never 'overheat' when measured against these foreign Standards. It is imperative therefore that guidelines make appropriate allowance for climate and culture. Such a need for local standards has recently been demonstrated for Pakistan; field-studies show that people are comfortable well beyond the limits of the ASHRAE comfort envelope [12,13], We look forward to Gul Najam Jamy telling us the implications of this finding [14].

3.3.3 Present Standards and inappropriate design

Once the decision has been made to provide air-conditioning, the need for careful climatological and thermal design becomes less pressing. It is possible to build highly glazed lightweight structures in hot climates, and make them habitable by means of massive cooling, with its consequent high energy consumption.

3.3.4 A fresh look at Standards

So the time is ripe for a fresh look at temperature standards for thermal comfort in buildings. The revision needs to go beyond the mere adjustment of numbers. What is needed is a change in the 'philosophy' underlying the standards. The Adaptive, people-centred way of regarding thermal comfort suggests that it would be advantageous to re-formulate temperature standards for buildings, so that they reflect the empirical relation between climate and thermal comfort, and make due allowance for human adaptability.

4 What might new guidelines look like?

4.1 Indoor temperatures, comfort temperatures and climate

What happens if we apply the 'Hobbit' method to humans? Fortunately many data have already been collected from many different countries and in different seasons. First we observe that humans have a wide range of habitat, and live in arctic, sub-arctic, temperate, tropical, equatorial or arid regions. Excluding the unusual conditions of polar research, we have thermal comfort field studies from populations living in regions where the outdoor monthly mean temperature was as low as -24 °C, and where it was as high as +33 °C. However, in many climates humans spend most of their time in cleverly constructed shelters, and so if we wish to know what conditions they like...

Table of contents

• Cover
• Half Title
• Title
• Copyright
• Contents
• List of participants
• Preface
• Introduction
• SESSION 1
• SESSION 2
• SESSION 3
• SESSION 4
• SESSION 5
• POSTER PRESENTATIONS
• Index