Table Of ContentThermal and Acoustic Insulation
R. M. E. Diamant, MSc, DipChemE, MInstE, CEng
Butterworths
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First published 1986
©R. M.E.Diamant, 1986
British Library Cataloguing in Publication Data
Diamant, R. M. E.
Thermal and acoustic insulation
1. Insulation (Heat)
I. Title
693.8'3 TH1715
ISBN0-408-01394-X
Library of Congress Cataloging in Publication Data
Diamant, R. M. E. (Rudolph Maximilian Eugen), 1925-
Thermal and acoustic insulation
Includes index.
1. Insulation (Heat) 2. Soundproofing. I. Title.
TH1715.D45 1986 593.8'3 86-4224
ISBN 0-408-01394-X
Photoset by Butterworths Litho Preparation Department
Printed in Great Britain at the University Press, Cambridge
Preface
As an impecunious young man, I felt the cold very badly. Living in
uninsulated and draughty 'digs', I must have suffered from permanent mild
hypothermia in those days. Having studied chemical engineering, I
specialized in heat transfer science, which gave me the thought that this
was a field in which I had a chance to make some sort of contribution - to
alleviate this problem for others.
Later on I came under the influence of some very brilliant continental
workers in this field, particularly Professors Ingemar Hoglund and Gosta
Brown of the Royal Institute of Technology, Stockholm. My own first
book in this field was Thermal and Acoustic Insulation of Buildings,
published as far back as 1965 by Iliffe-Butterworth. Over the years, as
technology in this field, as in all others, made enormous strides forward a
number of other books of mine on this subject appeared, the last being the
Insulation Deskbook, published in 1977 by Heating and Ventilating
Publications Ltd of Croydon.
Since then, further important developments have taken place. Asbestos,
which used to be one of the mainstays of the insulation industry, has been
totally discredited. A large number of new insulation materials have come
on to the market. But perhaps the most important development of all has
been the use of computers to carry out all the very complicated calculations
which had previously to be done the hard way - provided one had the
necessary mathematical ability and knowledge to do them. It is this
activity, the application of microcomputers to insulation and condensation
problems, which has been the field of my own personal interest.
Chapter 9 of this book describes some of the work I have done in this
field during the development of 33 specialist computer programs to solve
problems ranging from evaluating solar ingress through windows to curing
condensation in municipal dwellings.
My thanks are due to the large number of firms that have made available
to me their practical expertise. In particular, I am most grateful to
Pilkington Glass Ltd who supply most of the illustrations in, and a good
half of the text of, Chapter 6; to Thermovitrine Ltd, who supplied material
on the wind loading of double-glazed units in Chapter 6; and Mr Ralph
Shaw of Warrington Research Centre Ltd for help in preparing
Chapter 10.
R. M. E. D.
Units
This book has been written in its entirety in SI units, which are now used in
most parts of the world except the United States.
The following table summarizes some of the more important specialist
conversion factors used in heat transfer calculations. All the computer
programs detailed in Chapter 9 can be used with either SI units or with
Imperial (American) units.
Imperial or non-SI metric unit Factor SI unit Symbol
BThU/hour 0.2930.71 watt W(J/s)
BThU/pound 2.326 kilo j oule/kilogramme kJ/kg
BThU/foot3 37.2589 kilo joule/metre3 kJ/m3
therm/UK gallon 23.2080 giga joule/metre3 GJ/m3
therm/US gallon 27.8717 giga joule/metre3 GJ/m3
BThU/pound °F 4.1868 kilo joule/kilogramme K kJ/kgK
BThU/foot3 °F 67.0661 kilo joule/metre3 K kJ/m3K
BThU/foot2 hour 3.15459 watt/metre2 W/m2
BThU/foot2 hour op 5.67826 watt/metre2 deg C W/m2 °C
kilocalorie/metre2' hour °C 1.163 watt/metre2 deg C W/m2 °C
BThU/foot hour c T 1.730073 watt/metre deg C W/m°C
BThU inch/foot2 hour °F 0.144228 watt/metre deg C W/m°C
BThU inch/foot2 second °F 519.22 watt/metre deg C W/m°C
kilocalorie/metre hour °C 1.163 watt/metre deg C W/m°C
calorie/centimetre second °C 418.68 watt/metre deg C W/m°C
Chapter 1
General aspects of thermal insulation
Human beings operate effectively only within a very narrow temperature
band. In fact the range between a home being too hot and too cold is
probably no more than about 5°C.
It has been argued that the reason why technical progress has in the past
been made mainly in cold countries, rather than in tropical areas, is that
man discovered the use of fire to raise ambient temperatures before he
discovered the use of compressors and heat exchanger systems to lower
them.
According to the Second Law of Thermodynamics, a system has
appreciable exergy when there is a large temperature difference between
the two media at both sides of an interface. Such exergy is converted into
anergy, if the two temperature levels become the same.
Thermal insulation can thus be considered as a means of slowing down
exergy loss or, to express it in another way, of reducing the rate of entropy
increase. Put more simply, thermal insulation is intended to keep cold
areas and media cold, and hot areas and media hot. It does this by the
prevention of heat flow.
Theory of heat transmission
Heat passes through solid materials by means of conduction. The rate at
which this happens depends upon the thermal conductivity of the material.
Metals have very high conductivities, but even these may vary
considerably, as can be seen from Table 1.1. The thermal conductivity of
non-metals is a good deal lower. The lowest thermal conductivity is shown
by materials where the amount of solid matter is quite small, but where
there is a considerable proportion of so-called 'voids'; that is, small bubbles
of gas or air, which are not big enough to carry heat by means of
convection. Examples of such materials are mineral wool, glass wool and
the various plastic foams.
Other thermally insulating materials such as vermiculite, gas or aerated
concrete, etc., lie somewhere in between. The thermal conductivity is
largely dependent upon the percentage of void spaces in the structure
(Table 1.2).
Moisture immediately increases the thermal conductivity, the limit being
that of pure water, which has a conductivity of 0.6059 W/mK at 20°C.
1
2
TABLE 1.1 Metals: Thermal conductivities, densities and specific heats. All
data are taken at 20 C.
Metals Thermal Density Specific heat
conductivity kg/m3 J/kg K
W/mK
Aluminium 237 2702 900.2
Antimony 18.5 6684 209
Cadmium 93 8642 230.2
Chromium 91 7200 460.5
Copper 318 8920 419
Gold 315 17000 130
Iridium 147 22421 130
Iron 80.3 7860 452
Lead 34.6 11288 130
Magnesium 159 1740 1017.4
Mercury 84 13594 138
Nickel 90 8900 444
Platinum 73 21450 134
Plutonium 8 17700 134
Selenium 0.5 4810 322
Silicon 83.5 2330 718
Silver 427 10500 239
Tantalum 54 16600 142
Tin 64 5750 226
Titanium 20 4500 523
Tungsten 178 19350 134
Uranium 25 19050 117
Vanadium 60 5980 494
Zinc 115 7140 389
TABLE 1.2 Miscellaneous materials: Densities, thermal conductivities and
specific heats. All values are taken at a temperature of 20-30°C.
Materials Density Thermal Specific heat
kg/m3 conductivity J/kg K
W/mK
Bitumen 2120 0.70 1675
Boiler clinkers 750 0.151
Cardboard 700 0.20 1507
Chalk 2000 0.93 879
Clay (dry) 1800 0.81 837
Coal dust 1000 0.14 2650
Cork 180 0.046 1758
Glass 2550 0.85 779
Lime plaster 1600 0.70 837
Marble 2800 1.30 921
Mica 3000 0.52 879
Porcelain 2400 0.95 1088
Rubber (hard) 1200 0.16 1381
Rubber (soft) 1100 0.14 1381
Rubber sponge 225 0.20 1381
Wax 920 0.27 2600
Thermal conductivity of building materials 3
The coefficient of thermal conductivity k
This is defined as the heat flow in watts (joules/second) which takes place
across a cube 1 metre2 in cross-section and 1 metre in thickness, if the
difference in temperature between the two faces is 1°C. Its dimensions are
therefore watts/metre °C or W/mK (Figure 1.1).
1 metre
Figure 1.1 The thermal conductivity of a material is expressed in W m/m2 K or W/m K
This definition of k-values is used throughout this book. It is the correct
one according to SI rules and has replaced the old designations, which were
far more complex. In non-English speaking countries the k-value is
generally referred to as the λ-value.
Provided the shape of the heat transfer medium is rectilinear, that is the
two areas which face each other are similar in size and shape, we can
express the heat that passes from one area to the other as:
Ôcond = ~Ah - t) (1.1)
2
a
where ß d is the heat in W/m2 that passes from one face to the other per
con
m2 area by the process of thermal conduction, k is the fc-value of the
material in W/mK, d is the thickness of the insulation material in metres,
and ΐγ and t are the surface temperatures on the insulation material, kid is
2
also called the heat transfer coefficient for the insulation layer, or Λ, and is
expressed in W/m2K.
Thermal conductivity of building materials
One frequently sees figures quoted for the thermal conductivity of, say,
concrete, brickwork or similar materials. Inevitably these are simply
approxiations as their conductivities tend to vary extremely widely. The
two factors that govern the thermal conductivity of such materials are
primarily the bulk dry density and the moisture content.
4 General aspects of thermal insulation
Bulk dry density
Bulk dry density gives a measure of the percentage voids in the building
material. As the thermal conductivity of air is very much lower than the
thermal conductivity of silicous and aluminous materials, this is by far the
most important factor governing the thermal conductivity of building
materials. A very slight difference in density, as for example with different
batches of bricks, or slightly different formulations of concrete, makes an
enormous difference to the conductivity.
Table 1.3 correlates density with A>value in W/mK for perfectly dry
silicous/aluminous/calcium macro-molecules such as brickwork, breeze
blocks, concrete, mortar and similar materials.
Using a computer it is possible to evaluate the relationship as:
k = 0.053 636 4 + 0.000 212 7 D - 0.000 000 2 D2 (1.2)
where k is the conductivity in W/mK and D is the density in kg/m3.
Moisture content
The thermal conductivity of building materials rises considerably when the
materials are even slightly damp. The relationship between the moisture
factor and the moisture content is not a linear one, but it can be read off
from Table 1.4.
Example: The dry density of some brickwork is 1300 kg/m3. Determine its
thermal conductivity in a position where it is sheltered from the rain so that
its moisture content is 1%, and also in a position where its moisture
content has risen to 5%.
According to Table 1.3 the dry conductivity of this kind of brickwork is
0.260W/mK. When its moisture content is 1% by volume, its thermal
conductivity becomes 0.260 x 1.3 = 0.338W/mK, and when it contains
5% by volume of moisture its A>value becomes 0.260 x 1.75 =
0.455 W/mK.
Using linear regression analysis one can determine the moisture factor F
directly from the moisture content in % M.
F = 1.127 268 3 + 0.218 584 7 M - 0.027 932 7 M2 +
0.002 388 1 M3 - 0.000 097 5 M4 + 0.000 001 5 M5
For example, the moisture factor of brickwork containing 3.75% water can
be shown, using a pocket calculator, to amount to 1.662 approximately.
Thermal conductivity of water and other liquids
The thermal conductivity of water and other fluids is quite low, but this is
not really of primary importance as conductivity alone is not the main
factor when dealing with heat transfer problems. Water molecules are
capable of carrying heat with them as they move, and thus transfer heat by
convection. However, there are many cases where such convection
currents do not occur. Examples are waterlogged powders and thick
slurries, and also numerous hydrophilic colloids.
For example, in meat, fruit, jellified emulsion paint, etc. water
constitutes the most important item. Yet its movement is restricted to such
an extent that the true thermal conductivity of water applies in such a case.
5
TABLE 1.3 Correlation TABLE 1.4 Relationship between
between density and k- value moisture content and moisture factor
Density Conductivity Moisture content Moisture factor
kg/m3 W/mK % by volume
200 0.069 1 1.3
300 0.080 2 1.5
400 0.092 3 1.6
500 0.104 4 1.68
600 0.115 5 1.75
700 0.136 6 1.82
800 0.146 7 1.89
900 0.165 8 1.96
1000 0.185 9 2.03
1100 0.202 10 2.10
1200 0.238 15 2.35
1300 0.260 20 2.55
1400 0.323 25 2.75
1500 0.362
1600 0.415
1700 0.463
1800 0.546
1900 0.612
2000 0.708
2100 0.789
2200 0.908
2300 1.050
2400 1.146
TABLE 1.5 Properties of water at varying temperatures
Température? Density Viscosity Specific heat Thermal conductivity
°C kg/m3 cSt J/kgK W/mK
0 1000.63 1.6657 4209.17 0.5638
5 1000.19 1.5080 4202.87 0.5753
10 999.56 1.3601 4197.29 0.5861
15 998.76 1.2218 4192.42 0.5963
20 997.78 1.0933 4188.28 0.6059
25 996.62 0.9744 4184.85 0.6148
30 995.29 0.8653 4182.14 0.6231
35 993.77 0.7658 4180.14 0.6307
40 992.08 0.6761 4178.87 0.6377
45 990.21 0.5960 4178.31 0.6441
50 988.16 0.5257 4178.47 0.6499
55 985.94 0.4651 4179.35 0.6550
60 983.54 0.4141 4180.94 0.6595
65 980.95 0.3729 4183.26 0.6633
70 978.20 0.3413 4186.29 0.6665
75 975.26 0.3195 4190.04 0.6691
80 972.14 0.3074 4194.51 0.6710
85 968.85 0.3049 4199.69 0.6723
90 965.38 0.3122 4205.59 0.6730
95 961.73 0.3292 4212.21 0.6730
100 957.91 0.3559 4219.55 0.6724
105 953.90 0.3922 4227.61 0.6712
110 949.72 0.4383 4236.38 0.6693
6 General aspects of thermal insulation
As with other properties of water, thermal conductivity varies according
to the ambient temperature. Table 1.5 gives data for the properties of
water at different temperatures.
Table 1.6 gives data for a number of non-aqueous liquids.
TABLE 1.6 Properties of some non-aqueous liquid sat 20°C
Organic liquid Density Specific heat Thermal conductivity
kg/m3 J/kgK W/mK
Acetone 790 2152 0.130
Benzene 878.6 1754 0.1594
Ethyl alcohol 789.3 2847 0.1685
Glycerol 1261.3 2411 0.2965
Octane (Petrol or
Gasolene) 702.5 2420 0.1663
Paraffin (Kerosene) 890 2000 0.1507
Thermal conductivity of gases
The thermal conductivity of gases is very low indeed, but this is normally
not the main criterion involved in heat transfer. Heat is transmitted
through gases by convection and radiation.
High void insulation materials act as insulators because they restrict the
molecular movement of gases, thereby reducing convection heat transfer.
It is best if the solid material that surrounds the gas molecules divides them
up into sacs, which are below the 'mean free path' in dimension, and are at
the same time totally opaque to radial heat transmission. It is then
theoretically possible to approach the theoretical thermal conductivity of
the gas at the specified temperature. These values are given in Table 1.7.
For temperatures other than those given it is usually good enough simply to
interpolate or to extrapolate. For example, to determine the thermal
conductivity of nitrous oxide at 150°C:
k = 0.015 23 + (150/100) (0.023 88 - 0.015 23) = 0.028 21 W/m K
In practice, we distinguish between 'micro-convection' and 'macro-
convection' of gases.
Micro-convection is heat transfer when gas is present in sacs that exceed
in dimensions the mean free path of the molecules concerned. The mean
free path is very short indeed, being of the order of 10"4 for air at
atmospheric temperature. Some micro-convection therefore always takes
place except in high void insulators where air sacs are smaller than about
0.1 micrometre.
Macro-convection is heat transfer which necessitates the existence of
proper convection currents. Macro-convection mainly takes place when air
pores are open, because temperature differences cause gaseous molecules
to pass along open paths of this kind due to buoyancy differences. For
example, even if the pore sizes of gas sacs are the same in a sample of, say
polyurethane foam which has closed pores, and urea formaldehyde foam
