Building Physics - Heat, Air and Moisture : Fundamentals and Engineering Methods with Examples and Exercises

by Hens, Hugo S. L. C.

• ISBN13: 9783433030271
• ISBN10: 3433030278
• Edition: 2nd
• Format: Paperback
• Copyright: 2012-09-24
• Publisher: Ernst & Sohn

Summary

Bad experiences with construction quality, the energy crises of 1973 and 1979, complaints about 'sick buildings', thermal, acoustical, visual and olfactory discomfort, the move towards more sustainability, have all accelerated the development of a field, which until 35 years ago was hardly more than an academic exercise: building physics. Through the application of existing physical knowledge and the combination with information coming from other disciplines, the field helps to understand the physical performance of building parts, buildings and the built environment, and translates it into correct design and construction. This book is the result of thirty years teaching, research and consultancy activity of the author. The book discusses the theory behind the heat and mass transport in and through building components. Steady and non steady state heat conduction, heat convection and thermal radiation are discussed in depth, followed by typical building-related thermal concepts such as reference temperatures, surface film coefficients, the thermal transmissivity, the solar transmissivity, thermal bridging and the periodic thermal properties. Water vapour and water vapour flow and moisture flow in and through building materials and building components is analyzed in depth, mixed up with several engineering concepts which allow a first order analysis of phenomena such as the vapour balance, the mold, mildew and dust mites risk, surface condensation, sorption, capillary suction, rain absorption and drying. In a last section, heat and mass transfer are combined into one overall model staying closest to the real hygrothermal response of building components, as observed in field experiments. The book combines the theory of heat and mass transfer with typical building engineering applications. The line from theory to application is dressed in a correct and clear way. In the theory, oversimplification is avoided. This book is the result of thirty years teaching, research and consultancy activity of the author.

Author Biography

Prof. em. Dr.-Ing. Hugo S. L. C. Hens, Katholische Universit?t L?wen/Belgien, lehrte Bauphysik von 1975 bis 2003, Geb?udeplanung von 1970 bis 2005 und Technische Geb?udeausr?stung von 1975 bis 1977 sowie von 1990 bis 2008. Bis 1972 war er als Tragwerksplaner f?r Wohnh?user, B?ro- und Geschossbauten in einem Architekturb?ro t?tig. Er hat als Autor bzw. Koautor ?ber 150 Ver?ffentlichungen verfasst und hunderte Schadensgutachten erstellt. W?hrend zehn Jahren koordinierte er die internationale Arbeitsgruppe CIB W40 ?Heat and Mass Transfer in Buildings?. Von 1986 bis 2008 war er im Rahmen des Forschungsprogramms ?Energy Conservation in Buildings and Community Systems? der Internationalen Energieagentur IEA f?r die Erarbeitung von Annex 14, Annex 24, Annex 32 und Annex 41 verantwortlich. Er ist Mitglied der American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE).

Table of Contents

Preface  VII

0 Introduction  1

0.1 Subject of the book  1

0.2 Building Physics   1

0.2.1 Definition  1

0.2.2 Criteria   2

0.2.2.1 Comfort   2

0.2.2.2 Health    3

0.2.2.3 Architecture and materials                       3

0.2.2.4 Economy   3

0.2.2.5 Sustainability    3

0.3 Importance of Building Physics                     3

0.4 History of Building Physics                      5

0.4.1 Heat, air and moisture                         5

0.4.2 Building acoustics   5

0.4.3 Lighting   6

0.4.4 Thermal comfort and indoor air quality                  6

0.4.5 Building physics and building services                  7

0.4.6 Building physics and construction                    7

0.4.7 What about the Low Countries?                     8

0.5 Units and symbols   9

0.6 Literature  12

1 Heat Transfer   13

1.1 Overview  13

1.2 Conduction  15

1.2.1 Conservation of energy                        15

1.2.2 Fourier’s laws    16

1.2.2.1 First law  16

1.2.2.2 Second law  17

1.2.3 Steady state  18

1.2.3.1 What is it?   18

1.2.3.2 One dimension: flat assemblies                    18

1.2.3.3 Two dimensions: cylinder symmetric                  26

1.2.3.4 Two and three dimensions: thermal bridges                27

1.2.4 Transient regime   32

1.2.4.1 What?   32

1.2.4.2 Flat assemblies, periodic boundary conditions               33

1.2.4.3 Flat assemblies, random boundary conditions               45

1.2.4.4 Two and three dimensions                      48

1.3 Convection  49

1.3.1 Heat exchange at a surface                      49

1.3.2 Convective heat transfer                       50

1.3.3 Convection typology                         52

1.3.3.1 Driving forces    52

1.3.3.2 Flow type   52

1.3.4 Calculating the convective surface film coefficient             53

1.3.4.1 Analytically  53

1.3.4.2 Numerically    53

1.3.4.3 Dimensional analysis                        54

1.3.5 Values for the convective surface film coefficient             56

1.3.5.1 Flat assemblies   56

1.3.5.2 Cavities   59

1.3.5.3 Pipes    61

1.4 Radiation  61

1.4.1 What is thermal radiation?                      61

1.4.2 Quantities   62

1.4.3 Reflection, absorption and transmission                 62

1.4.4 Radiant surfaces or bodies                      64

1.4.5 Black bodies    65

1.4.5.1 Characteristics    65

1.4.5.2 Radiant exchange between two black bodies: the view factor         67

1.4.5.3 Properties of view factors                       69

1.4.5.4 Calculating view factors                       69

1.4.6 Grey bodies  72

1.4.6.1 Characteristics    72

1.4.6.2 Radiant exchange between grey bodies                 73

1.4.7 Coloured bodies   75

1.4.8 Practical formulae  75

1.5 Applications    77

1.5.1 Surface film coefficients and reference temperatures            77

1.5.1.1 Overview  77

1.5.1.2 Indoor environment                         77

1.5.1.3 Outdoor environment                        81

1.5.2 Steady state, one dimension: flat assemblies               84

1.5.2.1 Thermal transmittance and interface temperatures             84

1.5.2.2 Thermal resistance of a non ventilated, infinite cavity            88

1.5.2.3 Solar transmittance                         90

1.5.3 Steady state, cylindrical coordinates: pipes                93

1.5.4 Steady state, two and three dimensions: thermal bridges           94

1.5.4.1 Calculation by the control volume method (CVM)             94

1.5.4.2 Practice   95

1.5.5 Steady state: windows                        98

1.5.6 Steady state: building envelopes                    99

1.5.6.1 Overview  99

1.5.6.2 Average thermal transmittance                     99

1.5.7 Transient, periodic: flat assemblies                  100

1.5.8 Heat balances   101

1.5.9 Transient, periodic: spaces                      102

1.5.9.1 Assumptions    102

1.5.9.2 Steady state heat balance                      102

1.5.9.3 Harmonic heat balances                       103

1.6 Problems   107

1.7 Literature   120

2 Mass Transfer   123

2.1 Generalities    123

2.1.1 Quantities and definitions                      123

2.1.2 Saturation degrees                         125

2.1.3 Air and moisture transfer                      126

2.1.4 Moisture sources  128

2.1.5 Air, moisture and durability                     129

2.1.6 Link between mass and energy transfer                 130

2.1.7 Conservation of mass                        131

2.2 Air transfer    132

2.2.1 Overview   132

2.2.2 Air pressure differences                       133

2.2.2.1 Wind   133

2.2.2.2 Stack effects    134

2.2.2.3 Fans    135

2.2.3 Air permeances  135

2.2.4 Air transfer in open-porous materials                  139

2.2.4.1 Conservation of mass                        139

2.2.4.2 Flow equation   139

2.2.4.3 Air pressures    139

2.2.4.4 One dimension: flat assemblies                    140

2.2.4.5 Two and three dimensions                      142

2.2.5 Air flow across permeable layers, apertures, joints, leaks and cavities     143

2.2.5.1 Flow equations  . 143

2.2.5.2 Conservation of mass: equivalent hydraulic circuit            143

2.2.6 Air transfer at building level                     144

2.2.6.1 Definitions    144

2.2.6.2 Thermal stack   145

2.2.6.3 Large openings   145

2.2.6.4 Conservation of mass                        146

2.2.6.5 Applications    148

2.2.7 Combined heat and air transfer                    151

2.2.7.1 Open-porous materials                       151

2.2.7.2 Air permeable layers, joints, leaks and cavities              157

2.3 Vapour transfer   160

2.3.1 Water vapour in the air                       160

2.3.1.1 Overview   160

2.3.1.2 Quantities  161

2.3.1.3 Maximum vapour pressure and relative humidity             161

2.3.1.4 Changes of state in humid air                     166

2.3.1.5 Enthalpy of humid air                        166

2.3.1.6 Measuring air humidity                       167

2.3.1.7 Applications    167

2.3.2 Water vapour in open-porous materials                 172

2.3.2.1 Overview   172

2.3.2.2 Sorption isotherm and specific moisture ratio              173

2.3.2.3 Physics involved  174

2.3.2.4 Impact of salts   177

2.3.2.5 Consequences   177

2.3.3 Vapour transfer in the air                      177

2.3.4 Vapour transfer in materials and assemblies               179

2.3.4.1 Flow equation   179

2.3.4.2 Conservation of mass                        182

2.3.4.3 Vapour transfer by ‘equivalent’ diffusion                182

2.3.4.4 Vapour transfer by ‘equivalent’ diffusion and convection          197

2.3.5 Surface film coefficients for diffusion                 204

2.3.6 Applications    207

2.3.6.1 Diffusion resistance of a cavity                    207

2.3.6.2 Cavity ventilation                         207

2.3.6.3 Water vapour balance in a space: surface condensation and drying      210

2.4 Moisture transfer  211

2.4.1 Overview   211

2.4.2 Moisture transfer in a pore                      211

2.4.2.1 Capillarity  211

2.4.2.2 Water transfer   213

2.4.2.3 Vapour transfer   222

2.4.2.4 Moisture transfer  224

2.4.3 Moisture transfer in materials and assemblies              224

2.4.3.1 Transport equations                         224

2.4.3.2 Conservation of mass                        227

2.4.3.3 Starting, boundary and contact conditions                227

2.4.3.4 Remark  228

2.4.4 Simplifying moisture transfer                     228

2.4.4.1 The model  228

2.4.4.2 Applications    230

2.5 Problems   245

2.6 Literature   263

3 Combined heat-air-moisture transfer                 267

3.1 Overview   267

3.2 Material and assembly level                     267

3.2.1 Assumptions    267

3.2.2 Solution  267

3.2.3 Conservation laws                         268

3.2.3.1 Mass   268

3.2.3.2 Energy   269

3.2.4 Flow equations   272

3.2.4.1 Heat    272

3.2.4.2 Mass, air   272

3.2.4.3 Mass, moisture   273

3.2.5 Equations of state                         273

3.2.5.1 Enthalpy/temperature, vapour saturation pressure/temperature        273

3.2.5.2 Relative humidity/moisture content                  273

3.2.5.3 Suction/moisture content                      273

3.2.6 Starting, boundary and contact conditions                274

3.2.6.1 Starting conditions                         274

3.2.6.2 Boundary conditions                        274

3.2.6.3 Contact conditions                         274

3.2.7 Two examples of simplified models                  275

3.2.7.1 Non hygroscopic, non capillary materials                275

3.2.7.2 Hygroscopic materials at low moisture content              276

3.3 Building level   277

3.3.1 Overview   277

3.3.2 Balance equations                         277

3.3.2.1 Vapour   277

3.3.2.2 Air    279

3.3.2.3 Heat    279

3.3.2.4 Closing the loop  282

3.3.3 Hygric inertia   283

3.3.3.1 Generalities    283

3.3.3.2 Sorption-active thickness                      283

3.3.3.3 Zone with one sorption-active surface                 286

3.3.3.4 Zone with several sorption-active surfaces                287

3.3.3.5 Harmonic analysis                         288

3.3.4 Consequences   289

3.3.4.1 Steady state    289

3.3.4.2 Transient   289

3.4 Problems   292

3.5 Literature   303

Postscript  305

Problems and Solutions                      307