Second Edition
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2. completely revised Edition
Print ISBN: 978-3-433-03147-6
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To my wife, children and grandchildren
In remembrance of Professor A. de Grave who introduced Building Physics as a new discipline at the University of Leuven (KULeuven) Belgium in 1952
Until the first energy crisis of 1973, building physics was a rather dormant field within building engineering, with seemingly limited applicability. While soil mechanics, structural mechanics, building materials, building construction and HVAC were perceived as essential, designers sought advice on room acoustics, moisture tolerance, summer comfort or lighting only when really necesary or when problems arose. Energy was even not a concern, while thermal comfort and indoor environmental quality were presumed to be guaranteed thanks to infiltration, window operation and the heating and cooling system installed. The energy crises of the 1970s, persisting moisture problems, complaints about sick buildings, thermal, visual and olfactory discomfort and the move towards more sustainability changed it all. Societal pressure to diminish energy consumption in buildings without degrading usability activated the notion of performance based design and construction. As a result, today, building physics – and its potential to quantify related performance requirements – is at the forefront of building innovation.
As with all engineering sciences, building physics is orientated towards application, which is why, after the first volume on the fundamentals, this second volume examines performance metrics and requirements as the basis for sound building engineering. Choices have been made, among others to limit the text to the heat, air and moisture performances. Subjects treated are: the outdoor and indoor ambient conditions, the performance concept, performance at the building level, performance metrics at the building enclosure level and the heat-air-moisture material properties of building, insulation and finishing materials. The book reflects 38 years of teaching architectural, building and civil engineers, bolstered by close to 50 years' experience in research and consultancy. Where needed, information from international sources was used, which is why each chapter ends with an extended reading list.
The book uses SI units. Undergraduate and graduate students in architectural and building engineering should benefit, but also mechanical engineers studying HVAC and practising building engineers, who want to refresh their knowledge. The level of discussion presumes that the reader has a sound knowledge of the fundamentals treated in the first volume, along with a background in building materials and building construction.
The book reflects the work of many people, not just the author. Therefore, I would like to thank the thousands of students I have had during my 38 years of teaching. They have given me the opportunity to optimize the content. Also, were I not standing on the shoulders of those who precede me, this book would not be what it is. Although I started my career as a structural engineer, my predecessor, Professor Antoine de Grave, planted the seeds that fed my interest in building physics. The late Bob Vos of TNO, the Netherlands, and Helmut Künzel of the Fraunhofer Institut für Bauphysik, Germany, showed the importance of experimental work and field testing for understanding building performance, while Lars Erik Nevander of Lund University, Sweden, taught that solving problems does not always require complex modelling, mainly because reality in building construction is always much more complex than any model could simulate.
During my four decades at the Laboratory of Building Physics, several researchers and PhD students have been involved. I am very grateful to Gerrit Vermeir, Staf Roels, Dirk Saelens and Hans Janssen, colleagues at the university; also to Jan Carmeliet, professor at the ETH, Zürich; Piet Standaert, principal at Physibel Engineering; Jan Lecompte; Filip Descamps, principal at Daidalos Engineering and part-time professor at the Free University Brussels (VUB); Arnold Janssens, professor at the University of Ghent (UG); Rongjin Zheng, associate professor at Zhejiang University, China; Bert Blocken, full professor at the Technical University Eindhoven (TU/e); Griet Verbeeck, associate professor at the University of Hasselt; and Wout Parys, all of whom contributed through their work. The experiences gained as a structural engineer and building site supervisor at the start of my career, as building assessor over the years, as researcher and operating agent of four Annexes of the IEA, Executive Committee on Energy in Buildings and Communities forced me to rethink my engineering-based performance approach time and again. The many ideas I exchanged and received in Canada and the USA from Kumar Kumaran, the late Paul Fazio, Bill Brown, William B. Rose, Joe Lstiburek and Anton Ten Wolde were also of great help.
Finally, I thank my family, my wife Lieve, who manages to live with a busy engineering professor, my three children who had to live with that busy father and my many grandchildren who do not know that their grandfather is still busy.
Leuven, January 2016
Hugo S.L.C. Hens
This is the second volume in a series of four books:
Subjects discussed in this volume are: outdoor and indoor ambient conditions, performance concept, performance at the urban, building and building envelope level and the heat–air–moisture material properties. The book figures as a hinge between ‘Building Physics: Heat, Air and Moisture’ and the two volumes on ‘Performance Based Building Design’. Although it does not deal with acoustics and lighting in detail, they form an integral part of the performance arrays and are mentioned as and when necessary.
The outdoor and indoor ambient conditions and related design approaches are highlighted in Chapter 1. Chapter 2 advances the performance concept with its hierarchical structure, going from the urban environment across the building and building assemblies down to the layer and material level. In Chapter 3, the main heat, air and moisture linked performances at the building level are discussed, while Chapter 4 analyses related metrics of importance for a well-performing building envelope and fabric. Chapter 5 deals with timber-framed walls as an exemplary case, and Chapter 6 lists the main heat, air and moisture material property values needed to predict the response of building assemblies.
A performance approach helps designers, consulting engineers and contractors to better ensure building quality. Of course, physical integrity is not the only factor adding value to buildings. Functionality, spatial quality and aesthetics – aspects belonging to the architect's responsibility – are of equal importance, though they should not cause us to neglect the importance of an overall outstanding building performance.
Readers who would like to know more about the engineering field ‘building physics’, its importance and history, should consult the first volume, ‘Building Physics: Heat, Air and Moisture’. It might seem that adding the term ‘applied’ to this second volume is unnecessary – building physics is, by definition, applied. Rather, the word stresses the focus of this book: entirely directed towards its use in building design and construction.
The book uses the SI system (internationally mandated since 1977). Its base units are the metre (m), the kilogram (kg), the second (s), the kelvin (K), the ampere (A) and the candela. Derived units of importance when studying applied building physics are:
| Force: | newton (N); | 1 N = 1 kg·m·s−2 |
| Pressure: | pascal (Pa); | 1 Pa = 1 N/m2 = 1 kg·m−1·s−2 |
| Energy: | joule (J); | 1 J = 1 N·m = 1 kg·m2·s−2 |
| Power: | watt (W); | 1 W = 1 J·s−1 = 1 kg·m2·s−3 |
For symbols, the ISO-standards (International Standardization Organization) are followed. If a quantity is not included, the CIB-W40 recommendations (International Council for Building Research, Studies and Documentation, Working Group ‘Heat and Moisture Transfer in Buildings’) and the list edited by Annex 24 of the IEA EBC (International Energy Agency, Executive Committee on Energy in Buildings and Communities) apply.
Table 0.1 List with symbols and quantities
| Symbol | Meaning | SI units |
| a | Acceleration | m/s2 |
| a | Thermal diffusivity | m2/s |
| b | Thermal effusivity | W/(m2·K·s0.5) |
| c | Specific heat capacity | J/(kg·K) |
| c | Concentration | kg/m3, g/m3 |
| e | Emissivity | — |
| f | Specific free energy | J/kg |
| Temperature ratio | — | |
| g | Specific free enthalpy | J/kg |
| g | Acceleration by gravity | m/s2 |
| g | Mass flux | kg/(m2·s) |
| h | Height | m |
| h | Specific enthalpy | J/kg |
| h | Surface film coefficient for heat transfer | W/(m2·K) |
| k | Mass related permeability (mass could be moisture, air, salt…) | s |
| l | Length | m |
| l | Specific enthalpy of evaporation or melting | J/kg |
| m | Mass | kg |
| n | Ventilation rate | s−1, h−1 |
| p | Partial pressure | Pa |
| q | Heat flux | W/m2 |
| r | Radius | m |
| s | Specific entropy | J/(kg·K) |
| t | Time | s |
| u | Specific latent energy | J/kg |
| v | Velocity | m/s |
| w | Moisture content | kg/m3 |
| x,y,z | Cartesian coordinates | m |
| A | Water sorption coefficient | kg/(m2·s0.5) |
| A | Area | m2 |
| B | Water penetration coefficient | m/s0.5 |
| D | Diffusion coefficient | m2/s |
| D | Moisture diffusivity | m2/s |
| E | Irradiation | W/m2 |
| F | Free energy | J |
| G | Free enthalpy | J |
| G | Mass flow (mass = vapour, water, air, salt) | kg/s |
| H | Enthalpy | J |
| I | Radiation intensity | J/rad |
| K | Thermal moisture diffusion coefficient | kg/(m·s·K) |
| K | Mass permeance | s/m |
| K | Force | N |
| L | Luminosity | W/m2 |
| M | Emittance | W/m2 |
| P | Power | W |
| P | Thermal permeance | W/(m2·K) |
| P | Total pressure | Pa |
| Q | Heat | J |
| R | Thermal resistance | m2·K/W |
| R | Gas constant | J/(kg·K) |
| S | Entropy | J/K |
| S | Saturation degree | — |
| T | Absolute temperature | K |
| T | Period (of a vibration or a wave) | s, days,… |
| U | Latent energy | J |
| U | Thermal transmittance | W/(m2·K) |
| V | Volume | m3 |
| W | Air resistance | m/s |
| X | Moisture ratio | kg/kg |
| Z | Diffusion resistance | m/s |
| α | Thermal expansion coefficient | K−1 |
| α | Absorptivity | — |
| β | Surface film coefficient for diffusion | s/m |
| β | Volumetric thermal expansion coefficient | K−1 |
| η | Dynamic viscosity | N·s/m2 |
| θ | Temperature | °C |
| λ | Thermal conductivity | W/(m·K) |
| μ | Vapour resistance factor | — |
| ν | Kinematic viscosity | m2/s |
| ρ | Density | kg/m3 |
| ρ | Reflectivity | — |
| σ | Surface tension | N/m |
| τ | Transmissivity | — |
| ϕ | Relative humidity | — |
| α, ϕ, Θ | Angle | rad |
| ξ | Specific moisture capacity | kg/kg per unit of moisture potential |
| Ψ | Porosity | — |
| Ψ | Volumetric moisture ratio | m3/m3 |
| Φ | Heat flow | W |
Table 0.2 List with suffixes and notations
| Symbol | Meaning | Symbol | Meaning |
| Indices | |||
| A | Air | m | Moisture, maximal |
| c | Capillary, convection | o | Operative |
| e | Outside, outdoors | r | Radiant, radiation |
| h | Hygroscopic | sat | Saturation |
| I | Inside, indoors | s | Surface, area, suction |
| cr | Critical | v | Water vapour |
| CO2, SO2 | Chemical symbol for gasses | w | Water |
| ϕ | Relative humidity | ||
| Notation | Meaning | ||
| [], bold, | Matrix, array, value of a complex number | ||
dash (ex..:  |
Vector |