Building Physics - Heat, Air and Moisture

Fundamentals and Engineering Methods with Examples and Exercises

Hugo S. L. Hens

• Publisher: Ernst & Sohn
• Year: 2017
• ISBN:
  9783433608562
• Edition: 3
• Topic: Technology & Engineering
• Subtopic: Construction & Architectural Engineering
• Index: Technology & Engineering

Bad experiences with construction quality, the energy crises of 1973 and 1979, complaints about "sick buildings", thermal, acoustical, visual and olfactory discomfort, the need for good air quality, the move towards more sustainability - all these have accelerated the development of a field that, for a long time, was hardly more than an academic exercise: building physics (in English speaking countries sometimes referred to as building science). The discipline embraces domains such as heat and mass transfer, building acoustics, lighting, indoor environmental quality and energy efficiency. In some countries, fire safety is also included. Through the application of physical knowledge and its combination with information coming from other disciplines, the field helps to understand the physical phenomena governing building parts, building envelope, whole buildings and built environment performance, although for the last the wording "urban physics" is used. Today, building physics has become a key player on the road to a performance based building design. The book deals with the description, analysis and modeling of heat, air and moisture transport in building assemblies and whole buildings with main emphasis on the building engineering applications, including examples. The physical transport processes determine the performance of the building envelope and may influence the serviceability of the structure and the whole building. Compared to the second edition, in this third edition the text has partially been revised and extended.

1
Heat Transfer

1.1 Overview

1.1.1 Heat

A first description of what heat is comes from thermodynamics. That discipline describes how systems and their environment interchange energy. Anything can be a system: a material, a building assembly, a building, part of a HVAC system, even a whole city. Energy transmitted as ‘work’ is purposeful and organized, whereas as ‘heat’ it is diffuse and chaotic. A second description resides in particle physics, where the statistically distributed kinetic energy of atoms and free electrons stands for heat. In any case, heat is the least noble, most diffuse form of energy to which each nobler form degrades; consider the second law of thermodynamics.

1.1.2 Temperature

The temperature reflects the quality of the heat. Higher values reflect the increased kinetic energy of atoms and free electrons, resulting in higher exergy and the potential to convert more heat via a cyclic process into work. Lower temperatures and therefore less kinetic energy of atoms and free electrons result in less exergy. Higher temperatures require warming up, lower temperatures cooling down of a system. Like any potential, temperature is a scalar, which, as heat, cannot be measured directly. It is sensed and because many material properties depend on it, indirectly quantifiable. A mercury thermometer uses the volumetric expansion of mercury when heated and the contraction when cooled. In a Pt100 thermometer, the electrical resistance of platinum wire changes with temperature. Temperature logging with thermocouples uses the varying contact potential between metals.

The SI syst...