Preface
An understanding of building physics is, I suggest, fundamental for the design and construction of safe, functional, energy-efficient buildings. We need to understand how buildings work – not in the sense of circulation spaces, massing and the like but rather how a building will perform: for example how much ventilation will be required for the likely occupants and how can that be provided without compromising the thermal performance; or how will the layout and fenestration affect light levels and cooling demand.
My intention in this book is to give an overview of the main physical phenomena that affect the use and operation of buildings and to explore those effects and their interactions, but always with the aim of addressing the usability of buildings.
At this point I have to acknowledge that the term ‘building physics’ can be off-putting: it may have unwelcome associations with tedious experiments at school with mirrors and pins, inexplicable circuits or trolleys running down slopes. For the ancient Greeks physis – from which we get our word physics – was the whole of nature (which included not just the physical world – there’s the word again – but also beings and phenomena which we would classify as ‘spiritual’ or ‘supernatural’). Of course, academic subjects have divided and mutated over the years, and our understanding of the cosmos has changed, but physics remains a way of understanding the interactions of the materials and forces of which nature is composed.
And building physics is a division within that larger subject which seeks to understand how a building is affected by – and affects – its environment and its occupants. Perhaps the best way to explore that is to consider a building with which I am very familiar: my house.
It was built in 1925, with masonry cavity walls (with only air and wall ties in the cavities), a tiled roof with neither insulation nor underlay, and timber floors with a very well-ventilated void beneath. The outhouse for the laundry was roofed, but not entirely enclosed. The wood windows were single glazed, with opening casements. There were four fireplaces, and a kitchen range which also heated water for the bathroom.
It was not old-fashioned though: it had electric lighting from the start (in comparison, my father’s home town did not get mains electricity until about 1930) and the long greenhouse at the end of the garden was centrally heated by 4 inch diameter hot water pipes.
In the 90 years since it was built the house has been adapted and remodelled: the wall cavities have been filled with blown-fibre insulation; the roof has been re-covered and insulated, and most of the windows have been replaced with double-glazed units in frames made of synthetic material (uPVC). The fireplaces have been blocked up and central heating fitted; the house has been rewired twice and the laundry has become an enclosed utility room.
Those changes occurred not by random mutation, but as the result of deliberate (and frequently expensive) choices on the part of successive householders. Choices made, in part, on the basis of changing expectations of what a house should provide. Why be too hot in a room with a blazing fire and yet too cold elsewhere, when a central heating system can heat the whole house? And once that is done, why put up with the gusty draughts drawn up the chimneys?
Of course, there have been other motivations. Householders responded to the energy disruptions and price shocks of the 1970s by improving energy efficiency; the first few inches of roof insulation probably date from that decade. Snow drifted in through the leading of the windows and the traffic noise became increasingly intrusive: the wooden windows went, replaced by ‘maintenance-free’ uPVC. Even the cutting-edge incandescent lamps have been replaced by compact fluorescent lamps and light-emitting diodes (LEDs).
Despite all the changes the house still fulfils its original functions of keeping the occupants safe, dry and (reasonably) warm. And, despite all the changes, the fundamental physical processes that affect and are affected by the house operate as before: electromagnetic radiation from the sun passes through the glazing of the windows, lighting and warming the rooms; and large-scale air movement creates pressure differentials across the house, which in turn produce draughts.
Yet, although the ‘laws of physics’ are unchanged (even if our understanding of them has sharpened since the 1920s) the scope of their operation has changed. Rooms are heated by convection currents generated by central heating radiators, instead of the radiant heat from the fires. We probably wear fewer layers of clothing around the house than the first occupants, and keep a larger proportion of it at a higher temperature. There is substantially less air movement: there is no updraught generated by the chimneys, the new windows are well-sealed and – because we are using deodorant, washing more frequently and not burning coal – we do not fling the windows wide for fresh air. With less air movement the water vapour generated by bathing, showering (the shower is new) and laundry moves through the house and passes into the fabric by air leakage and diffusion, rather than being carried rapidly out of the house.
In addition, the behaviour of the building’s occupants has changed and any analysis of a building’s physics has to consider those occupants, because buildings have to work for people.
In Arthur C Clarke’s novel Childhood’s End the last man stows away on an alien spaceship to reach the aliens’ home planet. He has a miserable time there, in part because of what he discovers about humanity’s fate, but also because the alien buildings are made to suit the aliens’ abilities, one of which is flight: the stowaway’s explorations are curtailed by an absence of stairs.
We are used to stairs being the right size for human use, which is something that has been learned and eventually codified. But when it comes to other things we are less accustomed to acknowledging the human measure. In the huge range of temperatures that the universe offers we can survive in only a narrow range and are comfortable in an even narrower one. Our eyes can perceive a limited band of the electromagnetic spectrum, and exposure to other parts of the spectrum is harmful. We can see by sunlight and moonlight, but not by starlight.
Not surprisingly, we construct and operate our buildings to address those limitations as best we can. I am not primarily thinking of questions of why buildings are the way they are: there are plenty of books dealing with power structures, cultural imperatives and aesthetics. I am interested in the fundamentals of shelter, security and wellbeing, which we strive to achieve through the interaction of three things: human physiology, buildings and building physics, which is illustrated in the ‘building physics triangle’.
The Building Physics Triangle
That triangle of relationships underpins the rest of this book: it applies to a cabin in the Florida panhandle, a concert hall in Norway or a museum in Japan.
Looking at the first point of the triangle we can appreciate that, while the expectations of the cabin-dweller, concert-goer or museum visitor may differ, they are subject to the same physiological constraints. On the second point, as Scotty observed in Star Trek, ‘we cannae change the laws of physics’ which (in a descriptive–predictive sense) are the same across the planet. Air above Australia is affected by forces in the same way as the air above Alaska: in either place, if air that is already saturated with water vapour is cooled the result will be condensation. Which brings us to the third point of the triangle and the real challenge: designing buildings. That is the justification for looking at building physics.
But in thinking and writing about building physics we face the problem that occurs whenever we think and write about complex, interrelated phenomena: we have to divide them up to consider them one at a time. To write about the principles of heat transfer I have to isolate it from other phenomena such as moisture, even though I know that moisture content has an effect on heat transfer. Simply to get text into meaningful chapters I have to draw lines around subjects, deciding that this belongs in Light and that belongs in Heat, yet at the same time knowing it all happens at the same time. (Also, a book, unlike physical phenomena, has to have a boundary: for reasons of space some interesting topics, such as thermal movement, have had to be excluded.)
As a result, each chapter focuses on one of the five primary physical phenomena: heat, air, moisture, sound and light. (Key terms, given in bold, are defined in the Glossary pp. 134–37.) At the end of each chapter is a diagram that shows the main points of interaction between the subject of the chapter and the other phenomena, and identifies the main design implications of those interactions. If the diagrams do nothing else they will remind us that to think successfully about building physics we have to think holistically.