Thermal Comfort and Fabric

Thermal Comfort and Fabric

RESEARCH PATHWAY: personal reflections on a career in research

Robyn Pender (about to retire from Historic England) reflects on a career spanning physics to building conservation, and along the way rediscovers a forgotten approach to thermal comfort: the use of wall hangings. These are effective strategies for today but also raise important questions about how we measure and think about thermal comfort.

Education and training

My initial interest in the world of conservation sprang from a desire to reconcile dual passions for science and art. This had originally led me to study architecture as I was interested in how buildings were constructed and how they functioned for their users. However, it was not a happy experience for me: the fashion at the time was to consider ‘buildability’ as uncreative. I transferred to study physics, in a special degree that stressed field studies and experimentation, and the logical thought processes underpinning good research. Theory had to be grounded in observation of the real world.

The story of how I then came to study at the new Wall Painting Conservation Department at the Courtauld in London in 1991 is too long for here, but within weeks of arriving I was working with Professor Sharon Cather to develop methodologies for assessing building environments, including monitoring. As I’d suspected, the interaction between building fabric and environment – especially interior environment – was key (Cather et al. 1992). Most of our paintings were in churches and cathedrals, and a recurrent problem was the impact of heating. This was a Victorian innovation, and it caused wild swings in ambient conditions and consequent problems for the paintings (especially where the building had moisture problems as well).

The Courtauld also taught me the importance of history. Studying the past gives us clues to the present; and coupled with scientific investigation and practical surveying, the results could be astonishing. And, at times, deeply puzzling. On a research trip with Sharon to Normandy, we found 15th century wall paintings representing tapestries, but actual hooks were above it for hanging real tapestries. As the paint ran over the hooks, they were clearly contemporaneous. But why would you paint a tapestry on a wall, and then hang a real tapestry over it?

Thermal comfort and discomfort

Thinking about people and comfort over the next few months, the light suddenly dawned. We already knew that worshippers feel very cold when they sit next to a massive stone wall in a church, because they radiate their body heat into it. The wooden wall panelling common in churches was obviously intended to stop this, and of course cloth would have the same effect. It wouldn’t even have to be thick: it only needs to form a radiant barrier between the body and the wall. What’s more, you could take the cloth down in summer when losing some heat would be welcome. Checking the house records of stately homes, we found that is exactly what they did.

A whole series of other puzzling observations suddenly began to make sense. Physical puzzles, like the random hooks we found all over the walls in our buildings; and historical puzzles, like the guild that made painted cloths being one of the biggest in medieval London. In pre-Industrial times, everyone (rich and poor, house and tavern) had painted cloths. We used to wonder why. Now it was clear: the cloths (which were stretched on battens fixed to the wall) were primarily for thermal comfort (English Heritage 2014). Much later I was given another piece of the jigsaw by the head of the upholsters’ guild, who pointed out that actually their name is the Guild of Upholders!

I stayed on at the Courtauld for 12 years, before moving as a postdoc to the University College London’s Bartlett Graduate School. At the new Centre for Sustainable Heritage, I completed a scoping study into climate change impacts on the historic environment, which had been commissioned by English Heritage’s far-sighted chief scientist Mike Corfield (Cassar 2005). It soon became clear that older buildings – those predating the Industrial Revolution – were designed to run passively, with no significant input of energy (either in the form of fossil fuels or otherwise). They were also well-equipped to cope with extreme weather. On the other hand, problems loomed for the ‘modern’ construction that had evolved in a high-carbon world (Calder 2021). Lightweight facades, overglazing, and a dependence on control of air temperature to achieve comfort not only made them expensive to run but failed to deliver the promised level of building usability. Alas, even the older buildings, which could be run passively, were being subjected to this high-energy approach to comfort.

The ‘performance gap’ between prediction and reality came as no surprise: heating and cooling the air is apt to produce draughts and other discomforts, and to drive fabric problems. This is as true of ‘modern’ as it is of ‘traditional’ construction. Indeed, the former is even more sensitive to consequent environmental problems such as condensation (English Heritage 2012).

I was also sceptical from the physics point of view because the building models were (and are) fundamentally flawed. They cannot take proper account of the myriad complexities of heat and moisture transport in real buildings inhabited by real people, with three-dimensional construction and real joints, real leaks, real weather… Nor did modellers seem to understand the thermal physiology involved in comfort (Pallubinsky et al. 2019). The interactions between the human body and its surroundings are highly complex, with the body not just acting as the primary energy source but reacting in elaborate ways (e.g. by altering the blood supply to the skin, initiating shivering, or sweating and panting…). Heat loss by radiation has been shoehorned into the concept of ‘mean radiant temperature’, but this not only assumes steady-state conditions and surfaces that are alike in every direction, but also takes heat exchange to be a simple matter of differences in surface temperatures.

Are we asking the right questions?

The underlying assumption was that thermal comfort is given by an absence of thermal transfer between the body and its surroundings. But why should it be so? What happens if you are active and need to lose body heat? And how do you reconcile comfort for multiple users of one space, all doing different things and with different preferences?

In 2006 I moved to English Heritage (which later became Historic England) to continue working on climate change and building performance (not least to support the writing of its new Practical Building Conservation series), just as the UK government began to seriously recognise the importance of reducing energy demand in the built environment. With increasing alarm I watched the focus shift from 20th-century cavity walls, cement construction, and lightweight architecture (with its high demand for energy) onto Victorian solid-walled houses, which were perfectly able to operate passively and were certainly not designed to be sealed and given additional insulation (English Heritage 2014). This is a fine way to make a good building fail. It also meant that the true causes of increased energy use since the Second World War were being entirely overlooked.

Researching the ways the human body lost heat, it became clear that radiant breaks were potentially even more effective than I had thought. The estimates put against proportional heat loss by thermal physiologists are only from 2-22% into the air (with the latter being reached only with wet skin exposed to a strong cold wind), but 60-65% via radiation into the surrounding surfaces (Hill et al. 1916, Hardy 1937, Luginbuehl & Bissonnette 2009). Clearly, wall cloths had the potential to make a huge difference to comfort in cold climates, entirely passively and with no adverse impact on the building or the occupants. But they seem to have disappeared as the Industrial Revolution began. The only modern use I have found is in Shaker villages in the United States, where they still hang cloth from the peg rails in winter; otherwise the use of radiant breaks has been entirely forgotten. The reverse – using radiant-absorbing surfaces to cool people in hot climates – is still remembered and exploited, perhaps because the less industrialised countries are those in hotter climates.

This seemed to me to be a highly important piece of the puzzle of pre-carbon comfort and thus of energy use, but I found it frustratingly difficult to get the messages across, even within the conservation sector.

A breakthrough finally came in an unexpected way. I was asked to make a short presentation to the UK government’s Department for Culture, Media and Sport (DCMS), so I wanted to find a painting of a medieval fireplace in use. My search revealed hundreds and hundreds of images of medieval interiors. Almost all showed not just the fireplace, but draperies on the walls. Dropping ‘fireplace’ from the search, cloths proved almost ubiquitous in artworks right up to the 18th century, all over the world. Embarrassingly, many paintings were familiar to me from my wall-painting days: I had (wrongly) assumed that the draperies were purely decorative. Every seated Virgin in an Annunciation had a cloth behind her on the wall – often a cloak hung on pegs. Other draperies were looped upwards to make canopies to stop heat loss upwards (see Figure 1). No wonder the cloth trade was of such international importance, even though people had far smaller wardrobes than we do these days.

<strong>Figure 1:</strong> Montage of paintings showing wall cloths used in different settings to provide thermal comfort by reducing heat loss by radiation.
Figure 1: Montage of paintings showing wall cloths used in different settings to provide thermal comfort by reducing heat loss by radiation.

The DCMS were as excited as we were by the potential of this simple intervention, and keen to talk about ways of promoting it as part of a retrofit strategy. Alas, within just a few weeks the first Covid-19 lockdown commenced, but in subsequent presentations to professional audiences the images have proved equally engaging. Several private researchers and universities in the UK and beyond are interested in investigating various aspects of wall cloths, from the history of interventions of this type (including how they are talked about in contemporaneous sources), to assessing impact on comfort. One challenge will be to find way of quantifying thermal comfort that does not rely on measuring air temperature. As others have said, humans are the best comfort sensors: they are just difficult to calibrate.

Lessons for the future

Images of medieval domestic interiors often show the windows were left open in winter, presumably for health reasons. This should not be interpreted as people preferring much draughtier buildings than we do now or that they were more stoical. Instead, it is very possible that, with wall cloths addressing the bulk of thermal discomfort, the air temperature mattered very much less. Moreover, they were not relying on radiant breaks alone: these were combined with personal heating such as hot bricks or small charcoal burners. This is something we could certainly manage much more effectively and safely nowadays. Heated seat cushions, anyone?

Wall hangings have enormous potential for reducing the use of heating energy, at the same time as making our buildings (new as well as old) more comfortable and very much cheaper to run. Just as importantly, it means we would not have to seal the buildings to lock in heated air, avoiding all the problems that entails. Taking down the cloths in summer (as they used to) and installing awnings to cut solar gain (as they also used to) plus ceiling fans and other forms of personal cooling, can form part of an effective passive approach to the warming climate.

The first step is clearly to raise consciousness of the impact of radiant heat loss on comfort. The simple experiment of trying the effect is very powerful, as Sarah Khan showed in her research at the Architecture Association for her award-winning dissertation. Perhaps most important is for engineers and building physicists to find an alternative to air temperature as a measure of comfort (and by extension as a measure of the success of interventions). It is very easy to measure, but a very poor proxy for what we want to know.

Providence has pushed me into finding something that really could help us tackle the climate emergency, and with the speed, low cost and lack of risk that we need. My odd concatenation of physics, building conservation and art conservation allowed me to rediscover wall cloths – albeit terribly slowly! There are clearly many more such discoveries to be made, especially if we look at the past and at vernacular construction with open eyes and true respect (Calder 2021, Pender 2021, Pender & Lemieux 2020). Conservation is often considered a sideline in buildings studies, and even ‘ivory-tower’ by professionals interested in retrofit, but the reality is that every building becomes historic the moment the architect and engineers walk away: it is the conservators who need to find out how to keep it operating in good order. How much more interesting my architecture studies would have been if they had begun with instilling a deep understanding for the history and materiality of buildings, and especially of what the original features tell us of how their builders intended them to be operated! Understanding how a building envelope works is crucial to designing good buildings, and preventing failures such as the Preston Green Deal, Arbed, and Grenfell Tower.

My advice to young researchers is to have more than one disciplinary background, and draw on everything you know, no matter how far it might seem from your current work. There may well be important connections that only you can make. You might even provide a whole new way of looking at our most pressing problems.

References

Calder, B. (2021). Architecture From Prehistory to Climate Emergency. London: Pelican.

Cassar, M. (2005). Climate change and the historic environment. London: Centre for Sustainable Heritage, University College London.

Cather, S., Danti, C., Matteini, M. & Moles, A. (1992). Le pitture murali: tecniche, problemi, conservazione. Studies in Conservation, 37, 65. https://doi.org/10.2307/1506441

English Heritage. (2012). Practical Building Conservation: Glass and Glazing. London: Routledge.

English Heritage. (2014). Practical Building Conservation: Building Environment. London: Routledge

Hardy, J.D. (1937). The physical laws of heat loss from the human body. Proceedings of the National Academy of Sciences, 23,  631–7. https://www.pnas.org/content/23/12/ 631

Hawkes, D. & Lawrence, R. (2021). Climate, comfort, and architecture in Elizabethan England: an environmental study of Hardwick Hall.  Journal of Architecture, 26 (6), 861 - 892.

Hill, L.E., Griffith, O.W. & Flack, M. (1916). The measurement of the rate of heat-loss at body temperature by convection, radiation, and evaporation.  Philosophical Transactions of the Royal Society B Biological Sciences 207 (1916), 183–220. https://royalsocietypublishing.org/doi/pdf/10.1098/rstb.1916.0005

Luginbuehl, I. & Bissonnette, B. (2009). Thermal regulation. A Practice of Anesthesia for Infants and Children, 4th ed. Philadelphia: Saunders/Elsevier, 557–67.

Pallubinsky, H., Schellen, L. & van Marken Lichtenbelt W.D. (2019). Exploring the human thermoneutral zone: a dynamic approach. Journal of Thermal Biology, 79, 199-208. https://doi.org/10.1016/j.jtherbio.2018.12.014

Pender, R. (2021). Making good decisions: avoiding alignment problems and maladaptation in retrofit and construction. Journal of Architectural Conservation. https://doi.org/10.1080/13556207.2021.1965759https://doi.org/10.1080/13556207.2021.1965759

Pender, R. & Lemieux, D.J. (2020). The road not taken: building physics, and returning to first principles in sustainable design. Atmosphere, 11, 620. https://doi.org/10.3390/atmos11060620

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