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Building with Earth and the Meaning of Thermal Inertia

Building with Earth and the Meaning of Thermal Inertia 

 

Clément Gaillard

 

In 1821, Joseph Fourier published The Analytical Theory of Heat, ten years after the presentation of his work on heat flow at the Academy of Science in Paris. This publication was the groundwork of what later became thermal analysis, which has been used since then in every field of engineering, and particularly in building design. In the preliminary discourse of this ground-breaking book, Fourier emphasised the universality of thermal phenomena. Prigogine and Stengers state that Fourier’s theory of heat introduced “a new kind of science”, as universal as the Newtonian system, but remarkably simpler and understandable.1  After Fourier, all thermal phenomena have been explained in terms of gradient and irreversibility.2

Following this intuition, I want to develop the meaning of thermal analysis by studying an underrated thermal property of massive buildings called “thermal inertia”. This complex property can be easily explained in temporal terms and this dimension will be central to my discussion. This paper will study earth as a construction material and try to define how this material illustrates the thermal inertia property and the temporal dimension of this concept. Nowadays, we tend to consider that modern buildings need to be strongly isolated from thermal variations of the environment, merely due to climate and temperature swings. However, as the architecture historian Kiel Moe explained recently, the idea that a building needs to be isolated can be explained historically as a result of a false analogy between a building and refrigeration apparatus.3 For him, the refrigeration industry and contemporary building practice both illustrate “strategies that isolate interior from exterior through envelope design, coupled with a heat pump to manage internal convection”.4 As a matter of fact, when insulated or not, a building continuously exchanges thermal energy through its surface and to consider it as an isolated object is a huge simplification. Moe argues that buildings “are not thermos bottles” and “literal isolation is thermodynamically impossible to achieve in the context of buildings”.5

Earth cannot be defined as an insulative material from a thermal viewpoint but instead has multiple interesting thermal properties, the main one being thermal inertia. Earth also has the property to regulate indoor humidity by absorbing and releasing moisture. In order to be used as a construction material, earth must be composed of a fixed proportion of clay, sand and silt.6 This mix is called loam and needs to be mixed with other materials as straw or gravels to have mechanical strength. By adding water, this mix becomes plastic and can be processed using different techniques which depend of the country and local traditions. It can be placed in wooden forms and pressed to form bricks (adobe) or directly tamped in a formwork to build a wall (rammed earth or “pisé” in France). These techniques allow very thick earth walls to be made and there are examples of earth construction with nearly one meter/three foot-thick walls in vernacular architecture. As we will see, traditional troglodyte buildings, made by digging in the lœss have much thicker walls. By contrast, for structures with such massive building techniques, which use industrial processes and add lime, earth can be used as a thin board of about two centimetres (roughly one inch). As it is still the cheapest raw material of construction, it seems as if earth could now be used in light buildings, as well as massive ones. Earth can therefore provide a great variety of thermal behaviour when used in a building.

The “thermal behaviour” of a building expresses the way it loses and gains heat and therefore provides comfortable indoor temperatures to inhabitants. It represents the exchanges of thermal energy between indoors and outdoors, as a building is continuously exposed to varying climate elements, the main one being the intensity of sunlight. As described by the architect Baruch Givoni, the propagation of a heat wave into a construction element can be easily represented by mentally slicing this element into smaller layers. When the temperature of one of the surfaces of these element rises, thermal energy will flow and “fill” the first layer.7 When this first layer is at the same temperature as the surface, the heat flow progresses to the adjacent layer and so on, as long as there is a thermal gradient between the two surfaces of the element.2 The study of this type of heat flow – called conduction – occupied many scientists during the 18th century,8 and was clearly explained by Fourier at the beginning of the 19th century.

Following on from Fourier’s work, many engineers like Nessi and Nisolle in France or Mackey and Wright in the U.S.A. kept working on the mathematical analysis of the conduction of heat in constructions during the first half of the 20th century, in order to define more accurately the thermal behaviour of buildings using massive materials. They were preoccupied by the fact that the temperature of the surface of an element constantly varies over time, and consequently modifies the thermal gradient between the interior and the exterior surface, as well as the heat flow between them. Mackey and Wright developed what they called the “sol-air temperature” in order to consider the thermal effects of a building’s surrounding environment (sun, ground reflection, etc.) on part of a building.9 By introducing the “sol-air temperature” and the calculation of thermal inertia, Mackey and Wright mathematically explained that massive material like earth has the property of delaying the conduction of heat flow, but also reduces the external thermal amplitude create on the outside surface due to sunshine exposure. 

The thermal advantages of massive materials like earth were intuitively known way before the birth of thermal analysis. In a temperate climate like in France, the architect François Cointeraux (1740-1830) promoted earth construction for its thermal and economic advantages. Vernacular and traditional Indian Pueblo architecture in New Mexico10 or Nubian vaulted houses in Egypt11 provided fair comfort in hot-arid climates, because they were built with thick earth walls and roofs. Those climates are characterised by their great thermal amplitude: the diurnal thermal range can be as much as 22 °C (40 °F).12  Massive earth walls help reduce such variations. The thickness of traditional earth walls, more than fifty centimetres (nearly 20″), allows the heat to be transmitted with a roughly twelve-hour delay, so the earth structure slowly conducts heat during during day and the cold nights. At sunrise, the structure is cool and has achieved thermal equilibrium, then it heats slowly during the whole day, providing cool interior temperatures for the inhabitants.

Although earth was widely used on an intuitive basis, the work of Mackey and Wright and other engineers allowed the property of massive materials as earth – called thermal inertia – to be defined mathematically. Their work was synthesised by thermal engineers and architects during the 1950s, like the French engineer Jacques Dreyfus who worked in a tropical country in Africa in the middle of the 20th century. Dreyfus was a specialist of earth construction and conducted many theoretical studies on thermal inertia, based on Mackey and Wright’s work. He defined thermal inertia as the property of a material to slow down the transmission of the thermal wave through its thickness – thus creating what thermal engineers called a “thermal lag” or a “phase shift” – and to reduce inside temperature variations by dampening the thermal wave.13 These two properties that characterise thermal inertia can be accurately estimated by the calculation of what engineers call “thermal diffusity”14 which depends on the thermal conductivity of the material, its mass and its specific heat capacity. There is a robust analogy between the flow of heat through a massive wall and the flow of water through a dam: they both retain this flow and regulate its input variations so that output remains constant.15 As the architect Jean-Louis Izard has stated: “in the event of a flood, the dam fills up, thus reducing the downstream flow; if the flood follows a low flow (low water), the water stored by the dam is used to supply the downstream flow”.2 Like a dam, a massive wall regulates the diffusion of heat. Thus, increasing the thermal inertia of a building allows it to store a greater quantity of thermal energy. 

A convenient way to understand thermal inertia is to calculate or measure the thermal lag in hours. Dreyfus proposed many charts in order to design a structure with a proper thermal lag in tropical, arid climates. Choosing the proper thermal lag implies anticipating when the outdoor thermal energy will be beneficial to the indoor comfort. Dreyfus gave the following values: an earth wall of 10 cm (4″) has a thermal lag of 2 hours 40 mins; with 20 cm (8″) it is 5 hours 25 mins, with 30 cm (12″) it would attain 8 hours 10 mins, and so on.16 But designing by taking thermal lag into consideration is tricky because the sol-air temperatures of the different façades and the roof vary throughout the day. Locating a bedroom behind a west-facing wall with a 6 hour time lag would mean that it radiates thermal energy in the middle of the night, at the most inappropriate moment.17 The simplest solution is to increase the thickness of such a wall, in order to expand the thermal lag and delay the heat transmission. This implies designing a construction with different materials and thickness for each facade.

 After the work of Wright, Mackey and Dreyfus, the importance of thermal inertia was widely accepted by architects who worked in hot-arid climates. The famous Egyptian architect Hassan Fathy conducted experiments on earth construction techniques in Egypt and promoted these techniques in his book called Architecture for the Poor published in 1969. He emphasized the thermal qualities of earth in the Egyptian climate compared to much more expensive materials like concrete.18 The main difference between earth and other massive materials like concrete or stone is that there is theoretically no limit to the thickness of earth walls, because earth is a very cheap raw material. Increasing the thickness of an earth wall can add more and more thermal inertia and therefore reduce indoor thermal amplitude to a point where isothermal conditions may prevail. In theory, it is possible to build an earth wall of infinite thickness.

In practice, increasing the thickness of an earth wall up to infinity can be done by digging in loess soils,19 which are made of a soft sedimentary rock composed of silt, one of a three basic components of loam. There are plenty of examples of very thick loess walls and roofs in troglodyte houses and underground vernacular architecture (Tunisia, Spain, etc.). In China, in the Shaanxi province, troglodyte houses made by digging in the loess soil have at least three-metre-thick roofs and walls of unmeasurable thickness, and with infinite thermal inertia.20 At this depth, very little temperature variations may be felt. Although outside temperature may be as low as –20 °C (–4 °F) with a diurnal thermal range of more than 20 °C (36 °F), the temperature inside the troglodyte houses remains close to 9 °C (48 °F) throughout the year and can be warmed easily by a small furnace.21 

Such thermal performances of underground buildings can be calculated with the methods used by thermal engineers. Like a wall, the temperature of the earth’s surface is subjected to diurnal and seasonal variations due to the heat balance between the inflow and outflow of radiation. If we consider these variations as sinusoidal for the sake of simplification, it is possible to calculate at which depth these seasonal thermal variations are unnoticeable. In a book about insulative materials, Claude Huraux made some of these calculations.22 He found that if you consider the thermal diffusivity of the ground, at a depth of about nine meters (approximately thirty feet) the temperature is higher in winter than in summer.23 This means that at this depth there is a six-month phase shift (or thermal lag) of the thermal wave, as well as a reduction of the annual temperature range. At a depth of about twenty meters (approximately sixty-five feet) these seasonal variations become unnoticeable and the temperature is in a steady-state throughout the year. In architectural terms, this calculation means that the temperature inside an underground house with twenty-metre-thick walls will remain the same independently of the temperature variations outside. In practice, very little temperature variation occurs in underground or vernacular troglodyte buildings with thinner earth roofs and walls. Measures show that at a depth of five meters (approximately sixteen feet) in temperate climates, only very little temperature variations of 1° to 2 °C (2° to 4 °F) occurs, which remain practically unnoticeable.24

By considering thermal inertia, a built structure can be defined by the thermal lag it produces. Rather than seeing a massive structure as a static element, it appears that it has a thermal function which is to delay and reduce indoor thermal variations. The engineer Dreyfus considered thermal inertia as a form of “memory”.25 He stated that a massive construction “records everything it receives” and “any amount of heat absorbed by the walls will influence comfort for the next 10 hours or more”.2 Following this metaphor, massive earth walls have greater thermal memory than light ones. Other thermal engineers, like Gilles Olive, talk about the “thermal past” of massive structures, and the way this “past” influences the thermal present state of a structure.26 All these metaphors have a profound meaning as they insist on the time dimension of thermal phenomena.

But understanding thermal inertia is not the sole preserve of thermal engineers and architects, because our skin gives us all a feeling of this phenomenon. We commonly think that air temperature expresses accurately the thermal comfort zone. But our bodies are very sensitive to the temperature of surfaces that surround us (ground, walls, etc.). As a first approximation, these surfaces radiate thermal energy when they are hotter than our skin and absorb our body thermal energy when they are colder.27 Therefore, indoor thermal comfort depends greatly on the temperature of these surfaces, which is why we feel cold in a room with a very acceptable air temperature, but in which we are at the same time surrounded by colder surfaces, like single glass window in winter for instance. In fact, our skin cannot measure temperature. It can only measure very accurately the rate at which our body loses or gains thermal energy.28 Physiologist have found that this measure can be very precise.2 The faster we lose thermal energy, the colder we feel, and the faster we gain it, the hotter we are. All these energy exchanges depend greatly on the materials of the surrounding surfaces which have a certain thermal inertia, emissivity and effusivity. The fact that our bodies measure the rate of inflow or outflow of heat means that we intuitively understand thermal phenomenon in a time perspective.

Moreover, by considering the relationship between thermal comfort and thermal inertia, I also want to stress that choosing the right materials in architecture and urban planning could be decisive given global warming. The urban heat island effect, known since the 19th century, is principally caused by the thermal inertia of all building materials and their impermeability. Therefore, the thermal property that can be very beneficial has disastrous consequences on urban thermal comfort for a longer period each year. Every urban inhabitant has felt the warm mineral surfaces that radiate heat during summer evenings. But the urban heat island effect is not a fatality. It forces urban planners, architects and thermal engineers to consider and tackle this phenomenon, by choosing more porous and reflective materials. For them, as for all urban inhabitants, understanding thermal inertia means learning about the active role of surfaces and materials by feeling the different thermal lags they produce. These thermal lags are time signals. They illustrate the near past and the future thermal behaviour of a material. By considering the meaning of thermal inertia, I have wanted to show here that thermal phenomena can be explained precisely in clear terms, without using complex mathematical calculations, and that they have a direct influence on our daily lives. Just as there is a “poetics of space”,29 I believe that there is also a poetics of time in thermal phenomena.

 Bibliography :

– Bachelard, Gaston. Étude sur l’évolution d’un problème de physique : la propagation thermique dans les solides. 3rd ed., Bibliothèque des textes philosophiques, (Paris : Vrin, 2016).

– Bachelard, Gaston.The Poetics of Space, 2nd ed., trans. Maria Jolas, Boston: Beacon Press,1994.

– Bardou, Patrick and Varoujan Arzoumanian. 1978. Archi de terre. Roquevaire: Parenthèses.

– Dreyfus, Jacques. Le Confort dans l’habitat en pays tropical.  La Protection des constructions contre la chaleur. Problèmes de ventilation. Paris: Eyrolles, 1960.

– Fathy, Hassan. Architecture for the poor: An Experiment in Rural Egypt. 4th ed., Chicago: University of Chicago Press, 2000.

– Fourier, Joseph B. The Analytical Theory of Heat. Cambridge: Cambridge University Press, 1878.

– Givoni, Baruch. L’Homme, l’architecture et le climat. Paris: Le Moniteur, 1978.

– Huraux, Claude. Les Isolants. Que-sais je? Paris: PUF, 1968.

– Koenigsberger, Otto et. al., Manual of Tropical Housing and Building. Part 1: Climatic Design, Londres: Longman, 1974.

– Mackey, Charles O. and Lawrence T. Wright Jr. « Summer Comfort Factors as Influenced by Thermal Properties of Building Materials ». New York: John B. Pierce Foundation, 1943.

– Missenard, André. La Chaleur animale, Que sais-je? Paris : PUF, 1969.  

– Moe, Kiel. Insulating Modernism: Isolated and Non-isolated Thermodynamics in Architecture. Basel: Birkhäuser, 2014.

– Izard, Jean-Louis and Alain Guyot. Archi bio. Roquevaire: Parenthèses, 1979.

– Loubes, Jean-Paul. Archi troglo. Roquevaire: Parenthèses, 1984.

– Olive, Gilles. « Hygrothermique des enveloppes », Techniques & Architecture, n° 315 (1977): 49-51.

– Prigogine, Ilya and Isabelle Stengers. Order out of Chaos: Man’s New Dialogue with Nature. New York: Bantam Books, 1984.

  1. Ilya Prigogine and Isabelle Stengers, Order out of Chaos: Man’s New Dialogue with Nature (New York: Bantam Books, 1984), 104. []
  2. Ibid. [] [] [] [] []
  3. Kiel Moe, Insulating Modernism: Isolated and Non-isolated Thermodynamics in Architecture(Basel: Birkhäuser, 2014), 11-15. []
  4. Ibid., 38. []
  5. Ibid., 14. []
  6. Patrick Bardou and Varoujan Arzoumanian, Archi de terre (Roquevaire: Parenthèses, 1978), 5-6. []
  7. Baruch Givoni, L’Homme, l’architecture et le climat (Paris: Le Moniteur, 1978), 140. []
  8. Gaston Bachelard, Étude sur l’évolution d’un problème de physique : la propagation thermique dans les solides, 3rd ed., Bibliothèque des textes philosophiques, (Paris : Vrin, 2016). []
  9. Jacques Dreyfus, Le Confort dans l’habitat en pays tropical.  La Protection des constructions contre la chaleur. Problèmes de ventilation (Paris: Eyrolles, 1960), 131-133. []
  10. Patrick Bardou and Varoujan Arzoumanian, Archi de terre, 31-34. []
  11. Hassan Fathy, Architecture for the poor: An Experiment in Rural Egypt, 4th ed., (Chicago: University of Chicago Press, 2000), 45-49. []
  12. Otto Koenigsberger et. al., Manual of Tropical Housing and Building. Part 1: Climatic Design, (Londres: Longman, 1974), 27. []
  13. Jacques Dreyfus, Le Confort dans l’habitat en pays tropical, 77-80. []
  14. Ibid., 80-83. []
  15. Jean-Louis Izard and Alain Guyot, Archi bio, (Roquevaire: Parenthèses, 1979), 16. []
  16. Jacques Dreyfus, Le Confort dans l’habitat en pays tropical, 83. []
  17. Ibid., 142. []
  18. Hassan Fathy, Architecture for the poor: An Experiment in Rural Egypt, 45-46. []
  19. Jean-Paul Loubes, Archi troglo, (Roquevaire: Parenthèses, 1984), 17. []
  20. Ibid., 70. []
  21. Ibid., 69. []
  22. Claude Huraux, Les Isolants, Que-sais je? (Paris: PUF, 1968), 70-71. []
  23. Ibid., 71. []
  24. Jean-Paul Loubes, Archi troglo, 117. []
  25. Jacques Dreyfus, Le Confort dans l’habitat en pays tropical, 179. []
  26. Gilles Olive, « Hygrothermique des enveloppes », Techniques & Architecture, n° 315 (1977): 50-51. []
  27. André Missenard, La Chaleur animale, Que sais-je? (Paris : PUF, 1969), 13. []
  28. Ibid., 49. []
  29. Gaston Bachelard, The Poetic of Space, 2nd ed., trans. Maria Jolas (Boston: Beacon Press,1994). []