HEAT CAPACITY of BUILDINGS
Post 336 ⇒ by Gautam Shah →
Heat capacity is very important property of building materials. It is a dynamic property that is proportional to the size of the system, its mass, and how and when it flows in and out the material body. The building is affected how the materials constitute the shell, and what is the time relevance of the space in it. In other words, the shell and its space use both together determine Where and When the activities take place. The shell, the space and its environment also define How the activities flourish.
Heat capacity or Thermal capacity, is a measure that reflects the ratio of the heat added or subtracted from an object, to the resulting temperature change. The heat capacity is divided by the amount of substance, mass, or volume, so that the quantity is independent of the size or extent of the entity. Specific heat is the heat capacity per unit mass of a material. In very simple terms heat capacity gets doubled, when the amount of substance of a body is doubled.
Specific heat capacity shows how much heat a body can store. So a building shell made up of materials with high specific heat capacity, high density, and thermal conductivity will function well, if it is occupied with a suitable space occupation and appropriate time scheduling of activities.
Heat is conducted through various building elements, each having different capacity to store heat. Concrete and masonry have large capacity to heat. Wood is too slow in absorbing heat, but steel is very fast in warming up. Once the material object reaches certain level of heat storage, it begins to radiate the heat to the interior or exterior (or other adjoining entities) depending on the temperature conditions. On the exterior surface, if solar radiation and air temperatures are low, the heat will radiate to the outside. Alternatively, when interior conditions are comparatively cooler, the object will radiate heat on that face.
Typically in a hot climate area a building with substantially heavy masonry and concrete begins to warm up after sunrise and continues to absorb heat till 1100 to 1500 hours (depending on the solar temperature, wall density, etc.). Thereafter it begins to radiate the heat to the interior side (as exterior side may remain warm till 1800 hours). One can stay indoors by allowing breeze to evacuate the heat through openings, or shift to indoor cooler sections, or outdoor shaded areas. One may not be able to use the interiors till about 2100 hours, or later till the roof and walls have cooled down. (The cooling of the shell occurs with release of heat on interior as well on exterior face).
This cycle could change in a thin body sheet roof or wall building. The structure gets warm very fast, often within 1 hour of sunrise, making it nearly impossible to stay indoors. The thin body structure, however, cools down within 1 hour of sunset time, allowing quicker reoccupation of the interior space.
Very heavy body structures are economically not very viable. Heavy roof buildings (like stone, concrete, slate slabs, rammed earth terraces, etc.) are hazardous in earth quake prone zones. Thin body structures are faster to erect and economical. Light roof buildings (like of a tin or cement sheet, ceramic tiles, etc.) unless well framed get blown off in hurricanes.
The heat exchange process in the interior side of a building primarily depends on the form (layout) and materials of the building and the external environment. It secondarily depends on the colour and texture of the external surfaces, size, location nature (glazed, solid, etc.), shading devices and placement of openings. It also depends on evaporation of water, heat generating elements like cooking hearth, lights, workshop equipments, and density of human occupation.