|Natural Energy and Vernacular Architecture: Principles and Examples with Reference to Hot Arid Climates (UNU, 1986, 172 pages)|
|Part 1. Man, natural environment, and architecture|
|2. Architectural thermodynamics and human comfort in hot climates|
The concepts of thermal conduction and resistance are important in attempting to provide a comfortable environment for the inhabitants of hot, arid regions. These heat-flow concepts are based on the movement of a quantity of heat.
The specific heat of a substance is the quantity of heat energy required to raise the temperature of one unit mass of the substance by one degree of temperature.
When considering heat-flow concepts, the notion of rate of heat flow is useful. It equals the rate of displacement of a quantity of heat.
Conduction is the process by which heat flows through a material, or from one material to another with which it is in contact. Some materials, such as metals, are good thermal conductors, while others, like air, are poor thermal conductors. Thermal conductivity is a specific property of a material and is a measure of the rate at which heat will flow through a material when a difference in temperature exists between its surfaces. It is defined as the quantity of heat that will flow through a unit area in a unit time, or equivalently, as the rate of heat flow through a unit area, when a unit of temperature difference exists between the faces of the material of unit thickness, such as the wall shown in figure 1. The thermal conductivity varies with the density, porosity, and moisture content of the material and also with the absolute temperature. The quantity of moisture contained in a material can have a considerable effect on the thermal conductivity of the material; the higher the moisture content, the greater the thermal conductivity. This is important because rain penetration, high humidity within a building, and condensation may result in an appreciable amount of moisture in the building structure. The average temperature of a material is another factor influencing the rate of heat flow; the thermal conductivity may be considerably greater at high than at low temperatures. However, the variation of the thermal conductivity over the range of temperatures commonly occurring in buildings is comparatively small, and thus the thermal-conductivity values measured at normal atmospheric temperature are generally used when considering structural insulation.
In calculations, it is often convenient to use the reciprocal of the thermal conductivity which is called the thermal resistivity. The thermal resistivity may be regarded as either the time required for the transmission of one unit of quantity of heat through one unit area of a rectangular solid material of unit thickness, when the difference between the temperatures of the surfaces perpendicular to the direction of heat flow is one degree of temperature; or the number of degrees difference between these surfaces of the material of unit thickness when one unit of quantity of heat flows through one unit area in one unit of time. Thus resistivity, like conductivity, is a property inherent to a material and is independent of its thickness.
The thermal resistance is a measure of the resistance to heat flow of a material or a combination of materials. The thermal resistance may be regarded as either the time required for the transmission of one unit of quantity of heat through one unit area of material when the temperature difference between surfaces perpendicular to the direction of heat flow is one degree of temperature; or the number of degrees difference in temperature between these surfaces when one unit of quantity of heat flows through one unit area in one unit time. If the thickness of the material is increased there is a corresponding proportional increase in its thermal resistance. If several materials are placed together in layers, as, e.g., in a plastered and rendered solid brick wall, as illustrated in figure 2, the total thermal resistance of the wall may be obtained by adding the resistances for each component, i.e., of the plastering, rendering, and brick masonry.
The thermal conductance is the rate of heat flow through a material or a combination of materials and is therefore the reciprocal of the thermal resistances. The thermal conductance is the quantity of heat that will flow per unit time per unit area of a material for a one degree temperature difference between its surfaces. If the thickness of the material is increased, its conductance decreases proportionately.
The thermal conductance and resistance and thermal conductivity and resistivity already considered have been related to the tempera tures at the material surfaces. The surface temperatures of a building usually are not known. For purposes of heat-loss calculations, therefore, the inside and outside air temperatures are used. In this situation, heat transfer from the warmer to the cooler air mass occurs in three steps: first from the warmer air to the structure, then through the structure, and finally from the structure to the cooler air. Both the inside and outside air-surface interfaces provide some resistance to heat flow.
The thermal transmittance includes these surface resistances and is the rate per unit area at which heat will flow from the air on one side of the structure to the air on the other side. It may be defined as the quantity of heat that will flow per unit time per unit area through the material when one unit of temperature difference exists between the air on each side. In fact, the thermal transmittance may be regarded as the overall air-to-air conductance, which is the reciprocal of the overall air-to-air resistance. The thermal transmittance is of considerable practical importance. It provides a basis both for comparing the insulating capabilities of different wall, floor, and room constructions; and for calculating heat loss from a building for heating purposes in cold climates, and heat gain for cooling purposes in hot climates.