By Aman Hashmi.
Temperature is actually not a physical quantity but it can be thought of as a symptom-as the outward appearance of the thermal state of a body. If energy is conveyed to a body, the molecular movement within that body is increased and it appears to be warmer. Temperature is measured by the Celsius scale and heat exchange in buildings Just like the human body, the building can also be considered as a defined unit and its heat exchange processes with the out-door environment can be examined. The thermal balance, i.e. the existing thermal condition is maintained if: Qi + Qs ± Qc ± Qv ± Qm – Qe = 0 If the sum of this equation is less than zero (negative), the building will be cooling and if it is more than zero, the temperature in the building will increase.
The main function of the building envelope in hot climates is to minimize external heat stress. Indoor thermal control can only be achieved through understanding of the thermal performance of the building envelope in relation to relevant weather parameters. Most materials with high strength have relatively high density, and the strength of most building materials (e.g., concrete, wood, plastic) drops along with drops in their density. Hence, the need for low density (or more accurately, high porosity) reduces the structural capacity of most insulation. It is also becoming increasingly realized that much can be done to mitigate heat stress in unconditioned buildings and to reduce cooling and heating loads and the energy consumption of air conditioned buildings, through a proper choice of building envelope materials and envelope design.
When air is too humid, it needs to dehumidified to maintain occupant comfort. This dehumidification requires the removal of the latent heat and is an important function of HVAC systems. While less common, it is sometimes necessary to add humidity to buildings during very cold weather to compensate for the inability of colder air to hold moisture. Evaporation and condensation, although not usually listed as modes of heat transfer, represent the primary means by which latent heat is transfer and are an important determinant of human comfort.
Accordingly, low-density insulation layers—such as glass fiber , and foamed plastics—are used to control heat flow in most modern building enclosures, while high-density, high-strength, high-conductivity materials such as steel studs, and concrete are used to support structural loads. In the past, building materials such as adobe, log, and low-density brick were used in a manner that combined both moderate insulating and acceptable load-bearing functions. Buildings constructed of these materials had thick walls, both to provide a reasonable level of resistance to heat flow and to provide sufficient strength.
Rockwool fiber is thicker than glass (since the spun rock contains more impurities than glass), and hence conduction plays a larger role at lower densities. Rockwool products therefore tend to use higher densities to achieve the same thermal performance as spun glass. This extra density provides these products with greater strength and more resistance to convection and radiation effects. Even though more material is used for the same thermal resistance, rock wool products compete in applications that require these properties.
Buildings lose sensible heat to the environment (or gain sensible heat from it) in three principal ways:
1) Conduction: The transfer of heat between substances which are in direct contact with each other. Conduction occurs when heat flows through a solid.
2) Convection: The movement of gasses and liquids caused by heat transfer. As a gas or liquid is heated, it warms, expands and rises because it is less dense resulting in natural convection.
3) Radiation: When electromagnetic waves travel through space, it is called radiation. When these waves (from the sun, for example) hit an object, they transfer their heat to that object.
The Solar Heat Gain Coefficient (SHGC) is the window property used to rate the amount of energy allowed through windows. The SHGC is the fraction of incident solar radiation that passes through a window and becomes heat inside the building. For example, if the SHGC of a glazing unit is 0.50, and the sun is shining on the window with an intensity of 500 W/m2, 250 W/m2 will enter the building. The lower the SHGC, the less solar heat that the window transmits through and the greater its shading ability. In general, south-facing windows in houses designed for passive solar heating (with a roof overhang to shade them in the summer) should have windows with a high SHGC to allow in beneficial solar heat gain in the winter. East or west facing windows that receive large amounts of undesirable sun in mornings and afternoons, and windows in houses in hot climates, should have a low SHGC.
Solutions to control this form of thermal control include reduced window area, projecting horizontal shading (most effective on the south), exterior operable vertical shade, and solar control coatings on windows. Interior shades have a relatively small impact, but have the important role of controlling glare and providing privacy. Passive solar heating design is used to capture the heat of the sun in a beneficial manner—this requires that most of the windows face south, and that window area be limited to collect only as much energy as needed for heating and to warm storage. Modern passive buildings have better control of the thermal losses in cold weather and hence have almost normal ratios of window to wall area.
Interior Heat Gains
In a well-insulated building, the interior heat generated by occupants and activities can be quite important. In cold weather, this interior heat offsets the heat required to warm the space. In warm climates this heat adds to the cooling load. In smaller buildings (or buildings with a large enclosure surface area to interior floor area ratio) such as housing, interior heat gains do not play a large role in most cases. Only in very well insulated homes or mild heating weather (i.e., around 10 ºC or 50 ºF) do interior heat gains form a significant proportion of heat flows in a small building .Large boxy buildings (that is, those with a small ratio of enclosure surface area to floor area) are often dominated by internal heat gain.
Thermal flow in properly insulated commercial office buildings generally is dominated by heat gain and loss through windows at the perimeter (that is, within about 30 feet of the perimeter) and by interior heat gains in the core. By employing moderate areas of high performance (U<0.3 or 1.25 W/m2ºC) windows in a well insulated opaque enclosure, many commercial buildings will require little or no heating in below freezing weather when occupied.
All in all,it can be generally concluded that the control of heat flow in buildings requires insulation layers penetrated with few thermal bridges, an effective air barrier system, good control of solar radiation, and management of interior heat generation.
Other special glasses Whilst the heat absorbing glasses achieve a selective transmittance by selectivity in absorption, the heat reflecting glass achieves a similar selective transmittance by selectivity in reflection. The glass is coated by a thin film of metal (usually nickel or gold), applied by vacuum evaporation. Such glasses absorb very little heat, therefore the improvement in reducing the total solar gain is far greater, but unfortunately they are still rather expensive. Recently, several types of photo chromatic or light-sensitive glasses have been developed, containing submicroscopic halide crystals, which turn dark when exposed to strong light and regain their transparency when the light source is removed. Their transmittance may thus vary between 74 and 1%. When the technique is more developed and more economical, these glasses may have a future in solar control.