Thermal insulation

The term thermal insulation refers to materials used to reduce the rate of heat transfer. Thermal insulation is the method of preventing heat from escaping a container or from entering the container. Heat is transferred from one material to another by conduction, convection or radiation. Insulators are used to minimize the transfer of heat energy.

If the air outside is cold, you may want to protect your skin by wearing clothes that keep the cold out and the body warmth in. In any location where there are materials of two drastically different temperatures, you may want to provide an insulating barrier to prevent one from becoming the same temperature as the other. In such situations, the effort is to minimize the transfer of heat from one area to another.

Thermal insulators are materials specifically designed to reduce the flow of heat by limiting conduction, convection, or both. Radiant barriers are materials which reflect radiation and therefore reduce the flow of heat from radiation sources. Good insulators are not necessarily good radiant barriers, and vice versa. Metal, for instance, is an excellent reflector and poor insulator. Only a small fraction of radiant energy is absorbed by such a material, most being reflected back away. The effectiveness of a radiant barrier is indicated by its reflectivity, which is the fraction of radiation reflected. A material with a high reflectivity has a low emissivity, and vice versa. An ideal radiant barrier would have a reflectivity of 1 and would therefore reflect 100% of incoming radiation. Vacuum bottles (Dewars¹) are 'silvered' to approach this. In space vacuum, satellites use multi-layer insulation which consists of many layers of aluminized (shiny) mylar² to greatly reduce radiation heat transfer and control satellite temperature.

Most gases including air are poor conductors, but good insulators. Conductive barriers often incorporate a layer or pockets of air to reduce heat transfer. Conductive heat transfer is largely reduced by the presence of the air-filled spaces.

Convective heat transfer occurs between two objects separated by a moving interface of liquid or gas. The physical properties of the fluid or gas and the velocity at which the molecules travel influence the rate of transfer. Convection can be reduced by dividing the convective medium into small compartments to prevent large currents from forming.

The effectiveness of an insulator is indicated by its resistance value (R-value). The R-value of a material is the inverse of the conduction coefficient multiplied by the thickness of the insulator. Rigid fiberglass, a common insulation material, has an R-value of 4 per inch, while poured concrete, a poor insulator, has an R-value of 0.08 per inch.

Notes to the text:

1.Dewar ['djuə ] - a double-walled flask of metal or silvered glass with a vacuum between the walls, used to hold liquids at well below ambient temperature; named after Sir James Dewar.

2. Mylar ['mʌ ɪ lɑ: ] - a form of polyester resin used to make heat-resistant plastic films and sheets. It is made by copolymerizing ethylene glycol and terephthalic acid (терефталевая кислота); was produced in 1950s.