Materials or the methods and process used to reduce the rate of heat transfer are referred as Thermal insulation.

Heat energy can be transferred from a hotter object to a cooler object by conduction, convection and radiation. The flow of heat can be reduced by addressing one or more of these mechanisms and is dependent on the physical properties of the material employed to do this.

Insulation is used to minimize the transfer of heat energy. It acts as a barrier to heat flow and is essential to keep an enclosed area such as a building warm in winter and cool in summer. A well insulated and well designed home will provide year-round comfort and can reduce energy use for heating and cooling.

Units and Measures

The efficiency of insulation is determined by its thermal resistance or R value. The R-value of a material is the inverse of the conduction coefficient multiplied by the thickness of the insulator. Under uniform conditions it is the ratio of the temperature difference across an insulator and the heat flux (heat flow per unit area) through it. The bigger the number, the better the effectiveness of the insulation.

Following are the units for measuring thermal mass:

  • SI Units: square-metre kelvins per watt (m²•K/W) or square-metre degree Celsius per watt (m²•°C/W)
  • Imperial Units: Square feet degree Fahrenheit hour per British thermal unit (ft²•°F•h/Btu)

It is particularly easy to confuse SI and Imperial R-values, because R-values both in the US and elsewhere are often cited without their units, eg R-3.5. Usually, however, the correct units can be inferred from the context and from the magnitudes of the values.

Example Values

  • Snow - R-1
  • Cardboard - R-3
  • Rock and slag wool - R-2.5
  • Polyethylene foam - R-3

Useful References


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