Thermal insulation is defined as the reduction of heat transfer or as a method of preventing heat from escaping or entering an object. It helps in maintaining a certain temperature in a particular area.
Thermal insulation seeks to reduce the heat flow between two objects or areas of different temperatures. The property of a material to withstand heat flow is known as thermal resistivity (r). This is a reciprocal of thermal conductivity (k or which is the property of the material to conduct heat.
Thermal resistivity is measured in m.K/W in SI units and multiplying this with the thickness of the material gives us the thermal resistance R in m2.K/W. Adding all the thermal resistances for a particular barrier i.e. wall, roof or floor and taking its reciprocal gives us the U value known as thermal transmittance.