The substances which are used to retard the heat transfer between cold spaces and warm surroundings are commonly known as thermal insulating materials they isolate the warm surrounding from cold refrigerated spaces. The properties of the thermal insulating materials which are used in refrigerating systems to insulate refrigerated spaces from surroundings can be grouped as:- 

(1) Thermal properties 

(2) Mechanical properties 

(3) Physical properties

(1.) Thermal properties:- The ability of a material to retard the flow of heat is given by its thermal conductivity. The thermal conductivity is a property of a homogeneous material which changes with the variation of density. A compressible substance such as glass wool, if loosely packed, is a better insulator than if closely packed. The other important thermal properties are specific heat capacity, thermal diffusivity etc. 

(2.) Mechanical properties:- The mechanical properties of an insulating material includes strength in compression, tension, shear, impact and flexure. These properties become important when some insulating materials are also to be used as load bearing floors and to form self-supporting partitions. 

(3.) Physical properties:- These properties include permanence, low odour level, moisture resistance, safety to health, inflammability, repellence to insects & vermin.