This Technical Report supplements technical information on modelling of heat transfer in products of high and medium thermal resistance when the thickness effect may be relevant; by doing this it supplies minimum background information on the interpolating equations to be used in the procedures described in EN 12939 to test thick products of high and medium thermal resistance.All testing procedures to evaluate the thermal performance of thick specimens require utilities, which are essentially based on interpolating functions containing a number of material parameters and testing conditions. Interpolating functions and material parameters are not the same for all materials. Essential phenomena and common interpolating functions are presented in Clause 4, which is followed by separate equations for each material family.This Technical Report also gives diagrams derived from the above interpolating equations to assess the relevance of the thickness effect for some insulating materials.