The use of blowing agents as an alternative to conventional chloro-fluorocarbons (CFCs) and the quality of foaming process can lead to non-homogeneity of the foam structure with the presence of defects along the panel thickness. These defects appear as holes in cell windows or cavities. Accurate measurements of thermal conductivity were made on test specimens having different thicknesses obtained by cutting away the external faces of the panels.This type of characterization of foams blown with alternative blowing agents allowed us to understand their ageing behaviour (natural and accelerated) and the problems related to the scaling factor. Moreover this study pointed out how the effective diffusion coefficients of gases involved in the ageing process can have different values along the thickness of a sample. All these observations were related with morphological analyses.Our studies showed that sampling procedure and model design are very important for a correct analysis of ageing phenomena. Good agreement between experimental and theoretical data was found, allowing us to verify the validity of the method used to characterize non-homogeneous foams.