Ageing of rigid closed-cell cellular plastic is the key problem to be solved in developing new foamed insulation. Diffusion-tight facing is one of the strategies to slow down the ageing process. As an example, n-pentane-based foam insulation was selected to evaluate the possible improvement in thermal performance by using this strategy. It has been found that the average thermal conductivity increase after 10 years is only about 1 mW/m.K for a foam with perfect facing on upper and lower surfaces, whereas the corresponding value is about 4.5 mW/m.K for a commercially available foam with so called "diffusion tight" facing. Thus, further development in foam facing is recommended.