Compressive creep of insulating materials below ground. The compressive creep deformation for different types of insulating materials used below concrete ground plates have been studied for loading times up to 20000 h. Eight different insulating products are included, representing three commercial material types and one laboratory made material. Applicable for polystyrene cellular plastics, a test method is presented giving the extrapolated 50-year creep deformation from measurements during 3000-5000 h. Included types of insulating materials are: expanded polystyrene cellular plastics (EPS and XPS), rigid polyurethane foam (PUR) and mineral wool. The increase of the compressive deformation under constant load (creep test) was followed at specific load levels and temperatures. The creep behaviour confirms a hypothesis of a physical aging effect (Struik) in EPS and XPS materials. As a consequence of the physical aging the creep and hence the total deformation develop more slowly compared to conventional creep models (Findley). The method of extrapolation presented considers this physical aging effect.