A storage tank (100) with a pre-insulated outer tank (110) has been described, and a method for building such. For building a wall (112) of an outer tank (110), a formwork (90) is erected with an inner partition (92) to which PVC-foam plates (10) have been attached, which at their inner surface (2) are provided with a coating (3) provided with gravel (4). Subsequently, concrete (94) is poured into the inner formwork space (93), which concrete attaches firmly to the gravel sticking out of the coating. On a floor (111), a coating layer (121) is applied, over which a layer of PVC-foam plates (122) is applied. Subsequently, a secondary monolithic coating layer (123) is sprayed over those PVC-foam plates (122) and onto the inner surface of the coated PVC-plates 10 of the wall (112).