Atmospheric solar tanks made of welded mild steel are common. Inside the tank, the steel is first line with polyiso foam insulation, and then lined with the EPDM material. EPDM stands for Ethylene Propylene Diene Monomer. EPDM is a synthetic rubber that can be made in a variety of grades. Although manufacturers of EPDM lined tanks indicate that they have an indefinite lifetime and come with a warranty against leakage, many solar experts are concerned about EPDM for solar storage, stating that the material will deteriorate under the high temperatures of solar hot water systems. These tanks are generally the least expensive tanks overall.