The tepidarium was the warm (tepidus) bathroom of the Roman baths heated by a hypocaust or underfloor heating system. The speciality of a tepidarium is the pleasant feeling of constant radiant heat which directly affects the human body from the walls and floor.
There is an interesting example at Pompeii; this was covered with a semicircular barrel vault, decorated with reliefs in stucco, and round the room a series of square recesses or niches divided from one another by Telamones. The tepidarium in the Roman thermae was the great central hall around which all the other halls were grouped, and which gave the key to the plans of the thermal. It was probably the hall where the bathers first assembled prior to passing through the various hot baths (Caldaria) or taking the cold bath (Frigidarium). The tepidarium was decorated with the richest marbles and mosaics: it received its light through clerestory windows, on the sides, the front and the rear, and would seem to have been the hall in which the finest treasures of art were placed; thus in the thermae of Caracalla, the Farnese Hercules, and the Toro Farnese, the two gladiators, the sarcophagi of green basalt now in the Vatican, and numerous other treasures, were found during the excavations by Paul III and transported to the Vatican and the museum at Naples.