Tepidarium
The tepidarium (plural: tepidaria) was the warm room of a Roman bathing complex; a middle temperature between the hot caldarium and the cold frigidarium. The name is derived from the Latin tepidus, meaning moderately warm or lukewarm. Like the caldarium, the tepidarium was heated by air forced into the cavity of the hypocaust system beneath the floor of the room by the furnaces, the praefurnia. The hypocaust system was a series of stacked brick (or in some cases stone) pillars called pilae or suspensura, which raised the floor of the tepidarium above the foundations (recommended to be a height of two feet by Vitruvius) of the baths to create a space in which the heated air could circulate beneath the floor of the room, warming the floor. The tepidarium was typically situated farther away from the praefurnia than the caldarium, allowing the air to cool somewhat before reaching the tepidarium and creating a heated, but lower temperature environment than the hotter caldarium.
The vestiges of the hypocaust system can be one of the most recognizable features of a heated room in archaeological remains. Because of the relative fragility of the raised floor and negative space below it, the floor of heated rooms has typically mostly collapsed or otherwise not survived. Floors will sometimes be reconstructed at archaeological sites in order to illustrate the function of the hypocaust system. The supports of the hypocaust system can often be seen scattered around the room, or in many occasions reconstructed to some degree with the original materials. This is not always the case, however, and tepidaria (or caldaria) sometimes have no surviving elements of the hypocaust system displayed (though remnants may have been found during excavation).
