The tepidarium is a moderate temperature room, located between the rooms for hot and cold baths, to avoid sudden changes in temperature.
In the late antiquity domus, this space had to be separate from the adjoining massage room preceding it by a solid entrance door, to maintain the right temperature. It is different from the frigidarium due to the praefurni, large ovens where hot air was produced at a moderate temperature, located outside the apses.
Heat was spread evenly under the floors between small brick columns, the suspensurae, around which heated air circulated from tubules, hollow bricks inserted in the brickwork and equipped with lateral holes and grooves on the surface.
