The skytherm system, a roof covered with water filled plastic bags equipped with moveable insulation, is studied and a model is presented to predict its thermal behavior. A computer program is written to calculate hourly cooling and heating load requirements of a building and is validated by comparison with field data taken from an actual house in Shiraz, Iran. This program is then used to simulate metal and concrete skytherms. It is shown that for a 140.55 m(2) one story house, the skytherm system is capable of reducing heating demands by 86% and cooling loads by 52%.