Abstract: A building element which inhibits the change of the interior temperature of a building beyond prescribed limits approximating room temperature. A rigid porous matrix structure is provided in thermal communication with the building interior. A store of phase change material impregnates the matrix structure. The phase change material has a melting point within the temperature limits so that the material melts when the interior temperature of the building exceeds the temperature limits. Melting of the material absorbs heat and reduces the interior temperature of the building, until the material is entirely melted. Conversely, the material freezes when the interior temperature of the building falls below the temperature limits to release heat and increase the interior temperature until the stored material is entirely frozen. The store of phase change material is sealed within the matrix structure so that the material undergoes repeated changes in phase between liquid and solid.