The most global energy that directly related to building sector is the amount of energy consumption in the heating, and air-conditioning. One of the best ways that that enhances the reduction of this energy consumption and maintains the best comfort zone to the occupant is using a phase change material (PCM) with under floor heating/cooling system. In the present work, a new theoretical study involving comparative investigation among different PCMs on the energy consumption of under floor heating system has been performed. The most used organic and inorganic PCMs complete with their specified thermos-physical properties with the melting temperature as the main effective properties are studied. In addition, economical studies for all selected PCMs have been performed. Therefore, the finest PCM is selected carefully to satisfy law energy consumption with low cost. The results show that the organic PCM A39 with melting temperature of 39 °C is suggested the optimum as it has less negative effect on the environment with high power saving. The Cacl2-6H2O PCM is recommended with the restrictions of the impacts on the environments due to the low cost. In addition, it is found that the thickness of the PCM has a remarkable effect on the law energy consumption and so, the small thickness is recommended to give low cost. • Unlike using PCM in roofs and walls, in underflloor heating system application PCM melting temperature should be suitable for floor surface temperature and room air temperature