In this study, the condensation of capillary ceiling embedded with capillary radiant network (CRN) was simulated. Two types of condensation in capillary ceiling will be produced (i.e., surface and internal condensation of the enclosure). Temperature, humidity distribution and condensation of ceiling in both summer and winter were studied based on principle of coupled heat and moisture transfer. In this study, the effect of Expanded Polystyrene (EPS) insulation board thickness has been carefully analysed. In summer, the thickness of EPS must be greater than 10 mm to avoid wall surface condensation. If the thickness of EPS exceeds 20 mm, the surface relative humidity can be reduced to below 95%. In winter, in order to prevent the ceiling condensation at the internal interface between EPS and concrete, the thickness of EPS should better be less than 20 mm or more than 45 mm. Considering the fact that no condensation occurs within the 30 days in summer and winter, the thickness of the EPS should be greater than 45 mm. The results indicate that even if no condensation occurs on the surface of the capillary ceiling, the risk of internal condensation still exists, especially in winter. Increasing thickness of EPS insulation board can reduce the possibility of the radiant ceiling condensation.