In recent years, the demand for a highly efficient and comfortable hygrothermal environment has increased. Residential air conditioning using a system based on forced convection is prevalent in Japan. However, there are several problems associated with this type of system, such as unbalanced indoor temperature distribution, discomfort due to airflow, and diffusion of house dust. In addition, this system might also cause discomfort to the occupants, such as overcooling in the summer and excessive drying in the winter. An alternative is the radiation cooling/heating system, an air conditioning system that is already popular in Europe. This system is also being introduced into houses in Japan through mechanisms such as floor radiant heating. However, these systems tend not to be widely used due to unknown characteristics such as the delay in achieving the desired temperature, dehumidification due to condensation, and the radiant heat of the panel. In this study, we analyze indoor radiant heat and humidity characteristics using a stand-type radiation cooling/heating panel system installed in a highly insulated house. The characterization is performed using real-time measurements as well as numerical simulation. By using THERB for HAM (a residential building thermal environment simulation program for analyzing heat and moisture transfer), it is possible to predict the hygrothermal environment and the heat load of an entire building. We incorporate the algorithms for computing the parameters of the radiation panel system (heat convection, heat radiation, and two-dimensional heat conduction) in this simulation software.