Radiant cooling systems are commonly used in regions such as Europe, and many standards on their measurement and design have been developed. However, some standards do not apply to hot and humid regions such as Japan. The aim of this research is to investigate the effects of factors commonly found in such regions on the cooling capacity of radiant panels. Experiments were conducted in a test room made in accordance with the EN14240 Standard. Results showed that increase in air velocity increased both the cooling capacity and the downward heat flux. An increase in temperature within the ceiling chamber resulted in the decrease of the downward heat flux while the cooling capacity increased. It was concluded that proper control of the thermal environment within the ceiling chamber was vital when measuring the cooling capacity of radiant panels, and that the downward heat flux should be calculated when designing radiant systems.