Radiant system design and control standards and guidebooks currently classify radiant systems as a function of their structure and geometry. We assume that design solutions, testing methods, and control strategies of radiant systems can be more clearly described and classified based on their thermal parameters. In this study, we use the thermal response time to evaluate the dynamic thermal performance of radiant systems. We defined the response time (τ
95) as the time it takes for the surface temperature of a radiant system to reach 95% of the difference between final and initial values when a step change in control of the system is applied as input. The state space and thermal resistance models are used to calculate the response time for different radiant system types with a variety of configurations and boundary conditions. We performed 56,874 simulations. Concrete thickness, pipe spacing, and concrete properties have significant impact on the response time of thermally activated building systems, while pipe diameter, room operative temperature, water temperature and water flow regime do not. We find τ
95 < 10 min for radiant ceiling panels; 1 < τ
95 < 9 h for embedded surface systems; 9 < τ
95 < 19 h for thermally activated building systems. A preliminary radiant system classification scheme based on thermal response time is proposed.