Ventilated slabs are used to provide indoor comfort by taking advantage of thermal inertia. The underlying difficulty is that the computational cost is high, which is hardly compatible with an exhaustive study of dynamic behavior at room scale. This paper presents a methodology for obtaining a lumped model based on the identification of the transfer functions for constant airflow rate systems. The transfer functions were combined into a state space model, which is able to obtain the main heat fluxes with good accuracy at a much lower computational cost. The study moved to room scale in order to further understand how a ventilated slab influences the heat balance. Increasing the number of ducts promoted better indoor temperature stability but the impact was slight. The mass flow rate, on the other hand, appeared to be crucial. This result underlines the need to tailor a control law specifically to the ventilated slab.