Heat exchangers with phase change material (PCM) slabs and air gaps are a solution to store energy in building ventilation systems. Such storage unit can be used both for free cooling and peak load shaving in winter and in summer. However, in order to assess and optimize this system, numerical simulation is mandatory. For that purpose, a numerical model is developed taking into account both conduction in the PCM slab and heat transfer between the slab and the airflow. This model is presented in this paper and validated using our experimental data. (C) 2013 Elsevier B.V. All rights reserved.