The paper describes the numerical simulation of a novel ventilation cooling system. The ultimate aimof the mathematical model is to facilitate the design of the system for specific buildings and climates.The model directly takes account of time dependency in that the diffusion equation for the heat flow inthe fabric of the room is solved. This approach means that important parameters (e.g. ventilationrates and internal heat gains) can be arbitrarily specified as functions of time. For convenience themodel equations are solved with a spreadsheet, which means that the explicit method of solution has tobe used. The model was tested against an analytic solution and close agreement was obtained. Themodel has then been used to investigate the performance of the system under a wide range ofconditions. Examples of results relating to three important parameters are presented. It is also shownthat the behaviour observed in the field with simple thermostatic control is reproduced by the model.