This paper is concerned with proportional-integral-plus (PIP) control of ventilation rate in mechanically ventilated agricultural buildings. The PIP controller can be interpreted as a logical extension of conventional proportional-integral/proportional-integral-derivative (PI/PID) controllers, but with inherent model-based predictive control action. In particular, the paper considers the design of an optimal, scheduled gain PIP algorithm for a 22 m3 forced ventilation test chamber at the Katholieke Universiteit Leuven. Such a PIP approach proves more robust to pressure disturbances than an equivalent PID design and constitutes a preliminary step towards the development of the complete micro-climate controller.