This paper presents a model of the transient response of a short response time capacitive humidity sensor to evaluate its ability to measure the humidity level in exhaled air. This sensor will be used to increase diagnosis accuracy of pulmonary deficiencies. A dedicated humid air blower was developed to measure the transient response of the humidity sensor exposed to abrupt humidity variations. The model is proposed for extraction of humidity data from experiments or for the sensor response prediction in any humidity conditions. This model relies on the theory of gas diffusion in polymer films. It describes a bidimensional diffusion process as the combination of two one-dimensional mechanisms.