In Wireless Sensor Networks, where the communicating nodes are not mobile, the channel variations are mainly temporal and caused by the random displacement of human beings in the environment. In this paper, we propose to model these kinds of variations for transmissions at 433 and 868 MHz according to the number of persons in motion. The proposed approach is independent of the environment geometry and the spatial variability of the channel, but is complementary to the models that take into account these variations. It relies on a narrowband measurements campaign in order to predict the channel behaviour with a statistical identification method. Finally, a temporal modelling compliant to building monitoring applications is used to assess the impact of people motion on the transmission quality through the computation of the associated Bit Error Rates.