The objective of the paper was to draw up a mathematical model of heat and mass exchange in air inside a big-size greenhouse, where a commodity cultivation of plants is carried out. During formulation of the model, inter alia, models described in literature and results of experimental research were used. A developed mathematical model was implemented to MATLAB/Simulink programme and simulations carried out with a computer model were used for carrying out graphical and statistical validation of a model. Analysis of simulation results allows statement of logical correctness of the developed model and makes possible to determined critical points of failure to adjust the model. Degree of simplification of the developed heat exchange model influences precision of the developed model. In order to use the developed model e.g. for control purposes, it requires to be more detailed.