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Recent changes in mushroom growing systems have made it necessary to re-examine airflow in the insulated polyethylene growing tunnels that are used in commercial production in Ireland. Even for the existing systems, improvements in the control of the air conditioning have made it more important that the correctly conditioned air be delivered to the cropping surface at the correct speed and as uniformly as possible throughout the cropping area. Air is delivered by means of a polyethylene distribution duct mounted at the roof of the tunnel. Typical delivery speeds are 4 to 7 ms -1 and these have to be reduced to 10 to 30 cms -1 at the cropping surface. New systems generally involve a change from single layer growing at the floor of the tunnel to the use of a variety of staging and shelving systems in both two and three layers. Largely because of the very low speeds involved and various disturbances such as heating and solar gain through the tunnel walls, it has proven difficult, using empirical methods, to characterise, and then to arrive at a solution for, all of the system options. In order, therefore, to gain insight into the system characteristics and to obtain guidance for future experimental work, the airflows were modelled using PHOENICS CFD software. The model was built up in stages with the Virtual Reality Interface module of the software, tuning the solver parameters and verifying that a physically realistic result was obtained in each case, until, finally, numerical convergence was obtained for the basic single layer growing system. A number of the alternative systems were then modelled and the main qualitative features of the flows were successfully reproduced. Broad agreement was found between the quantitative output from the models and the limited experimental data available to date.