In churches, intentional airing may be a measure to evacuate temporarily high levels of contaminants, emitted during services. Crucial contaminants include moisture and other emissions that may deteriorate and/or soil painted surfaces and other precious artefacts. Most old churches do not have any mechanical ventilation system or any purpose provided openings for natural ventilation, but the ventilation is governed by air infiltration. Enhanced airing may be achieved by opening external windows or doors. Thus, models provided in energy simulation programs should predict this kind of airflows correctly, to get a proper estimation of the total energy use. IDA Indoor Climate and Energy (IDA-ICE) simulation program is examined here and its model for airflow through large vertical openings is investigated. Moreover, field measurements were performed for airing rate in a historical church including. The simulated single-sided flows were of the same magnitude of the measured ones, but the effect of wind direction was less considered by the simulation program. At cross flow, when wind is approaching the opening it has a choice; flow through the opening or around and above the building. This process is not considered in the IDA-ICE model, which can contribute to the discrepancy between measurements and predictions.