Air compression inside a flooded tank of a damaged ship can significantly delay the flooding process if the ventilation is restricted. This is a notable issue that needs to be considered for example in the design of cross-flooding devices. The problem is studied through systematic full-scale tests with a decommissioned ship, where the ventilation level of a flooded tank could be altered. Comparative time-domain flooding simulations have been performed by using Bernoulli's equation for compressible fluid for calculation of the air flows. The analysed measurements are used to validate the applied numerical method and relevant conclusions are drawn.