Experimental method of tracer gas technique and Computational Fluid Dynamic (CFD) analysis are used to study the transientexfiltration/infiltration process into a walk-in cooler. Initially, the "natural exfiltration/infiltration" of cold air (through cracks and seals) in awalk-in cooler is measured to establish the baseline (sink term) for the CFD analysis and also correcting the experimental data because of theexistence of natural infiltration at all times. It was found that the exfiltration of cold air from the cooler or infiltration of warm air into the cooleris a transient process never modelled or measured in the past. The time dependency of the process starts with an initial significant amount of coldair rushing out and its gradual decay depending on the duration of time that the door stays open. The hybrid modelling/experimental approach fordifferent cooler's inside and outside temperatures will not only capture the transient behavior of the entire process, but also assists in accurateprediction of the cooling load.