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Vent

Hi My house is about 140 square meters with very high subfloor. However one of the rooms smells musty occasionally when humidity is high even in summer time. I noticed that the subfloor is very secluded and has no ventilation. I need an estimate number of vents required and installing them. The subfloor walls are like Hardie boards (white ones) that can be cut using a electric rotating cuters. I need to fix the gap in the following picture with a water resistant weatherboard as well. I have some of the hardies boards mentioned above if that can be used. Thanks