First off you say you are exhausting in to a crawl space above your grow room, Is the crawl space vented ,if not that is the start of your problem , you have to first understand basic building concepts, to be more specific ,when a house or building is built it is built to move air through the whole foot print this is done for the reason of keeping the material of the structure from rotting,expanding ect ect , when you change the flow of things like adding fans ,you must take appropriate mesures to insure you do not hinder the natraul flow of the design it was intened for .if you gave more detail on what you have setup I would be able to give you more advice on rectifying your issue.