Not sure man. If the air outside is also humid and you already have decent circulation in the grow box then not sure what you can do. I'm running @ 60% rh right now due to the weather but just open my closet door a bit to keep the air fresh, but I'm working with much more room than you. I don't think you have much to worry about just yet, but you do want to take note of any water droplets building up on the walls etc as a potential gauge of your air circulation, or maybe check for moisture where leaves overlap each other and are touching. Can you take a few pics of the fridge and your exhaust, if just for my own curiosity?