First, you need to have it in some sort of a container - this will reducte dust and ensure protection from outdoor enviroment (water, bugs etc). Of course the level of protection depends on the material used. Second, you need good ventilation. This is a must for rigs in all enviroments, so a no brainer. Third, remember that condensations occurs on an object with lower temperature than the air. So as long as your rig is running it's ok because GPUs will probably be at least 50 degrees. Motherboard might be at risk here however. Having that in mind power outtages will be a problem, so prepare. So you must set the rig BIOS to power on when energy is restored (and have a miner open at startup). And if rig was off for a long time and some condesation occured you have to dry it out inside before running it (better safe than sorry). Of course the question is how safe you want to be? I'm running my rig in enviroment with around 60% humidity but I get anxious when it goes above 70%