That is not very good idea, if outside is really COLD like < 10 C, you can make condensation. If during the day or whenever, something happens, you loose electricity from unknown reason. And they sit there idle for 5-10 minutes. You will have drops of water. And when electricity comes, you can just say bye bye to your miner...
Correct me if I am wrong, But I do believe you are incorrect. Condensation only forms when the object is colder than the environment surrounding it. By definition condensation is "water that collects as droplets on a cold surface when humid air is in contact with it." For example, a glass of ice water. You always see this because the water inside of the glass is colder than the surrounding ambient air temperature. In this circumstance the S9 will be much hotter than any air being drawn in. Even if the miner goes down it is located inside so the temperatures should never be colder than the air being drawn in. Summer ac could potentially be an issue however.