If you do some reading/research on the Carcoon bike bubble, or any of their units, they actually recirculate most of the air in the bubble once it's inflated, that way the air in the bubble doesn't fluctuate as much as the surrounding air with changes in humidity & such. My carcoon also has a zipper/vent on the opposite end as the fan so that if you wash your bike put it away less than perfectly dry you leave the vent open to continuously pull in / push out fresh air to remove any remaining moisture, then close vent after a number of hours, overnite. This all sounds good, I'm just posting what I can remember from when I did all my searching around the net/forums. Makes sense to recirculate & filter the air instead of just pulling in outside air. When I say filter the carcoon has a charcoal filter screen as well as the outer foam filter. I don't want to get over my head & say something wrong, it's all on their site...