Good Question ...and I'm not sure. The fan works as designed, we haven't experienced any problems with wet bedding. However, keep in mind once the sensor gets wet, the fan won't come on again until the sensor dries out. You can manually open the vent, but on a hot, still, humid night you'll want the fan on to move the air. Although it might only takes 15 - 20 minutes to dry out, on a 90+ degree muggy night it feels like an eternity. Seems like it always rains when we go camping and as luck would have it we were normally at primitive sites w/o electric so we can't use the AC. So, if I had to do it over again, I think I might opt for the standard fan and invest in the detachable rain cover that is discussed elsewhere on the forum.