Why cover the fans? That is your only ventilation. Just power them up and turn them on.
If it is too muddy/snowy for the chickens to go outside, get a bale or two of straw and spread it around the building to give them something dry to walk on.
If it is too muddy/snowy for the chickens to go outside, get a bale or two of straw and spread it around the building to give them something dry to walk on.