If you are seeing light between boards in a lot of places, that might be wise to do something about given that you live in Michigan (if you lived in, like, Tennessee I'd say who cares ). The ideal thing would be to staple in some vapor-barrier plastic, wrapping it *around* wall studs so it is...
I'd like to help but I'm not really visualizing the structure of your coop, from your descriptions -- any chance of posting some *pics*? That would be really helpful and then we could doubtless make some specific suggestions that might help.
In general you will want to shut any vents or sources...