My pole building (which now houses all 4 cars) is unheated, with a concrete floor. It gets pretty cold here in the Philly area – especially this year 🙁 thus a dehumidifier is not really an option.
I use those "blue tarps" for a vapor barrier, also try to put something (cardboard or old conveyor belting) under the plastic… hopefully to keep the moisture under the plastic. I have no "authority" on this… just trying to keep the moisture away as best I can. All cars are covered with breathable covers. I am definitely getting some light "surface rust" on exposed steel parts over time.