Heating plus air leaks results in dry air. As you heat some air the relative humidity drops. If your room is not leaky, the relatively humidity will rise as i

Heating plus air leaks results in dry air. As you heat some air the relative humidity drops. If your room is not leaky, the relatively humidity will rise as it cools back off, but if you're constantly leaking air your indoor humidity will approach that of the outside air raised to the indoor temp. This is why energy recovery ventilators exist-- to exchange outside air without losing or gaining as much relative humidity change.