Aotearoa / New Zealand

The amount it increases will be dominated by the humidity of the outside air unless the room is reasonably sealed.

There is an osmodic pressure that will normalize the different humidities; the speed at which the normalization will occur is determined by the airflow and permeability of the barrier.

e.g. the bathroom will steam up because of the shower quite quickly, but opening the window will allow the steam to flow out; with an open window and no wind the bathroom will still exhaust the steam.