The purpose of the bathroom vent fan is to remove excessive humidity from the air to avoid rot and mould problems. Passive ventilation doesn't do the job. To avoid removing an excessive amount of conditioned air, you can put a timer on the fan; some controls allow for a choice of pre-programmed exhaust times, automatic operation when the room is occupied and even humidity sensing controls.

I know the OP, so let me clarify their situation.

The house is circa 1920 and the bathroom has a tall, louvered skylight (original) which extends upward roughly 4' above ceiling plane). The skylight is opened and closed by way of a pair of hinged glass panes that fold open (vertical) or closed (horizontal) near the ceiling.

The OP's intention is to permanently close the louvers (to prevent dust and leaves), install a vent fan in the skylight, and use the vent fan for longer periods to vent and cool the house (instead of using the central AC all the time).

This house is one of the typical two-story townhomes (row houses) in Washington DC which has a hot, humid summer season most of the time.

I will reserve my opinion at this time, but suggested that they post here to get other opinions. They have been told that installing a vent fan into the skylight on a thermostat is a good idea (much like an attic vent fan).

Thank you.