Is it true that downstairs bathroom "need ventilation" more than upstairs, to avoid mold growth? I know at my house the downstairs bathroom has a vent fan but the upstairs does not. I'm wondering if there's a reason for this, and also wondering if I need to add some ventilation to my downstairs bathrooms but not upstairs ones at a 4plex (these bathrooms do have windows). Thank you.
By Code any bathroom without a window needs a vent. I would add a vent to every full bathroom though. You dont want to open window in negative degree weather.