My house currently is sitting at 35% humidity while being very poorly ventilated (~900ppm CO2). In the summer, it’s around 50% with the same level of ventilation. This generally has been the case everywhere I’ve lived; in the summer, you’re cooling air, which (all else equal) increases the relative humidity of that air. In the winter, you’re heating air, which decreases the relative humidity of that air.
It's “less than ideal” level of ventilation, but it's also very far from “very poorly ventilated” (it's not even above the target level set by workplace regulation in my country, which is 600ppm above the baseline).
