Maintaining proper humidity in an indoor garden in Ohio can be difficult, but you can take steps to keep your plants happy. The quickest solution is to just add a cool mist humidifier to the area. Position the humidifier in the room so that it receives enough ventilation and is not misting directly onto the plants. Humidifiers work well in areas where the temperature is well controlled , but can be difficult if your garden experiences cold nights. When the air temperature drops 10°F degrees or more, humidity in the air can condense on your plants. This can lead to bud rot, or plant disease like powdery mildew or botrytis.
If you are going to use a humidifier in your garden, you should do your best to minimize temperature swings in the environment greater than 10°F It is also important to maintain adequate airflow at all times. The airflow helps to keep the water from accumulating on the leaf surfaces, as well as driving transpiration in the leaves Adding a space heater to the area on a thermostat can help with the cold nights. Maintaining proper humidity and temperature can be very energy intensive depending on the outside environment. This is why many commercial greenhouses cease production for a few months in the winter if they are located in a cold area. Providing insulation can help to maintain the warmth of the lights during the night cycles. A room that is well insulated will need proper ventilation to prevent overheating.
The real enemy of humidity in an indoor garden in Ohio is the cold. When the air is cold, it has a low ability to absorb water. This means any water the air was holding when your gardens lights were on can be deposited back onto your plants. When this happens over and over the health of your plants will suffer.