You ask how ventilation helps with reducing humidity. Here's how:
- An enclosed space, such as a garage, holds the air inside it fairly still. At the same time, it's warmed from the outside by the sun for at least part of the day on some days.
- This heats the air inside the garage; if you put a thermometer there, you'll find that even without any form of heating, it tends to be a bit warmer inside than outside.
- Warm air can hold more moisture than cold air can. If you take cold air at 100% humidity, and then you warm it by a few degrees, it will no longer be at 100% humidity.
Given these facts, here's how ventilation will make the garage dryer:
- Cold air moves into the garage.
- The air heats up; this makes it capable of absorbing more moisture.
- The air absorbs some of the moisture from the garage.
- Warm moist air moves out of the garage.
This obviously works best if you have at least two openings for ventilation. Having them on opposite walls, and one low and one high will help. The opposite walls help because then any wind present will help move air, and the low/high helps because warm air rises, so even without wind warm air will exit the top ventilation hole and be replaced by colder air drawn in at the bottom ventilation-hole.