My detached garage needs some sort of vent fan for the warmer weather as it gets incredibly humid in there and creates rust problems. I am going to install a vent fan but I am wondering if I also need a vent on the opposite wall to allow airflow or would putting this in just pull in more humidity? Would having just the vent fan with no way for crossflow even help?

Venting a closed area can make it more comfortable, and cross ventilation is probably best allowing more efficient air flow. However, the humidity is more a factor of the outside environment when there is no climate control. I guess it would help lingering humidity on some days, it would speed the drying of the inside when it is more arid outside.

Some details about the construction of your garage and your climate could inspire other ideas that may help. I am in Florida and there is no escaping the humidity without AC. In my garage, opening the windows at night and closing them before sunrise makes a bit of difference with humidity, and a big difference on temperature. It is insulated though and has cement floors. When I lived in NY, simply running a dehumidifier in our detached garage would do the trick.

Where are you located? Is the humidity coming out of the floor of the garage or is the air humid everywhere

The garage is built into a hill. Half of it is brick, the upper half is wood...not insulated. The floor seems to be damp in the summer and humidity just lingers in there with the sun beating down on the roof. I have tools and some equipment in there that I am concerned will be affected by this.

You can install a powered attic fan and set the temp for what you need. A garage is not so air tight that you need to create another opening, you can if you want but it won't do a whole lot. There is probably no vapor barrier under the floor so you are stuck with a damp floor but you could paint it, walls too. Paint & Accessories | Liquid Coatings | DRYLOK® Concrete Floor Paint Gull Gallon | B762575 - GlobalIndustrial.com

Thanks for the help. I can't give you rep as I have to spread it around.