How can I get rid of moisture build up and mold in my 20×40-foot, window-free cottage garage?

Increasing ventilation is what you need to do, and anything that lets in a small flow of fresh air will make all the difference. The trick is installing some kind of louver that can be opened and closed, since you don’t always want to admit outside air. It also needs to keep out wind driven rain and snow, even if you’re not around to close the vents.

Here’s an option that’s worth considering: install an operable, sliding louver in one side of the garage, up high on a wall and protected by the eaves of the roof. I’d make it 12″ x 12″ or so. Next, install a quiet exhaust fan in the other end of the garage connected to a variable speed control dial. I have a fan like this in my workshop and it moves about 250 cubic feet of air per minute, full blast. You could leave the fan running very slowly while you’re gone, or crank up the speed to deal with large amounts of moisture as needed. Extra ventilation is only necessary when the garage is closed, especially during spring, when the air starts to warm up.