I live in a coastal area of NC. What's a good humidity reading in my crawl space?
I'm seeing some mold on the joists.
Visible mold is never a good thing, so reducing humidity is key here. That will happen with several steps, starting with exterior drainage. Your downspouts and gutters should be clear of debris and divert water away from the foundation. The exterior of your home should slope away from the foundation as well. Next, the floor of your crawlspace needs vapor barrier. This can be as simple as thick plastic sheeting. If you use more than one piece, make sure you are overlapping the pieces where they meet. This will reduce evaporation of moisture up into the space. You also need to make sure your crawl space vents are open on all sides.
Lastly, you might want to think about a humidistatically controlled exhaust fan This is a special fan that fits into one of the crawlspace vent openings. It will come on automatically when a certain setting is reached - I recommended a humidity level of 50%. When it kicks on, it will pull in the drier outside air.
You need to make sure the moisture reading in the wood of your floor joists is low. If it reaches above 25%, decay organisims can attack. You need a moisture meter to read that.
I also recommend an inspection done to determine the integrity of the floor joists. Keep in mind that rot from the inside out, so it wouldn't be immediately apparent. You should treat floor joists with a product like Concrobium to kill the mold spores. Remember to follow all the label instructions during application.
Good luck and let us know how it turns out!