Just like anything else with the crawl space, the need for dehumidification depends on what region of the country you live in. Before you look into dehumidifying, you want to make sure you don't have water seeping into your crawl space and at a minimum a basic vapor barrier throughout. Ideally an encapsulation would be the best option. If these two conditions aren't taken care of before hand, dehumidifying will be very difficult if not impossible.
In areas like here in Western Kentucky, the humidity outside is very high, especially in the July and August months. Since most people in this area either don't have the money or don't want to spend the money on an encapsulation, this means that usually the crawl space has just as high of humidity as the outside. This can be very bad for your home since humidity levels over 60% offers prime conditions for mold to grow. This could be all types of mold, including: black mold, white mold, yellow mold, wood-rot fungus, and many more. Once mold has established, it will just continue to spread and in many cases eat away at your home's structure. You can test your crawl space for humidity levels with a relative humidity meter, bought at your local hardware store or found online.
If you have come across mold or have a high humidity levels underneath your home, the quicker you take action the better. The cost for mold remediation can range anywhere between $500-$15,000, depending on the size of the crawl space, how much mold is present, and how much space there is to work in. If it has gotten to the point where the floor joists are rotted out, you could be looking at another $5,000-$20,000 in structural repairs. Yikes! What we're getting at here is that preventative actions should be taken to ensure that your crawl space stays dry, especially in those warmer, more humid months.
The most common reason for humidity in the crawl space in high humidity areas is open foundation vents. But this is also a catch 22 scenario though. If you open your crawl space vents and the humidity outside is higher than the crawl space humidity, then the humidity will increase in the crawl space. Just like if you were to open the windows in your house. But if you leave them closed without proper ventilation, you will be left with moist, stagnant air which is also no good. So if you plan to close your vents, you need to take the proper steps in either ventilation or dehumidification that also gets the air circulating within the area.
Some people will vent their HVAC system into the crawl space which blows dry air into the crawl space and helps with dehumidifying. The problem with this is that you are now paying to heat and cool a large area under your home and it is not a true dehumidifier, so it may not bring your humidity levels down far enough. Especially in the "swing seasons'', spring and fall, where the HVAC system may not run very often.
Your best option is installing a dehumidifier specifically for a crawl space. Now don't go out and buy one of those cheap home dehumidifiers from Walmart thinking it will fix your humidity problems. Those are small scale, looking to dehumidify a room that already has lower humidity from being inside the house where the HVAC system is running. You will need a good crawl space dehumidifier, such as an Aprilaire or Sante Fe, and it will need to be drained properly to either a sump pump or to the outside since they can pull around nine gallons of water out of the air every day. They also have a built in 200 CFM fan to circulate hot, dry air around your crawl space. Just be sure that you have a good vapor barrier down and that you seal all your vents and any other intrusions you have from the outside. Because no matter how hard you try, you cannot dehumidify the outdoors.
Make sure that if you do decide to get a dehumidifier installed in your crawl space that you consult a local professional. You will need an electrician to install a new outlet for your dehumidifier and draining can be a tricky task, often needing a separate pump for the water. Routine maintenance is also something to take into account, as the dehumidifiers have filters that need cleaned or changed on a regular basis.