Crawl spaces are located beneath your home's bottom floor. Usually 32 to 36 inches high, they must be navigated by crawling on your hands and knees. Crawl spaces can be nasty, cramped and muddy, especially if certain precautions are not taken to preserve the area. Usually, a crawl space will have certain protective measures in place to prevent moisture from reaching the surface--moisture seeps up through the soil and onto the surface of these spaces. The problem is that if you don’t install the correct preventative measures, you may as well not install any at all.
Gravel will serve only to mask the moisture in a crawl space, rather than to eliminate it. If you want to eliminate or reduce the moisture, you need to do a combination of things.
Apply heavy mil polyurethane plastic directly over the dirt floor of your crawl space. The plastic will trap the moisture and prevent it from entering the usable space. Plastic installation is the norm for any home. Whether it's put beneath a concrete home slab before the concrete is poured, in the crawl space or in a basement, plastic will keep the moisture from transferring onto the bottom of floor joists and accumulating on your concrete floor or in your basement.
Add sand over the top of the installed plastic in order to cushion your hands and knees as your crawl across the dirt floor of the crawl space. Once you have eliminated the moisture problem using plastic, sand will stay dry and also serve as an insulator between the dirt floor and the crawl space. Use course river sand for the application so that it will not compact and hold moisture.
Install crawl space fans beneath your home in order to circulate air, which will eventually dry out your saturated crawl space, as well. Make sure that your foundation vents are open so that the air circulates in and out of your crawl space. Although you can add gravel to your crawl space, it will be painful to crawl on and won’t serve any purpose other than to hide the mess below.