If you are thinking of purchasing, or own a home with a crawlspace, there are many things you should know concerning how to keep things in proper order down there. I find the following issues most often while performing a home inspection in St. Louis and St. Charles, Missouri.
First off, you must have access. Who knows what is going on down there if no one can get in? I recently had to enter through a very small window well to properly inspect the crawlspace. I found many issues, and I have no idea how normal sized people (I'm about as skinny as it gets) will get in and out, or how they will get materials in there to fix the issues, for that matter. If it's hard to get in, repair prices, of course, go up too.
The crawlspaces worst enemy? Moisture.
An under floor-system vapor barrier's purpose is keep moisture that may enter the crawlspace from entering the home. If the floor system is insulated, it is most effective for the vapor barrier to be below the insulation. The ground vapor barriers purpose is to reduce the amount of moisture entering the crawlspace. I know that sounds really simple, but I'm no longer surprised when I inspect homes with both missing entirely. Both are important.
Proper ventilation of the crawlspace is necessary to reduce moisture buildup. Humid days (I did say inspections in St Louis and St Charles, Missouri, right? Our humidity runs high), the ground itself, storm water runoff, a sweaty A/C condenser line, and more, all add to the moisture level in the crawlspace. Keeping a good cross breeze through the space helps keep that level down. Sometimes more vents are needed, and sometimes power vents are needed. Make sure vents are screened to keep the rodents out.
It was a good thing I went in that tiny access mentioned above. One of the things I found was the return air trunk line, with a large hole rusted through, due to excess moisture in the space in combination with the duct work being below the vapor barrier and left uninsulated. The heating and cooling system was drawing unconditioned air from the unvented crawlspace and pumping it directly into the home. If there is duct work in the crawlspace, it must be insulated.
Often the water supply comes through the crawlspace. If the home is in a freezing climate, steps need taken to ensure the water supply line does not freeze. Some solutions include chases, insulation wrap, and temperature controlled heated pipe wrap ('heat tape').
Much like in a basement, most homes on a crawlspace have similar beams running down the center of the home, and joists will overlap there. These beams are generally set on blocking in a crawlspace, instead of posts. The blocking is often concrete block, and the most common problem I find is these block being set laid on their sides, so you can see through the holes. A concrete block is much stronger when set with the holes running vertical.
Many crawlspaces are prone to flooding, and sump pumps are popular for this reason. Sump pits are buried below grade with a pump in the bottom of the to dispell the water that may accumulate at this low point in the crawlspace. They can be tested by running a hose and filling until the pump kicks on.
Many American homes are built on crawlspaces, and can last a long time if properly set up and maintained.