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Our 1938 one-story home in NC has a dirt-floor crawl space. (No insulation.) The general moisture level is around 14% in the crawl space. Should we install a moisture barrier?
Yes, Consider simply installing a 6 mill plastic on the drt and taped the seams if any with "Tuct" tape or a 50 year silicone caulk and then on the walls caulk the plastic to the walls. This way the moisture will not travel up into the home. Be sure to leave exposed any drain opening if there is one_ a drain would be unusual but just a heads up.
For 74 years your crawl space didn't have any moisture barrier. What made you inquire about moisture barrier? Do you have moisture damages? is your home energy inefficient?
A barrier will not protect you from a flood. It will protect you from moisture coming up from the ground, and 14% humidity is kind of low.
Finally, if you want to install such a barrier, go ahead. Just consider the benefits in relation to the cost. If you hire a professional, don't pay him in full until you inspect the job, to make sure every inch is covered.
If you regularly have high humidity in your home (above 40%), a vapor barrier on the ground in the crawl space may help reduce the humidity. Moisture coming up from the ground can enter the home, and create mold problems when it condenses inside exterior walls and attic ceilings. The moisture problems aren't necessarily in the crawl space, but the other parts of your home.
Yes should install a moisture barrier in your crawl space to prevent it from mold. You can even install sump pump in the crawl space to drain the accumulated water.