A vapor barrier, which is a more sophisticated way to say plastic sheeting, is a barrier that helps prevent the entry of moisture and soil gas into a home.
Crawlspaces should have a properly installed vapor barrier with complete coverage to help regulate humidity, insects, and radon from entering the home. A vapor barrier will also help provide a cleaner crawl space which is easier to navigate. It’s important to note 2 things about vapor barriers:
A properly installed vapor barrier needs to be sealed to the foundation wall and sealed at all seams and penetrations.
If a radon mitigation system needs to be installed on a home with a crawlspace, a vapor barrier will need to be installed as part of that system.