A vapor barrier, by its nature, resists the passage of air. By stopping air movement from the ground, it can also turn your crawlspace into a semi-conditioned space – one that is close to the temperature of the living spaces above. When that occurs, your floors feel warmer in winter and your energy bills go down.
A vapor/air barrier can also slow or block most of the movement of harmful gases, such as radon, from the soil. Coupled with a radon mitigation system (in the event that radon is present in higher than acceptable levels), a vapor barrier can significantly reduce radon levels in the home.