Can you cover the vents to the crawlspace under the house?
All vents that lead to the outside should be completely covered. An unconditioned crawlspaces has very poor air quality. Opening the vents just adds to the problem. When the outside temperature hits the crawlspace temperature, condensation occurs which results in excessive humidity. The correct fix depends on where you live geographically but is basically the same. The crawlspace does need air. It needs air from your hvac system. That is how to avoid dead air.
