When humid air is allowed to enter the environment of a cool crawlspace, the result is high humidity which causes the furnace pipes to sweat. Properly controlling all sources of moisture will prevent this from happening. Sources of moisture include outside air entering through foundation vents, water penetrating through the foundation, broken or leaking pipes, furnace drains improperly drained, springs in the crawlspace, inadequate or missing vapor barrier and clothes dryers venting into the crawlspace. The result is high humidity and the perfect environment for mold and fungus to thrive.