Once a moisture problem has been detected either by an obvious effect of elevated crawlspace moisture or through wood moisture readings in the crawlspace, the next step will be to determine the possible cause of the moisture issue.
Every crawlspace is different, but here are some general causes to moisture problems:
Water coming into the crawl space is an obvious issue. The source of the water could be from the rain runoff sloping toward the house and into the crawlspace. It can also be from misdirected gutters and downspouts that aren’t getting the water far enough away from the house. Water intrusion into the crawlspace will require waterproofing or proper drainage of some kind. Ideally, the water intrusion would be solved by keeping the water outside the house. Often, that is not feasible and an interior drain system is required. Having an interior drain system is an important part of the overall solution to the water problem, yet these drain systems often create moisture issues in the crawlspace that must also be addressed. While the water is in the crawlspace drain system, it has the opportunity to evaporate and condense as water elsewhere in the crawlspace.
Ground moisture occurs in most dirt crawlspaces – in some areas of the country more than others. It is especially prevalent when the ground underneath the house has a high water table, when the outside dirt is higher than inside the crawlspace, and when the dirt naturally allows water to push back up. This moisture is created in such a way that it is difficult to waterproof this from outside the crawlspace and generally will need to be managed as it cannot be eliminated. This ground moisture is the primary reason that ATMOX strongly recommends a complete vapor barrier on the floor of the crawlspace.
Limited height and airflow blockage
In the last few decades, crawlspaces have often been built with lower heights. This explains why very old homes often do not have moisture problems in the crawlspace as they have adequate ventilation naturally. Additionally, the airflow in these low crawlspaces is getting further hindered by duct lines and handlers from the HVAC systems. Airflow blockages are also often created in house additions where the crawlspace is now in various compartments and some sections are okay and other sections are getting virtually no air flow. Houses on the west coast built for earthquake protection also experience this issue as the crawlspace is not getting adequate ventilation naturally through the divided sections.
Decks and porches
As houses get more interesting from an architectural standpoint, they often have more issues underneath. Once again, when large sections of the house do not have vent openings to the outside due to decks, porches or garages, the crawlspace will not get natural air flow to all sections. Vent openings that are under decks and porches lead to even greater moisture coming into the crawlspace. Decks are designed to allow water to flow underneath them. Then after a rainfall, when the sun hits the deck, the air underneath begins to heat up and allows the water to evaporate into the air. Most of this air is trapped below the deck and with open vents to the crawlspace will drift into the crawlspace. Now this warm and moist air from underneath deck comes into the much colder crawlspace. The cold air of the crawlspace cannot hold the moisture causing it to condense back into water when in the crawlspace.
HVAC Systems that are in the crawlspace lead to the lower temperatures in the crawlspace in the summer months as the air-conditioned air leaks from ducts and HVAC system. This cooler air cannot hold as much moisture as the air that is outside in the summer. Crawlspaces will often have their greatest issues right around the HVAC system.
Sprinkler systems are often installed to water landscaping right in front of the house. If improperly installed, the sprinkler systems can pour water directly against the foundation allowing water intrusion in the crawlspace and creating higher moisture levels in the air directly around the crawlspace vents. This seems like common sense but sprinkler systems have led to crawlspace water issues that would not have existed from rain alone.