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CONTROLLING MOISTURE INTRUSION-Moisture entry into crawlspaces can be controlled in a number of ways...

Evaporation:
A vapor barrier is a waterproof membrane such as plastic which is spread out across the soil to help prevent moisture in the soil from evaporating into the crawlspace air. This waterproof membrane is also called a "soil cover" or "ground cloth".

Groundwater
A sump pump is a float-activated water pump which is installed in a pit dug into the floor of the crawlspace. As groundwater enters the pit, the pump is activated automatically and water is pumped through pipes to the outside of the home. More than one pump may be installed.

Ventilating Crawlspaces-Passive Ventilation...Passive vents are simply ventilation holes installed in a crawlspace. Passive ventilation depends upon natural air movement to move moisture-laden air outof the crawlspace.Advantages to installing passive vents are...The system is simple and requires no electricity or maintenance.Air movement can carry off hot air during the summer.Disadvantage to ventilating crawlspaces. In moist climates, warm moist air entering the cool crawlspace will condense and the resulting moisture will be absorbed by wood framing and insulation. Under certain conditions materials may absorb enough moisture for fungi (mold) to become active.

Mechanical ventilation...-A humidistat is a device similar to a thermostat, except that it monitors humidity levels and activates a fan when moisture levels in the air exceed the humidistat setting. The fan pushes crawlspace air to the outside. This air must be replaced, and replacement air must come either from
the great outdoors, from inside the home or from out of the soil. Pulling replacement air from inside the home can create a whole new set of problems such as Negative pressure- Low pressure in the living space can suck toxic combustion gasses from water heater and furnace vents into the living space. Sounds nasty

Air temperature- Replacement air coming into the living space may need to be heated, cooled, humidified or de-humidified depending on the climate and time of year.

Crawlspace Recommendations-Insulate heating ducts- You can lose up to 35% of the heat from uninsulated  heating ducts in an unheated crawlspace. Insulate water supply pipes- Insulating pipes helps keep them from freezing in especially cold weather and helps save money on your hot water heating bill.

 
Plastic soil cover vapor barrier- helps prevent moisture from evaporating out of the soil into crawlspace air.
Insulate crawlspace walls rather than the home floor- This will help prevent damage from pipes freezing. The idea is that conditions in the crawlspace will be easier to control if the crawlspace is treated as indoor space.

Moisture in the Living Space
Moisture enters the home living space from various sources...
1)Moisture (humidity) levels inside the home are raised by...
2)Water-using appliances such as clothes and dishwashers.
3)Plumbing fixtures such as showers.

Moisture in its liquid form comes as a result of leakage from plumbing fixtures and pipes and from the roof. Living space air lost through ventilation, leakage and combustion is replaced by moist crawlspace air.