Obvious symptoms of excessive moisture passing upward through the floors may include any of the following: a musty odor; mold on the walls near the floor, in corners and in closets; moisture condensation on insulated windows or storm windows; and moisture condensation in the walls with resulting paint peeling. Some of the more common ways moisture gets into crawl spaces are:
 ground water moving through the foundation walls or up through the earth floor;
surface water moving through the foundation walls or flowing in through ventilators;
 capillary rise of ground moisture;
 “green” slabs, or concrete foundations that weren’t fully cured when the rest of the house was built above it;
Building codes in most areas requires cross-ventilation in a crawl space equal to at least 1.5 per-cent of the square foot area within the crawl space.
In addition, a ground cover of 6-mil polyethylene sheets (preferably black-clear will sometimes sweat) should be placed over the entire area of the crawl space soil, lapped at least 6 inches and held in place by bricks or other weights. (Black polyethylene works better than clear, because the black plastic slows the growth of plants, which can occur even in a relatively dark crawl space. Clear many time will sweat, creating an additional moisture concern. In addition, some wood floor-ing manufacturers are now recommending 8-mil poly in lieu of 6-mil.)
In cases where concrete is poured to create a floor in the crawl space, the 6- or 8-mil polyethylene cover is still required.
Capillary action causes moisture to travel upward into the slab. Gravel beneath the slab can slow the movement of moisture, but 6-mil plastic below the slab will provide a more effective vapor re-tarder.
CAPILLARY RISE OF GROUND MOISTURE: Moisture travels upward by capillary action–as much as 14 to 18 gallons per day have been noted under a 1,000 square-foot house–and evapo-rates within the crawl space. Capillary rise occurs in nearly all areas where the soil is clay or silt.

Capillary action is the effect of surface tension that causes water to rise up a narrow tube, against the effect of gravity. In building construc-tion, capillary action can occur between two sur-faces placed together, or within porous materi-als. This relates to the installation of wood floor-ing in that moisture can be drawn through both the subfloor and the concrete below it.
It is best not to build below the highest expected water table, for to do so is to have water under pressure trying to enter through any crack or weakness in the construction. Porous granular filling material around and under the building, and connected to drainage lines, can be used to divert drainage water away from the structure.
Making these adaptations is the responsibility of the general contractor. However, the flooring in-staller who has been hired by the general con-tractor to do the floors in a new home or building should be aware of these details.
MOISTURE FROM THE HOUSE: There are many sources of moisture from within the house. Mopping the floor in a 150-square-foot kitchen can release the equivalent of 4 1/2 pints of water into the air a shower or bath about 1/2 pint; washing the dinner dishes about 1/2 pint. Also, a family of four gives off about 1/2 pint of water per hour just breathing (this is why bedrooms are unexpected moisture sources). As moisture is released in a house, it moves to all rooms by natural air movement or by forced air movement from furnace or air conditioning.
MOISTURE FROM MECHANICAL SYSTEMS: Moisture is sometimes introduced into the crawl space from the mechanical systems within the house. To avoid this, make sure the clothes dryer is vented to the outside. Also, condensated water from cooling systems and water from auto-matic ice makers should be discharged away from the building.
RELATIVE HUMIDITY: When humidity in-creases, the effect on the wood floor can be damaging. This occurs most frequently in homes in which occupants are there for a short period of time, such as a weekend home or vacation cabin, or in rooms that are closed off (not heated) to save energy.
If air conditioning or heating is not used or is shut off, ventilation is a must even when the home is not occupied. Otherwise, the floor will expand in the high humidity, and cupping and buckling will occur. This “greenhouse effect” will be exagger-ated even more when a plank floor has been in-stalled, because wider boards react to moisture with more movement.
Minimizing moisture from wood subfloors
A heavy moisture invasion can seep up through a wood subfloor. It may occur slowly, but its ef-fects are damaging. Proper installation of flooring calls for checking subfloors for moisture.
To protect against moisture rising through sub-floors, the installer needs to make sure there is a proper moisture-vapor retarder.

In any case, the moisture content of solid strip flooring should be within 4 percentage points of the subfloor. (That is, if the subfloor is measured at 10 percent moisture content, the strip floor-ing should have no less than 6 percent moisture content and no more than 14 percent.) For solid plank flooring, the difference should be no more than 2 percentage points. Solid strip or plank flooring requires a felt paper moisture retarder between the floor and subfloor. If a wood subfloor is laid over an existing slab, the moisture retarder can be cemented to smooth, clean-swept concrete.
In joist construction, a vapor retarder of 15-pound saturated felt paper should be laid between the wood flooring and the wood subfloor. If the wood subfloor is laid over a concrete slab, the felt paper moisture retarder can be cemented to smooth, clean-swept concrete. Other vapor barriers or retarders may also be appropriate over concrete slabs.
Venting will reduce crawl space moisture levels. In reality, venting will only help reduce crawl space moisture levels when the outside air is dryer than crawl space air, or when enough hot outside air enters and warms the crawl space. Outside air in the summer may actually con-tain more moisture than crawl space air, and may make the situation worse, not better. In win-ter, venting will help dry a crawl space, sometimes to a detrimental extreme.
Hardwood floors over crawl spaces often experience cupping problems in the summer. Wood expands when it gets wet. The typical scenario I see is that air conditioning is keeping the liv-ing space moisture lower than the crawl space moisture. This results in uneven moisture levels on the upper and lower surfaces of the wood. The lower, wetter surface expands and causes the boards to cup.
A common solution is to add ventilation to the crawl space, to reduce the moisture levels. Guess what happens in the winter? The boards cup the opposite way. Now our crawl space is vented with relatively dry air such that things in the crawl space really dry out. (As you heat air, its relative humidity drops.) The more ventilation we add to cure summer cupping, the worse the reverse winter cupping. We are slamming the wood moisture levels from one extreme to the other. Other moisture related wood movement such as swelling and shrinking of doors happens in the house as well.
Flooring contractors also should make sure all basement areas and crawl space areas are dry before the wood flooring is delivered as well, with proper vapor barriers and ventilation se-curely in place and working. In general, crawl spaces should be a minimum of 18 inches from the ground to the underside of the floor joists. Crawl space vapor barriers should be a mini-mum of 6 mil black polyethylene. Perimeter venting should be equal to a minimum of .1600 of the crawl space square footage, with vents properly positioned to foster adequate cross venti-lation for the entire crawl space area.

Special Thanks to JJ Haines for the info