The mechanism, with no leak involved
A vented crawl space works by letting outdoor air move through it. That is the entire theory behind the foundation vents, and in a dry climate it holds up. In this one it does the reverse. Warm outdoor air carries a great deal of water. When that air enters a crawl space and touches joists, girders and the underside of a subfloor held cool by ground that never really warms up, the air cannot hold what it is carrying, and the water comes out onto the wood.
Nothing has failed for this to happen. No pipe is weeping, no shingle is missing, no gutter is overflowing, and the water bill has not moved. The vents that were installed to dry the space out are the mechanism that is wetting it, and they are doing exactly what they were designed to do. That is why an owner who has checked every fixture in the house and found nothing is not missing something obvious. There is nothing to find.
What it looks like from the hatch is a white or dark film across the joists, beads of water standing on the wood, insulation batts sagging out of the floor framing, rusting duct straps, and a subfloor that reads wet on a meter across the whole middle of the house. Upstairs it shows as a musty smell and floors that feel cool and slightly damp in August.
The region is low and flat, which is the whole reason
There is nowhere here to escape the humidity into. Memphis sits at around 260 feet. Tiptonville, out on the northwestern edge, is lower. The highest ground anywhere on this site is Pilot Knob in Benton County at 656 feet, which is the far eastern corner and not where most of the population lives. Between those points the region is a plain, and the crawl space under a house on it is cut into ground that holds its temperature through the summer.
That is what makes this a lowland problem rather than a mountain one. It is not rain running downhill into a foundation. It is a high water table and warm damp air meeting cool ground, in a place with no elevation to change the arithmetic. A house in the loess belt near the bluffs and a house on the flat further inland behave the same way here, because the driver is the air rather than the slope.
Why July and not March
March feels wet. It rains, the yard is soft, the ground is saturated, and every instinct says the crawl space must be at its worst. It usually is not. Cool air holds very little water, so March air moving through a crawl space is not carrying much to give up, and the temperature difference between that air and the framing is small.
July inverts both halves. The air outside is hot and close to saturated, the ground under the house is still cool, and the gap between them is at its widest. The EPA puts the ceiling for controlling indoor mold at 60 percent relative humidity and recommends holding a building between 30 and 50. Summer air in this region sits above that ceiling outdoors for months at a stretch. Push months of it through a cool crawl space and the wood underneath the floor spends the summer wet.
Which is why the seasonal pattern is diagnostic on its own. A smell that is strongest in high summer and faint in early spring, in a house where nothing is leaking, is describing condensation rather than plumbing. Clean the joists in April, leave the vents open, and the film is back before September.
Adding vents makes it worse
The instinct, once somebody has been under the house and seen the water, is to improve the ventilation. More vents, bigger vents, a fan pushing air through. In this climate that is pouring more of the same humid air across the same cool wood, and the result is more condensation rather than less. The old advice about cross-ventilating a crawl space was written for places where the outdoor air is drier than the space it is being sent into. Here it frequently is not.
The direction that holds is the opposite one. Close the vents, seal a heavy ground cover across the earth and run it up the piers and the stem wall, move the insulation to the perimeter, and put a dehumidifier in the space with a drain that actually goes somewhere rather than into a bucket nobody empties. That keeps the space inside the range the EPA recommends and stops the seasonal cycle instead of resetting it every spring.
Two things come before any of that, though. If there is standing groundwater under the house, it needs drainage first, because sealing over water that is still arriving just puts it under plastic. And if a supply line or a drain is leaking down there, the plumber goes first and the sealing waits.
The part you should not call anybody about
A small patch of mildew on the seal around a bathtub, on shower grout, on a caulk line or on painted trim is your job, not a contractor's. Hard nonporous surface, an area you can cover with an outstretched hand, and no water arriving from behind it: that is a spray bottle, a stiff brush, gloves, the exhaust fan running and an open window for twenty minutes. The scrubbing does most of the work. Nobody needs to be dispatched for it and nobody should be quoting you for it.
The line moves when the substrate does. Growth on wallboard, on its paper facing, on wood trim or on a cabinet base means the material is holding water rather than carrying it on the surface, and cleaning the face of it accomplishes nothing durable. It also moves when the same patch returns within weeks of every cleaning, which is the building telling you the water has not stopped.
One question this page will not answer is what any of it does to a person. That is a medical question, it belongs with a doctor, and a restoration contractor is not a useful source on it. What can honestly be described here is where the water is coming from, what condition the wood is in, and what it takes to keep the space dry through a West Tennessee summer.