Moisture beneath a manufactured home can come from several different sources, including the ground, rainwater, plumbing leaks, heating and cooling equipment, appliances, or humid outdoor air. Because the causes are so different, the first step is not simply to add vents, close vents, install a dehumidifier, or cover the soil. Rather, it is to figure out where the moisture is coming from.
Standing water, wet insulation, a sagging underbelly, musty or moldy odors, soft flooring, and recurring condensation are all signs that the problem should be investigated. Left alone, excess moisture can damage insulation and wood components, corrode metal, attract pests, and eventually affect the living space above.
Start by looking for active leaks
Plumbing is one of the first things to rule out because even a small leak can keep the area beneath a home wet, not to mention increasing your water bill. Check the water-service entrance, visible supply lines, drain connections, toilets, tubs, sinks, washing machine, water heater, and refrigerator water line.
A pressurized supply-line leak may drip continuously, while a drain leak may appear only when a particular sink, tub, or appliance is being used. If the source is not obvious, dry the area as much as possible and operate fixtures one at a time while watching for new water.
Heating and cooling equipment can also produce water through condensation. Condensate drains should carry that water away from beneath the home rather than allowing it to collect there. The same principle applies to drains associated with furnaces, air conditioners, and water heaters: water should be discharged to an appropriate location away from the crawlspace.
Watch what happens when it rains
If the moisture appears after storms, look at how water moves around the home during or shortly after a rain. The ground should carry water away rather than allowing it to collect beside the skirting or flow underneath the home.
Look for low spots in the soil, disconnected or undersized downspouts, erosion channels, roof runoff landing too close to the home, and patios, sidewalks, or other hard surfaces that slope toward it. Even a relatively small drainage problem can send a surprising amount of water under the skirting during a heavy rain.
Correcting the problem may involve extending downspouts, improving the grade, filling eroded areas, or providing another suitable drainage route. Avoid piling soil or mulch against siding, skirting rails, vents, or exposed wood, since that can create additional moisture problems instead of solving them.
Do not overlook moisture coming from the ground
The soil beneath a home can release moisture into the air even when the ground looks dry and no standing water is present. Over time, that moisture can contribute to damp conditions beneath the home.
A ground vapor retarder can reduce the amount of moisture entering the crawlspace from the soil. Federal manufactured-home installation standards allow a substantially lower amount of crawlspace ventilation when the ground is covered with an appropriate vapor retarder, such as six-mil polyethylene or another acceptable material.
That doesn’t mean every existing manufactured home should be sealed or ventilated according to one universal formula. Climate, drainage, the home’s original installation instructions, combustion equipment, local requirements, and other conditions all matter. A ground vapor retarder is intended to cover the soil beneath the home and should not be confused with the underbelly material attached to the bottom of the floor system.
Make sure ventilation and exhaust are working as intended
Skirting closes off the area beneath the home, which makes proper ventilation important. Unless the space was specifically designed as a different type of enclosed or conditioned crawlspace, it needs a way for moisture-laden air to escape rather than becoming trapped underneath the home.
Vent openings are generally distributed around the crawlspace so air can move through the area, and they should be protected with screens or covers that help keep rodents and other animals out. In colder climates, some systems use adjustable vents, but they should be operated in accordance with the home’s design, local conditions, and manufacturer instructions.
Also make sure appliances are not dumping moist air beneath the home. A clothes dryer should exhaust beyond the skirting, not into the crawlspace. Combustion-air inlets must remain open, and furnace, water-heater, and other appliance exhaust systems should terminate where they were designed to terminate. Blocking or rerouting these openings can create safety problems in addition to moisture problems.
Inspect the underbelly, insulation, and ductwork
Sometimes the moisture problem isn’t limited to the open space beneath the home. A torn or damaged underbelly can allow humid air and water into the insulated floor cavity, where the problem may be harder to see.
Wet insulation loses much of its insulating value and can become heavy enough to sag against the underbelly material. Damaged or leaking ductwork can also create cold surfaces and pressure differences that contribute to condensation, particularly during warm, humid weather.
Do not simply patch a wet underbelly and close everything back up. First determine where the water came from and whether the insulation or ductwork has been damaged. Repair leaks, allow the area to dry, replace contaminated or badly damaged insulation when necessary, and restore the underbelly with compatible material. Large wet areas, mold, sewage, electrical damage, or duct problems that are difficult to evaluate are good reasons to bring in a professional.
Let the location of the moisture help identify the cause
Where and when the water appears can tell you a great deal about where it is coming from. Moisture that shows up in one area after a fixture or appliance is used points toward plumbing or drainage. Water concentrated along one side of the home after a storm suggests grading, downspouts, or roof runoff. Damp soil throughout the crawlspace without an obvious leak may point toward ground moisture.
Condensation on cool ductwork, pipes, or other surfaces during hot and humid weather suggests a different problem involving moisture in the air and temperature differences. In some homes, more than one of these conditions may be occurring at the same time.
Take photographs, note the weather and which fixtures or appliances were being used, and check the area again after each repair. Moisture problems are often solved by following the evidence rather than assuming there is a single cause. Fixing a leaking pipe will not correct poor drainage, and adding more ventilation will not stop water from entering through a broken drain.