Why is my crawl space wet?

A crawl space gets wet from one of five places: rain running toward the foundation, groundwater coming up or through the walls, a plumbing or condensate leak, humid outdoor air condensing on cool surfaces, or bare soil evaporating into the air. Most wet crawl spaces have two or three of these at once. The fastest way to sort them out is to notice when the water shows up and what it looks like.

Wet after rain: water from outside

If the space is dry most of the time and wet after storms, the water is coming from outside. Virginia Cooperative Extension calls this kind of leak, rain through holes, cracks and gaps, the one that matters most: "This is the most important of the four modes of moisture migration because bulk moisture leaks represent pathways for liquid water to immediately move into building cavities".^[3]

Common entry points:

• Roof runoff. All roof runoff must be directed away from the building a minimum of 10 ft or via an underground drain system.^[1] A downspout dumping at the corner of the house is a frequent cause. See how to extend a downspout away from the foundation.
• Grading. The ideal is a 5% slope away from the foundation (6 in. within 10 ft).^[1] Settled soil, flower beds and patios that tip toward the house send water to the wall. See why water pools near a foundation.
• Low vents and doors. Foundation vents and access doors near the soil let surface water pour in. The Department of Energy's crawl space guide says If the vents fall near or below grade, they will need to be filled in with block, brick, or concrete and sealed to prevent water entry,^[1] and puts the access door at a minimum of 6 in. above exterior grade to prevent runoff or snowmelt from entering.^[1]
• Groundwater. If the yard is already graded and drained and water still seeps up through the floor or in at the base of the walls, suspect groundwater. That usually means a site with high groundwater or water flow against the foundation that cannot be remediated in any other way,^[1] and it calls for perimeter drains, waterproofing or a sump.

Wet in one spot: a leak

A puddle in the same place in dry weather is usually plumbing. Typically, standing water indicates a leaking pipe from above or groundwater entering the space from outside the building or under the floor.^[1] Check under bathrooms, the kitchen and the laundry, and follow the air conditioner's condensate drain to where it ends. The DOE guide notes that an auxiliary drain pan with a separate drain beneath any cooling or evaporator coil is recommended to prevent condensate from flooding the crawlspace^[1] if the main line clogs.

Wet in summer, no puddles: condensation

Dripping ducts, sweating pipes and damp joists in July with no rain point to condensation. Research for the Department of Energy found that vented crawlspaces located in mixed or hot-humid climates tend to increase the moisture level within the space instead of keeping it drier.^[1] The guide explains why: This increased moisture then condenses on the colder wood surfaces of the floor framing, promoting fungal growth and wood damage.^[1]

It is the same thing that happens on a cold glass. At 100% relative humidity, the air is saturated and water vapor will condense into a liquid; the temperature at which this happens is called the dew point.^[3] Humid outdoor air comes in through the vents, meets a cold supply duct or a cold water pipe, and leaves water behind. Opening the vents wider in summer tends to make this worse, not better.

Damp all the time: the soil and other sources

Bare dirt never stops giving off water. Moisture in damp soil, such as in a crawl space, can evaporate into the air, raise the relative humidity in that space, and enter the building as a vapor.^[6] The EPA's guidance says damp basements or crawlspaces may add as much water vapor to the air in a day as all the other internal sources combined.^[4]

A ground cover only works when it is complete: Poor sealing practices of the polyethylene vapor barrier allow ground moisture to infiltrate through gaps or open seams in the barrier.^[1] Other everyday sources:

• An open sump pit. Missing lids and an absence of sealing will allow moisture to enter and increase the relative humidity of the crawlspace.^[1]
• Exhausts. Dryer vents and bath fans belong outdoors; the closed crawl space guidance lists exhausts from kitchens, bathrooms and clothes dryers that terminate outside the crawl space^[2] as a basic requirement.
• Appliance discharges. The DOE's inspection list includes water softener discharge^[1] among interior sources.

Why it matters

Moisture does damage slowly and then all at once. Fungi (molds) grow in air over 60% relative humidity,^[3] and The moisture content of the wood in the crawlspace must remain below 17%.^[1] Wet wood also draws insects; Virginia Cooperative Extension notes that when carpenter ants nest in a house, their presence indicates that there is water damage somewhere in the building.^[7] After a leak, dry things out quickly; the EPA says that if wet materials are dried 24-48 hours after a leak or spill happens, in most cases mold will not grow.^[5]

Once you know the source, see how to lower crawl space humidity for the fixes, and why a house smells musty from the crawl space if the smell has reached the rooms above.

Stop and call a pro if

• Water runs in at the walls or rises through the floor after rain, even with downspouts and grading corrected.
• A pipe is leaking, or the condensate line or pan is overflowing.
• Ducts are dripping, which may mean the air conditioner or the duct insulation needs an HVAC technician.
• Joists or the subfloor are soft, stained dark or sagging.
• You see mud tubes, winged insects or sawdust-like piles under wood.
• Mold covers more than a small patch.
• There is a sewage smell or a failed sump pump.

To decide who to call, see whether to hire a pest control or a waterproofing company. More is under pest control answers, and you can browse pest control businesses near Blacksburg.

Describe your wet crawl space once and get quotes from licensed pros.