Water in the crawl space: causes, risks and fixes

Why a crawl space matters

It is easy to ignore a space nobody goes into. But air moves upwards through a house, drawn in at the bottom and out at the top. A good share of the air on the main floor has come from below. A damp crawl space means musty smells, higher humidity and mold spores upstairs.

The floor structure is down there too: joists, beams, posts and subfloor, along with plumbing, ducts and wiring.

Signs you can notice from upstairs

• A musty smell, strongest in summer
• Floors that feel cold, damp, bouncy or uneven
• Hardwood that cups, or gaps that open and close with the seasons
• High indoor humidity and condensation on windows
• Mold on the lower part of walls or in closets on the main floor
• More insects or rodents
• Higher heating bills, from wet insulation

What you see below

• Standing water or mud
• Damp, darkened or white-fuzzed wood
• Insulation hanging down, wet and heavy
• Condensation dripping from pipes and ducts
• White salt deposits on the foundation walls
• Rusted metal supports, straps and ductwork
• Torn or missing plastic on the ground

Where the water comes from

Surface water. Ground that slopes towards the house, downspouts emptying at the foundation and blocked eavestroughs send rain and meltwater through vents, hatches and wall cracks.

Ground water. A high water table rises through the floor of the crawl space in spring and after long rain, particularly on low-lying lots and near lakes and rivers.

Plumbing leaks. Supply lines, drain pipes and their joints run through the space. A slow leak can go on for months. A burst pipe floods it in minutes. Drain leaks mean the water is contaminated.

Condensation. In summer, warm, humid outside air enters through the vents and meets cool surfaces. Water condenses on pipes, ducts and wood. This is why a vented crawl space can be wettest in July with no rain at all.

Evaporation from bare soil. Exposed earth releases a surprising amount of moisture into the space every day.

What it does over time

• Mold on joists, subfloor and insulation
• Wood rot in sills, joists, beams and posts, which weakens the floor
• Rust on fasteners, hangers, supports and ducts
• Ruined insulation. Wet fibreglass loses its value and falls down.
• Pests. Carpenter ants, termites in the parts of southern Ontario where they occur, and rodents are drawn to damp wood.
• Odour and poor air quality upstairs

Safety before going in

A crawl space is a confined space with real hazards.

• Do not enter standing water if there is wiring, a pump or an appliance down there. Switch the power off first.
• If the water may contain sewage, stay out and call a professional.
• Wear gloves, eye protection, coveralls and a suitable respirator. Expect mold, rodent droppings and fibreglass.
• Old pipe insulation or loose granular insulation may contain asbestos. Do not disturb it.
• Tell someone you are going in.

Getting it dry

1. Stop the source where possible: shut off a leaking supply line, divert surface water.
2. Pump out standing water with a submersible pump, discharging well away from the house.
3. Remove wet, fallen insulation and debris. Wet insulation holds moisture against the wood.
4. Take up the old plastic if water is trapped on top of it or it is torn and dirty.
5. Dry the space with dehumidifiers and air movers. Opening vents does not dry a crawl space in humid weather. It adds moisture.
6. Check the wood with a moisture meter. Framing needs to come down to a normal moisture content before the space is closed up.
7. Deal with mold on the framing by cleaning it, and replace wood that has rotted.
8. Clean up contamination if the water came from a drain line.

Keeping it dry

Outside first. Slope the ground away from the house, extend the downspouts, keep eavestroughs clear, and seal or protect vents and hatches that take in surface water.

A proper vapour barrier. Heavy polyethylene laid over the whole floor of the crawl space, overlapped and taped at the seams, and sealed up the walls and around posts. This stops evaporation from the soil.

Drainage and a sump pump where ground water rises. A perimeter drain to a sump pit, with a pump and ideally a battery backup, removes water before it stands.

Closing or conditioning the space. Building scientists now generally favour sealing crawl space vents, insulating the walls instead of the floor above, and controlling the humidity with a dehumidifier or a small supply of conditioned air. This is often called encapsulation. Done properly, it avoids the summer condensation problem. Any fuel-burning appliance in the space, and radon, have to be taken into account, so get advice before sealing a crawl space.

Fix plumbing and insulate cold water pipes and ducts to limit condensation.

Monitoring

Leave a humidity gauge with a remote display in the crawl space, and a water alarm on the floor. Look in a couple of times a year and after heavy storms.

Insurance

A sudden plumbing failure in the crawl space is commonly covered for the resulting damage. Ground water seepage, surface water without an overland endorsement, condensation, rot and long-term dampness generally are not. A photographic record of the cause helps where the source is in question.

More: water damage restoration.