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Crawl space waterproofing deals with liquid water — puddles, mud and seepage — before crawl space mold treatment or encapsulation. A liner laid over standing water just traps it.
Where Crawl Space Water Comes From
| Source | Clue | Fix |
|---|---|---|
| Surface water | Water after snowmelt or irrigation, near one wall | Grading, downspouts, sprinkler adjustment |
| Plumbing leak | Wet area under a bathroom or kitchen | Repair the leak |
| Groundwater | Water across the whole floor after long wet periods | Perimeter drain and sump pump |
| Condensation | Wet ducts and joists, no puddles | Encapsulation and dehumidification |
Plumbing leaks are found with the same tools we use for thermal and moisture-meter leak detection.
Signs of Water Under the House
- Mud, puddles or a dark wet band on the soil along one wall.
- Water lines or white crust on the foundation walls.
- Rusty pipe hangers, nails or duct straps.
- Musty air at the access hatch, worse after snowmelt or watering.
- Insects and rodents, which follow moisture.
Check after a big spring melt and again after the sprinklers have been running for a few weeks; those are the two times Eagle Mountain crawl spaces get wet. Our guide to snowmelt and sprinkler water under the house covers the outside causes.
Waterproofing Options
| Solution | Typical cost |
|---|---|
| Grading and downspout extensions | $300 – $3,000 |
| Interior perimeter drain | $2,000 – $6,000 |
| Crawl space sump pump | $1,000 – $3,000 |
| Wall sealing and crack repair | $500 – $2,500 |
A drained crawl space usually ends at a sump pump with a sealed lid.
How a Perimeter Drain and Sump Work
A perimeter drain is a perforated pipe in a gravel trench around the inside edge of the crawl space, sloped toward the lowest corner. Water that seeps in follows the pipe to a sump basin, and a pump sends it outside, away from the foundation. A drainage mat under the liner keeps any remaining water moving toward the drain instead of sitting under the plastic.
Winter tip: crawl space sump discharge lines freeze just like basement ones. Slope them so they drain, bury them where possible and add a freeze-guard outlet near the house.
Our Process
- Find the sourceTiming, moisture readings and a look at the outside grading.
- Fix the outsideGrading and downspouts first — the cheapest fix that often solves it.
- Drain what remainsA perimeter drain and sump remove groundwater that still gets in.
- Clean and dryMold is removed from wood; the space is dried.
- SealEncapsulation or a vapor barrier finishes the job.
Standing water and wiring don't mix. Do not enter a flooded crawl space with live electrical lines or equipment. Shut off power to the area first.
Plumbing and Irrigation Lines Under the House
Water under a crawl space is often not from outside at all. Supply lines to a bathroom or kitchen, a slow drain leak, a water heater or softener in the crawl space, and irrigation lines that run under an addition can all keep the soil wet in one spot. A wet area right under a bathroom, with dry soil everywhere else, points to plumbing. Fix the leak, dry the area and then decide whether you need anything more.
What Crawl Space Drainage Costs
Interior perimeter drains are usually priced by the foot of wall, often in the range of $40 to $100 per linear foot for a crawl space, plus the sump basin and pump. A 1,000 sq ft crawl space with about 130 feet of wall typically lands in the $2,000 to $6,000 range before encapsulation. Outside fixes come first because they can make the drain unnecessary.
Grading Checklist for Crawl Space Walls
- Soil slopes away at least 6 inches in the first 10 feet, as the IRC calls for.
- Downspouts discharge several feet out, not next to the crawl space wall.
- Sprinkler heads and drip lines are not soaking the soil against the foundation.
- Window wells or access wells on the crawl space wall have working drains.
Water under the floor often means mold above it — see basement mold removal in ZIP 84005.
Frequently Asked Questions
Is standing water in a crawl space bad?
Yes. It raises humidity, feeds mold on joists and subfloor, attracts pests and can weaken the structure over time.
Can a crawl space have a sump pump?
Yes. A sump basin at the lowest point, fed by a perimeter drain, removes groundwater that collects under the house.
Should I waterproof before encapsulating?
Always. Encapsulating over standing water traps it against the soil and wood.
How much does it cost to waterproof a crawl space?
Outside fixes like grading and downspouts cost $300 to $3,000. An interior perimeter drain runs $2,000 to $6,000, and a crawl space sump pump $1,000 to $3,000.