Crawl space repair
Musty air, cupping floors, high humidity, standing water, mold on the joists — here's what's actually happening under your house, what each fix costs, and how to hire a crawl space contractor who solves the water problem instead of just wrapping plastic over it.
About a fifth of American homes sit over a crawl space, and almost none of the owners have been down there. That's exactly why it's the easiest place in the house to be oversold: nobody is going to crawl 30 feet through a vented dirt space on their belly to check whether the photos match their house.
The physics are simple. Warm, humid outside air enters a vented crawl space and hits cooler surfaces underneath your floor. It condenses. Wood above about 20% moisture content grows mold and attracts fungus and pests. Meanwhile, roughly half the air you breathe on the first floor came up through that crawl space. A wet crawl space is a whole-house problem.
This page covers what the real fixes are, in the order a competent contractor should do them — water out first, then seal, then condition, then repair the damage — plus national cost ranges, the signs that mean you have a problem, and the questions that separate a real contractor from a plastic-and-a-promise sales pitch.
The work
01
Nothing else works until the water stops. That means downspout extensions and grading outside, then a perimeter drain in the crawl space, a sealed sump basin and a pump with a check valve, and a discharge line routed well away from the house. A quote that starts with encapsulation and never mentions where the water is coming from is skipping step one.
02
A vapor barrier is polyethylene sheeting over the dirt floor, typically 6 to 12 mil, loosely laid. Full encapsulation is a heavier liner — usually 12 to 20 mil reinforced — sealed at the seams, run up and mechanically fastened to the foundation walls, and sealed around piers, with the vents closed. Encapsulation is the version that actually controls humidity; a thin barrier alone mostly keeps your knees clean.
03
A sealed crawl space needs its air managed. A dedicated crawl space dehumidifier with a condensate drain is the standard approach, sized to the square footage, ideally with a humidity readout you can see without going under the house. Target relative humidity is roughly 50 to 60 percent. Some builders instead supply conditioned air from the HVAC system; either way, code generally requires one or the other once you close the vents.
04
In an encapsulated crawl space, insulation belongs on the foundation walls — rigid foam board, usually with a code-required thermal or ignition barrier — not stapled between the floor joists where fiberglass sags, holds moisture and becomes a rodent hotel. If a contractor proposes sealing the space and leaving wet fiberglass overhead, they're doing half a job.
05
Mold on joists and subfloor gets cleaned, treated with a borate or antimicrobial, and then the moisture source gets fixed — in that order, or it comes right back. Anything with structural rot needs a carpenter, not a sprayer. Ask what the contractor's plan is for existing growth before the liner goes down and covers it.
06
Bouncy or sloping floors over a crawl space usually trace to a failed interior support — a crushed shim stack, a settled pier, a split girder, or joists softened by years of moisture. Fixes include new concrete footings and piers, adjustable steel supports, sistering joists and replacing girder sections. This overlaps directly with foundation repair and is often quoted together with it.
07
Sealing a crawl space is also the moment to fix rodent entry points, replace a rotted access door, add a proper sump lid, and consider radon — sealed soil-gas barriers and a passive or active vent stack are far cheaper to install while the liner is going in than after.
Plan the budget
Ranges below are national planning numbers for a typical 1,000 to 1,500 square foot crawl space. Real pricing depends heavily on headroom, access, how much debris and old insulation has to come out, and whether water has to be managed first. Check your city page for local pricing.
| Scope | Planning range | What changes it |
|---|---|---|
| Crawl space inspection (independent) | $150 – $400 | Many contractors inspect free as a sales call |
| Debris and old insulation removal | $500 – $2,500 | Priced by volume and access |
| Vapor barrier only (6–10 mil, laid) | $1,200 – $3,000 | Minimal humidity control on its own |
| Full encapsulation (12–20 mil, sealed) | $5,500 – $15,000 | Liner, wall attachment, seam sealing |
| Crawl space dehumidifier installed | $1,500 – $3,500 | Sized unit plus condensate drain |
| Perimeter drain + sump pump system | $3,000 – $9,000 | The step most quotes underprice |
| Battery backup sump pump | $700 – $1,800 | Worth it where outages follow storms |
| Foundation wall insulation (rigid foam) | $1,800 – $5,000 | Replaces sagging joist fiberglass |
| Mold remediation | $1,500 – $6,000 | Scope depends on surface area affected |
| Joist or girder repair / sistering | $1,500 – $8,000 | Rot repair priced per member |
| New support piers (per pier) | $800 – $2,200 | Footing, pier and shimming |
| Radon mitigation system | $900 – $2,800 | Cheapest installed alongside encapsulation |
Headroom: an 18-inch crawl doubles labor time versus a 4-foot crawl. It's the biggest hidden multiplier.
Square footage and pier count — every pier is a hand-sealed detail in an encapsulation job.
Liner thickness and whether seams are taped or heat-welded, and whether the liner is mechanically fastened to the walls.
Water first: a space that needs drainage and a sump before sealing can easily add $4,000 to $9,000.
Cleanout: decades of debris, torn fiberglass, or rodent contamination is billed separately and is rarely in the online price.
Access: no exterior door, a crawl reached only through a closet hatch, or a tight yard for the discharge line.
Regional climate: humid Southeast and Mid-Atlantic markets price full encapsulation as standard; drier Western markets often need far less.
Know the signs
You don't have to go under the house to see most of these. Half of them show up on your floors, your air and your energy bill.
A musty or earthy smell on the first floor, strongest near floor vents or an interior stair.
Hardwood floors cupping, buckling, or gapping seasonally.
Bouncy, springy, or visibly sloping floors above the crawl space.
Indoor humidity that stays above 60 percent in summer no matter what the AC does.
Insulation sagging out of the joists or lying on the ground when you open the access door.
Standing water, damp soil, or a water line on the piers after heavy rain.
Visible mold, white fungal growth, or dark staining on joists and subfloor.
Rusting ductwork, water heater or metal strapping under the house.
Rodent droppings, chewed insulation, or a crawl door that no longer seals.
Rising heating and cooling bills with no other explanation.
No pressure
You don't need to diagnose the job before you ask for help. Tell Henry what you see and compare up to three local crawl space contractors.
Get matched with a proHire with confidence
This category attracts both excellent building-science contractors and aggressive one-product sellers. The difference shows up in the first ten minutes of the inspection.
Require dated photos or video of your crawl space, with a location described for each shot — not stock images or 'a house like yours.'
Ask for a moisture reading on the joists and a relative humidity number. Real contractors carry a meter.
Make sure the proposal handles water first: grading, downspouts, drainage and sump before liner and dehumidifier.
Get liner specs in writing: mil thickness, reinforced or not, seam method, wall attachment method, pier wrapping.
Confirm the dehumidifier is sized to your square footage and has a real condensate route — not a bucket someone has to empty.
Ask how existing mold and any rotted wood get handled before the liner covers them.
Verify license or registration in your state, plus general liability and workers' comp certificates.
Collect three quotes and compare scope line by line — the cheapest is usually a bare vapor barrier compared against a full system.
Read the warranty: what's covered, is it transferable, does it require paid annual service to stay valid.
Ask about a post-install humidity check at 30 to 60 days, and get it in the contract.
A same-day price that drops if you sign before the salesperson leaves.
Encapsulation quoted with no drainage plan in a crawl space that has standing water.
Photos that can't be tied to your house.
"Mold" quoted as a whole-house health emergency with a five-figure remediation and no lab or moisture data.
Fiberglass batts left in the joists after the space is sealed.
A lifetime warranty voided unless you buy an annual inspection plan.
Related work
Sagging floors, settling piers and cracked walls under the same house are foundation work — read that guide next if your symptoms are structural.
Questions homeowners ask
For a typical 1,000 to 1,500 square foot crawl space, full encapsulation with a heavy sealed liner generally runs about $5,500 to $15,000. Adding drainage and a sump pump, a dehumidifier, cleanout or mold work commonly pushes a complete project past $15,000. A plain vapor barrier alone is far cheaper — roughly $1,200 to $3,000 — but controls much less moisture.
In humid climates, generally yes: it stabilizes indoor humidity, protects the wood structure, discourages pests and typically trims heating and cooling costs. In dry Western climates a properly drained space with a good vapor barrier is often enough. The deciding factor is your measured crawl space humidity and moisture content, not the sales pitch.
Almost always. Once you close the vents you own the air in that space, and codes generally require either mechanical dehumidification or conditioned supply air. A sealed crawl space with no humidity control can end up wetter than a vented one.
Modern building science favors sealing vents and conditioning the space in most climates, because outside summer air condenses on cool surfaces underneath the house. Sealing only works as part of a package: ground liner, air sealing, insulated walls and humidity control.
Laying a vapor barrier is a realistic DIY job if you have headroom and patience. Full encapsulation — mechanically fastened liner, sealed seams and penetrations, drainage, sump and a properly drained dehumidifier — is a professional job, and it's the sealing details that decide whether it works.
Most jobs run two to four days. Add a day or two for debris and old insulation removal, and more if drainage, a sump system or structural repairs are part of the scope.
Typically not. Insurers exclude gradual moisture damage, mold from long-term humidity and groundwater intrusion. A sudden event like a burst pipe under the house may be covered. Check your policy's water damage and mold sublimits.
No. Sealing controls moisture; it doesn't lift anything. Sagging or bouncy floors need structural work — new piers and footings, adjustable steel supports, sistered joists or girder repair — which should be done before the new liner goes in.
Aim for roughly 50 to 60 percent relative humidity, with wood moisture content under about 15 to 18 percent. Ask your contractor to leave a remote humidity monitor so you can verify it without crawling under the house.
A sealed liner reduces soil gas entry, but it isn't a mitigation system on its own. If you test above the EPA action level, add a vent pipe and fan — it is dramatically cheaper to install while the liner is being placed.
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Local market guides
Each city page covers local labor pricing, weather, permits, licensing, neighborhoods and the contractors Henry can match in that market.
Tell Henry what you need. We’ll send your project to no more than three vetted local pros — and you decide whether any of them earns your business.
Sources: EPA — indoor air quality and moisture control · U.S. Department of Energy — crawl space insulation guidance · Angi — crawl space encapsulation cost data · HomeAdvisor — true cost guides
The Henry Standard
Call back. Keep appointments. And if something changes, communicate it.
Clear scope, materials, and pricing in writing—so you know what you're agreeing to.
See the job through, including the drainage and moisture control that was quoted.
Nothing revolutionary. Just the way we believe doing business should still be done.
