Short answer: it is not a choice, it is a sequence. Vapor barrier first, dehumidifier second. The barrier stops moisture entering from the soil, which is usually the largest single source in a crawl space. The dehumidifier removes what is left.
Run a dehumidifier over bare dirt and you are fighting an unlimited supply of ground moisture, forever, on your electricity bill. If your budget only stretches to one of the two, buy the barrier.
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Search for a crawl space moisture fix and you will find two camps: encapsulation contractors who talk about liners, and appliance sites that talk about dehumidifiers. Both are partly right. They do different jobs, and doing them in the wrong order is the most expensive way to end up with a damp crawl space anyway.
The two things do different jobs
| Vapor barrier | Dehumidifier | |
|---|---|---|
| What it does | Blocks water vapor moving out of the soil into the crawl space air | Removes water vapor already in the air |
| Addresses | The source | The symptom |
| Running cost | None | Electricity, continuously, for as long as you own the house |
| Moving parts | None | Compressor, fan, pump, controls |
| Lifespan | Decades if not damaged | Roughly 5–10 years for consumer units; longer for a good crawl space unit |
| Handles humid outdoor air | No | Yes, but only if you stop importing it |
| Handles standing water | No | No |
A crawl space takes on moisture from three places: the soil below, the outdoor air coming through the vents, and the house above. The barrier deals with the first. Sealing the vents deals with the second. The dehumidifier deals with what remains, including the third. Skip either of the first two and the machine never gets to a steady state.
Why the barrier goes first
Soil under a house is never dry. Groundwater moves upward through the soil by capillary action and evaporates from the surface into the crawl space, continuously, all year. It is not a leak you can find and fix — it is the entire floor area, working steadily.
Put a sealed sheet of polyethylene over that soil and you cut off most of it in an afternoon, with no ongoing cost. This is the highest-value dollar in the whole project. Nothing else you can buy gives that ratio.
Now consider the alternative. You install a dehumidifier over bare dirt. It pulls the air down to 55%, the drier air increases the rate of evaporation from the soil, and the moisture load goes right back up. The machine never satisfies its humidistat, so it runs continuously, uses far more electricity than it should, and wears out years early. You have bought an appliance to hold back the ground.
Worth knowing: the same logic applies to the vents. A dehumidifier in a vented crawl space is dehumidifying outdoor air as fast as the wind supplies it. Sealing the vents costs a fraction of what the electricity will, and our page on crawl space humidity levels explains why open vents in humid weather actively make things wetter.
What a crawl space vapor barrier actually is
A vapor barrier — more precisely a vapor retarder — is a sheet material with very low permeability to water vapor, laid over the crawl space floor and usually run up the foundation walls. Polyethylene sheet in the class typically used here is rated as a Class I vapor retarder, meaning a permeance of roughly 0.1 perms or less. In practical terms, almost nothing passes through it.
Thickness classes and where each belongs
| Material | Typical use | Notes |
|---|---|---|
| 6 mil polyethylene | Basic ground cover in a vented crawl space; the common code minimum | Cheap and effective as a vapor retarder, but tears easily. Fine where nobody walks on it and nothing is stored down there. |
| 10–12 mil reinforced | The usual choice for a durable liner or a light encapsulation | String-reinforced so it resists puncture from rocks and knees. A reasonable DIY sweet spot. |
| 15–20 mil reinforced | Full encapsulation, storage crawl spaces, rough or rocky ground | What most encapsulation contractors install. Heavy, tough, expensive, and worth it if the space gets used or worked in. |
Installation details that decide whether it works
- Seams overlap and get sealed. A loose sheet laid down with open seams is a partial fix at best. Overlap generously and tape the seams with a tape intended for the job.
- Run it up the walls. The foundation wall is also a moisture path. Turn the liner up the wall and mechanically fasten it with a sealed termination.
- Leave a termite inspection gap. Many jurisdictions require the liner to stop a few inches short of the sill plate so the wood remains visible for inspection. Check locally before you tape to the top.
- Seal around piers and penetrations. Piers, plumbing and posts are where a careless job leaks.
- Clear the ground first. Sharp rock, construction debris and old insulation come out before the liner goes down.
Liner vs full encapsulation
A ground liner covers the dirt and stops there. It is the cheap, high-return version, and in many crawl spaces it is enough.
Encapsulation is the whole envelope. It adds:
- A heavier liner carried up and sealed to the foundation walls
- All seams and penetrations sealed
- Foundation vents closed and sealed
- The band joist air-sealed and usually insulated
- Wall insulation in colder climates, since the crawl space is now inside the conditioned envelope
- Humidity control — a dedicated dehumidifier, or a small supply of conditioned air from the house
The distinction that matters: an encapsulated crawl space is a sealed, controlled space, so it needs humidity control as part of the design. A liner in a still-vented crawl space does not necessarily. If someone seals your vents and does not provide a way to manage moisture, that is an incomplete job, not a bargain.
What each costs, honestly
These are the ranges commonly quoted in the US. They are ranges, not quotes, and crawl space work varies enormously with access, headroom, debris, drainage and regional labor rates.
| Job | Commonly quoted range | What drives it |
|---|---|---|
| DIY 6 mil ground cover | Low hundreds of dollars in materials | Area, tape and fasteners. The cost is your weekend, not the plastic. |
| Professionally installed liner | Roughly $1,500–$5,000 | Area, headroom, cleanout, liner thickness |
| Full encapsulation | Roughly $3,000–$15,000 | All of the above plus vent sealing, band joist, insulation, dehumidifier |
| Crawl space dehumidifier | Several hundred to a few thousand dollars for the unit | Capacity, pump, ducting, brand tier; installation extra |
| Drainage and sump, if needed | Added on top, often substantially | Whether liquid water enters at all |
If a full quote is out of reach, stage it. The ground cover is the part to do now; the dehumidifier is the part to defer.
When a barrier alone is enough — and when it never will be
A barrier alone is often enough when:
- You are in a dry climate where summer outdoor dew points stay well below crawl space temperatures
- The site drains well, the grading slopes away, and gutters discharge clear of the foundation
- There is no ductwork in the crawl space to condense on
- Readings after the liner goes in stay around 50–55% through the humid season
A barrier alone will never be enough when:
- You have humid summers — the eastern and southern US, the Gulf, the Pacific Northwest coast — and the crawl space stays vented
- There is standing water after rain, which is a drainage problem no sheet of plastic addresses
- AC ducts run through the space and sweat every July
- The crawl space connects openly to a basement or to the house, so house moisture reaches it
The honest test is measurement, not argument. Put a hygrometer down there, cover the ground, and see where the readings land through August. Buy the dehumidifier if the numbers say you need one.
The order of operations for the whole job
- Fix liquid water. Grading, gutters, downspout extensions, and interior drainage with a sump if water enters. Nothing downstream works until this is done.
- Clear the crawl space. Debris, sharp rock, old sagging insulation, anything that has been wet.
- Lay and seal the ground vapor barrier, up the walls, around piers, leaving the termite inspection gap where required.
- Seal the vents and the band joist if you are going to a closed crawl space. Check local code first.
- Measure for a season. You now know what the real remaining load is.
- Install a dehumidifier if the readings call for it — a low-temperature crawl space dehumidifier with an integral pump, not a portable. The crawl space dehumidifier guide covers selection and installation, and what size dehumidifier for a crawl space covers capacity.
What to do about the vents
Foundation vents exist because building codes once required them, on the theory that outdoor air would carry moisture away. In humid climates it does the opposite: warm outdoor air cools below its dew point in a crawl space and deposits water on the framing.
Modern codes recognize closed, unvented crawl spaces as an alternative, with conditions attached — typically a continuous ground vapor retarder plus either a small amount of conditioned air from the house or a dehumidifier. Requirements vary by jurisdiction and some regions have specific rules, so check locally before sealing anything.
What you should not do is close the vents on a bare dirt crawl space and walk away. That traps ground moisture with nowhere to go and no way to remove it. Vents come out of the plan as part of a sealed system, not on their own.
Frequently asked questions
Do I need a dehumidifier if I have a vapor barrier?
Sometimes. Measure after the barrier is in. In a dry climate with good drainage, a liner often gets readings to 50–55% on its own. In a humid climate, and especially in an encapsulated crawl space where you have deliberately closed off the ventilation, you will usually need one.
What thickness vapor barrier does a crawl space need?
6 mil polyethylene is the usual code minimum for a ground cover and blocks vapor perfectly well, but it tears. For anything you will walk on or store in, a 10–12 mil reinforced liner is a better buy, and encapsulation contractors typically use 15–20 mil. Thickness is about durability, not vapor performance.
Is crawl space encapsulation worth it?
In a humid climate with a chronically damp crawl space, generally yes — it addresses the cause rather than running an appliance against it forever, and it usually improves comfort and floor temperature upstairs. In a dry climate with a crawl space that already measures fine, it is an expensive solution to a problem you do not have. Measure first.
Can I install a crawl space vapor barrier myself?
A ground cover, yes, if you can physically work in the space and the ground is reasonably clear. It is unpleasant but not technical. Full encapsulation — wall terminations, band joist air sealing, insulation, vent closure — is more often worth hiring out, and any sign of standing water, structural rot or significant mold growth should be assessed before you seal anything in. Existing mold has to be dealt with, not covered over.
Will a vapor barrier get rid of the musty smell?
It removes the moisture that feeds the smell, so over time it usually helps a lot. It does not remove growth that is already on the joists or the insulation, and it does not fix a smell whose source is somewhere else in the house. Cover the ground, dry the space, remove the growth — in that order.
Next steps
- What readings to aim for: crawl space humidity levels explained
- Choosing the machine: the complete crawl space dehumidifier guide
- Getting capacity right: what size dehumidifier for a crawl space
- If mold is already present: what a dehumidifier can and cannot do about mold
- Where the mold risk begins: what humidity level prevents mold
