Comparison
Crawl space encapsulation vs a dehumidifier
This is the wrong question, and answering it as posed is how people end up paying for a major piece of work and still having a wet crawl space. Encapsulation and dehumidification are not two ways of doing the same job. They are two halves of one job. Encapsulation stops new moisture coming in from the soil and the outside air. A dehumidifier removes the moisture that is already down there, plus whatever still finds a way in after you have sealed everything you can reach. Neither does the other’s work.
The consequence matters more than the definition: sealing a crawl space without adding a way to dry it frequently makes the space worse than it was. The vents were doing something — badly, unreliably, and only in the right weather, but something. Close them and the air stops moving. If the moisture source is still active, and in most crawl spaces some of it always is, you have built a sealed box around it.
We have never encapsulated a crawl space and we have never run any of these units. What we publish instead is the arithmetic — on both dehumidifier rating scales — so you can check it against your own space. Product links below are affiliate links; see our affiliate disclosure. Last reviewed 21 August 2026.
What encapsulation actually is
“Encapsulation” is a contractor’s word for closing a crawl space off from the ground and the outdoors. Done properly it is four or five separate pieces of work, and skipping any of them undoes most of the benefit of the others:
- A heavy vapour barrier over every square foot of exposed soil, seams lapped and taped, run up the piers and the stem wall and mechanically fastened there. Bare soil in a crawl space evaporates water continuously; this is the single largest source in most houses, and it is the piece a homeowner can realistically do.
- Sealed foundation vents. The vents exist because older codes assumed outside air dries a crawl space. In a humid climate it does the opposite, for a reason we can compute below.
- A sealed rim joist and sill plate. The band of framing where the floor meets the foundation leaks air continuously and is usually the coldest wood in the house.
- A sealed, insulated access hatch, and often insulation on the interior face of the foundation wall rather than between the floor joists.
- Whatever drainage the space needs first — grading, downspout extensions, a French drain, sometimes an interior perimeter drain and a sump pump.
Three of those five are not products. They are labour, on your knees, in a space you may not be able to stand up in, and in most houses they are bought as a contractor package. We cannot help you price that half honestly. Quoted encapsulation jobs vary by more than the size of the crawl space explains — clearance, debris, standing water, existing damage, and how much drainage work is bundled in all move the number, and we have no dataset that would let us give you a range worth trusting. Get three quotes and make them itemise the drainage separately from the barrier.
The part we can price is the DIY vapour barrier, because those are ordinary products with published dimensions. Worked through further down, a 1,200 sq ft crawl space comes to roughly $299.97 in sheet and tape. That figure is materials only and assumes a crawl space you can actually work in.
What each one fixes, and what it doesn’t
Lifting this out of prose is the clearest way to see why the “versus” framing fails. Read down the two columns and notice that they barely overlap.
| Moisture path | Encapsulation | Dehumidifier |
|---|---|---|
| Evaporation from bare soil | Stops it | Removes the result, forever |
| Humid outside air through vents | Stops it | Fights it, continuously |
| Air leaking in at the rim joist | Stops most of it | Fights it |
| Moisture already in the wood and air | Does nothing | This is its entire job |
| Vapour through uncoated concrete | Reduces it | Removes the result |
| A plumbing leak or standing water | Does nothing | Does nothing |
| Groundwater and poor grading | Does nothing | Does nothing |
The bottom two rows are the ones people skip. Neither product on this page moves liquid water. If your crawl space has puddles after rain, a dehumidifier will run continuously and lose, and a vapour barrier will float on top of the problem. That is a drainage job, and it comes first.
Why sealing without drying goes wrong
Here is the physics, and it is the reason the vents were a bad idea in the first place. Warm air holds more water than cold air. On a humid summer afternoon at 85°F and 75% relative humidity, outdoor air has a dew point of 76.2°F. Your crawl space, shaded by the house and sitting on ground that stays cool all summer, is well below that. Ventilating with that air does not dry the crawl space — it delivers water to it, and the water condenses on the first surface it touches. That is why sealing the vents is usually right.
But sealing changes what the air is, not how much water the space contains. The soil keeps evaporating unless it is covered. The concrete keeps passing vapour. The house above keeps pushing air down through the floor in winter and pulling it up in summer. With the vents shut and nothing removing water, relative humidity climbs and stays there — which is exactly the condition that supports mould, and the reason we treat 60% as the line that matters rather than a comfort number.
Outside useful range. Above the mould-growth threshold. Sealing the vents alone lands most crawl spaces here.
Bands are the target range this site sizes against: 30–50% ideal, 60% as the mould-risk line. The reading shown is an illustrative figure, not a measurement we took.
Relative humidity on its own does not tell you whether anything will actually get wet. Dew point does. At 72°F and 72% RH, the air in that example reaches saturation at 62.5°F — so every surface in the crawl space colder than 62.5°F is collecting liquid water. In a summer crawl space the cold surfaces are the air conditioning ducts, the cold water lines, and the sections of the slab or soil in deepest shade. This is computed with the Magnus formula, the same one weather services use, and you can check it against any dew point table.
| Relative humidity | Dew point | Anything colder than this sweats |
|---|---|---|
| 50% | 52.3°F | Chilled surfaces only |
| 55% | 55.0°F | Chilled surfaces only |
| 60% | 57.4°F | Cold water lines and uninsulated ducts |
| 65% | 59.6°F | Cold water lines and uninsulated ducts |
| 70% | 61.7°F | Ductwork, cold water lines, most of the floor structure |
| 75% | 63.6°F | Ductwork, cold water lines, most of the floor structure |
| 80% | 65.5°F | Ductwork, cold water lines, most of the floor structure |
Read the table upwards. Pulling the crawl space from 80% down to 50% drops the dew point by about 13°F. That is the entire mechanism by which a dehumidifier stops condensation: it does not warm the surfaces, it lowers the temperature at which they would start to sweat. A vapour barrier cannot do that, because it does not touch the air that is already in the space.
If you can only do one this year
There is a real order here, and it depends on one thing most articles never separate: covering the ground and closing the vents are two different decisions with two different risk profiles.
The order that actually holds
- Fix bulk water first. Gutters, downspout extensions, grading away from the house, any plumbing leak. Neither a barrier nor a dehumidifier is a substitute, and doing this first sometimes reduces everything that follows.
- Lay a ground vapour barrier — vents open. This is the highest-value single step and it carries almost no risk, because you have not removed any ventilation. You have only stopped the largest moisture source. If your budget stops here, stop here.
- Buy the dehumidifier before you seal the vents, not after. The moment the vents close, the space depends entirely on mechanical drying. Sealing first and buying later is the sequence that produces the horror stories.
- Seal vents, rim joist and hatch last, with the dehumidifier already running and a hygrometer down there so you can see what the change did.
Steps 2 and 3 are the two purchases on this page, and note the order they come in. If you have a bare-soil crawl space with open vents and a limited budget, the sheet plastic is the better first spend. If you have a crawl space that someone has already sealed — a previous owner, a partial job, a contractor who closed the vents and left — then you are already past step 3 with nothing running, and the dehumidifier is urgent rather than optional.
How much of each a real crawl space needs
Both halves can be sized. Here is a 1,200 sq ft crawl space with damp spots on the joists and soil you can leave a handprint in — the very damp class — worked through end to end.
Worked example
1,200 sq ft crawl space, very damp, about to be sealed
Bare soil, open foundation vents, no standing water. Roughly 35 ft square. Sizing uses the AHAM dampness classes with a crawl-space uplift, and prints both dehumidifier rating scales.
- 1Floor area to coverEvery square foot of exposed soil, not just the accessible part1,200 sq ft
- 2Barrier needed, with laps and run-up+15% for seam overlap and running the sheet up piers and stem wall1,380 sq ft
- 3Rolls of SPIRECOVER 10 ft × 100 ft sheetListing states 10' × 100', so 1,000 sq ft per roll2 rolls
- 4Seam and perimeter tape4 strips across a 35 ft run gives 105 ft of seam, plus 140 ft of perimeter245 ft
- 5Materials subtotal2 × $129.99 sheet + $39.99 tape (3 rolls, 540 ft listed)$299.97
- 6Drying requirement — legacy chart scaleWhat a pre-2019 sizing chart would print for this space28 pints/day
- 7Drying requirement — modern nameplateThe same requirement, expressed in today's post-2019 DOE labels20 pints/day
Two things to take from this. First, the materials are affordable and the labour is the expensive part — which is the honest reason DIY encapsulation appeals and also the reason it goes wrong.
Second, and less expected: the drying requirement is small — about 20 pints a day on today’s labels. Every purpose-built crawl space unit in our data clears that several times over. In a crawl space, capacity is almost never the binding constraint. What is: whether the unit will run at crawl space temperatures, and whether it can get the water out without a bucket you have to crawl in and empty.
One caution on that requirement figure. It is printed on both scales for a reason. In 2019 the US Department of Energy moved the dehumidifier test from 80°F to 65°F, and a dehumidifier removes less water from colder air — so the same machine now earns a lower number. A pre-2019 “70 pint” unit wears roughly a “50 pint” label today without a single part changing. Most sizing charts still circulating were written before the change or copied from something that was, and they almost never say which scale they are on. Ours does, on every figure.
ALORAIR ALORAIR Wi-Fi 70 PPD Dehumidifier with Pump for Crawl Space and Basement
Purpose-built for crawl spaces with an integral pump — the part that matters when there is no floor drain and no one to empty a bucket. Rated 70 pints/day on the modern scale against a 20 pint/day requirement, so it is deliberately oversized for duty cycle rather than capacity.
$375.35 · 309 reviews
To be straightforward about that pick: it is not the cheapest way to remove 20 pints a day. Several units in our data cost less than half as much and are rated for more. They are portable basement machines with a collection bucket and a control panel you have to see to use, and a crawl space has nowhere to empty a bucket and nowhere comfortable to stand while you read a display. The pump, the remote monitoring and the low ambient duty are what you are paying for, and if your crawl space has a floor drain below the unit you can honestly skip the pump and spend less.
For the barrier half, the sheet in the worked example is SPIRECOVER 10 mil, 377 reviews and the seam tape is a 3-roll pack, 93 reviews. Ten mil is the common DIY weight. Heavier woven barriers exist — the 15 mil sheet in our data works out at $0.26 per square foot against $0.13 for the 10 mil, or 2.0× the cost. We have not laid either and cannot tell you how they hold up under knees and gravel over five years — that is a question for a contractor or a building science forum, not for us.
When a dehumidifier alone genuinely is enough
The whole argument above assumes bare soil and open vents. Plenty of crawl spaces are not in that state, and in these cases buying a dehumidifier and nothing else is the correct, complete answer rather than a compromise:
- The crawl space has a poured concrete floor. Concrete is not a vapour barrier, but it is a large reduction on bare soil, and there is no sheet to lay. This is common in newer construction and in converted basements.
- A barrier is already down and intact. Walk it, look for tears at the piers and gaps at the perimeter. If it is sound, encapsulation is done as far as the ground is concerned; the missing piece is drying.
- The vents are already sealed. Then the decision has been made for you and the dehumidifier is the outstanding half, not an alternative to anything.
- You rent, or you are selling within a year or two. A dehumidifier leaves with you and starts working the day it arrives. Encapsulation does not, and the money is gone.
- The dampness has one identifiable cause you have already fixed — a burst supply line, a failed sump, a season of blocked gutters. What is left is water in the wood and the air, which is a drying job with an end date.
There is a fifth case worth stating because nobody selling encapsulation will: some crawl spaces should not be sealed at all without professional input. If there is an atmospherically vented gas appliance — an older water heater or furnace that draws combustion air from the crawl space and vents up a masonry chimney — closing the vents can affect how it draws, and that is a carbon monoxide question, not a moisture question. Radon is a second one; sealing changes pressure relationships and some jurisdictions require a passive vent stack in an encapsulated crawl space. Both are reasons to have a professional look before you close anything, and neither has anything to do with which dehumidifier you buy.
What we can and cannot tell you
We can size the drying half. The requirement above comes from published dampness classes with a crawl-space uplift, printed on both rating scales, from one shared file so that no two pages here can disagree. You can read the derivation and check it.
We can price the DIY materials half, because sheet plastic and tape are ordinary products with stated dimensions and current listings. Prices are a snapshot taken 21 August 2026 and will drift.
We cannot price or judge the contractor half at all. Encapsulation is a service, not a product, and no amount of listing data substitutes for someone looking at your clearance, your drainage and your framing. We also will not pretend the DIY route suits every crawl space — under about two feet of clearance, over standing water, or with damaged framing or suspected asbestos or vermiculite, it does not, and dragging plastic around in it is a bad afternoon at best.
We have never run any of the units on this site and we do not claim to have. What we do instead is publish the arithmetic, on both scales, so you can check it. If you want to know how a particular unit behaves in year three, ask a building science forum — they have years of that and nothing to sell you.
Related: the other choice people face once they have decided to dry the space is desiccant versus refrigerant, which turns on how cold your crawl space gets in the season you need it dry.
Where this leaves you
If the vents are shut, this is the half that is missing
- The requirement for a 1,200 sq ft very damp crawl space is 20 pints/day on the modern DOE scale — 28 on the scale legacy charts use.
- Rated 70 pints/day, with an integral pump so the water leaves without a bucket.
- 309 Amazon reviews at time of writing; ratings are Amazon's, not ours.
- Skip it if your crawl space still has open vents and bare soil — cover the ground first, that is the cheaper and safer order.
Affiliate link. Price snapshot 21 August 2026; check the listing before buying. Damp Down has not tested this unit and makes no first-hand performance claim.
ALORAIR Wi-Fi 70 PPD Dehumidifier with Pump for Crawl Space and Basement
$375.35
100+ bought last month