BuzzSkito Mosquito & Tick Control Specialists · Published August 22, 2026
Crawl Space Dehumidifier vs Vapour Barrier: Which One First?
Lay the vapour barrier first. In a Canadian crawl space with an exposed earth floor, moisture evaporating out of the soil is the largest single source of humidity in the space, and unlike weather it never pauses. A dehumidifier set over bare ground is therefore competing with the earth itself: it will hold the reading down while it runs, and the space will refill as soon as it stops. Covering the soil removes that source outright, for a materials cost in the low hundreds rather than the four figures a proper crawl space dehumidifier commands.
The reason this article exists is that the honest answer to “which one” is often both — but in a specific order, with a measurement in between. Barrier, then measure for a month, then decide about the machine. Do it the other way around and you buy the expensive half to solve a problem the cheap half would have removed.
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Where Crawl Space Moisture Actually Comes From
There are only three sources worth thinking about, and they need different responses.
- The soil. Ground under a house is rarely dry, and water evaporates continuously from its surface into the air above it. On a bare-earth crawl space this dwarfs everything else, which is why the ground cover is the highest-leverage thing you can install.
- Outdoor air through the vents. Counterintuitively, this is a source rather than a cure for much of a Canadian summer. Warm humid July air entering a crawl space whose surfaces are still cool drops below its dewpoint and gives up water onto joists, ducts and foundation walls.
- Bulk water. Poor grading directing runoff at the foundation, downspouts discharging beside the wall, a seasonally high water table, a weeping-tile failure, or a leaking supply or drain line. This is liquid water, not vapour, and it outranks both of the others.
That ranking is the whole strategy. Fix bulk water, then cover the soil, then deal with the air. Skipping to the air is what makes people say a dehumidifier “did not work” in a crawl space — it worked exactly as designed against a problem it was never going to win.
The Order of Operations
Work through these in sequence, and stop when the humidity reading is where you want it. Each step is cheaper than the one after it.
- Fix the bulk water. Extend downspouts, correct grading so the ground falls away from the house, repair any plumbing leak, and add a sump if groundwater genuinely rises into the space. Nothing downstream matters until this is done.
- Cover the ground. Rake out rock and construction debris, then lay a reinforced liner across the entire earth floor, lapping the sheets generously, taping every seam, and running the material up the foundation walls to a mechanically fastened termination.
- Decide about the vents. This is the step that needs local advice rather than an internet opinion — see the warning above. Sealing is common practice in encapsulation work, but it changes how the space and any combustion appliances behave.
- Insulate, if you are going that far. In a sealed crawl space the insulation typically moves to the perimeter walls rather than the floor above. This is where a project stops being a weekend job.
- Dehumidify what is left. Only now does an appliance make sense, and only if a month of readings says the space still sits above about 60% relative humidity.
Getting the Vapour Barrier Right
The barrier is simple in concept and easy to do badly. Thickness first: 6-mil polyethylene is the common minimum and is generally accepted as a ground cover, but it tears where it passes over gravel and will not survive anyone crawling on it. If the space will ever be entered — and someone will eventually need to chase a pipe or a duct — a reinforced liner in the 10 to 20 mil range is the better buy. The woven scrim between the film layers is the difference between a barrier that lasts decades and one that is in shreds by the second year.
Then the detailing, which is where DIY jobs fail. Clear the ground of rock and debris before anything goes down. Overlap adjacent sheets generously and tape the full length of every lap with a tape actually formulated for polyethylene — general-purpose tape lets go in a cool, damp, low-airflow space within a season. Run the liner up the foundation walls rather than stopping at the perimeter, and hold it there with fasteners and a termination strip so it cannot sag back down and reopen the path you just closed. Cut and seal neatly around piers and posts. None of this is skilled work; all of it is the difference between a barrier and a tarp.
Getting the Dehumidifier Right
If the readings say you need one, the specification that matters most is not the pint rating — it is the minimum operating temperature. Crawl spaces run cool, and a standard room dehumidifier condenses moisture on a refrigerated coil that frosts over and stops extracting once the space drops to around 5°C. That is the same physics that decides the purchase in an unheated garage, which we cover in detail in our guide to the best dehumidifier for a cold garage. Purpose-built crawl space units are designed for low-temperature duty and for being left alone for a season, which is what this application demands.
Plan the drainage before you choose the machine, because it often decides which machine you can use. A bucket is not an option in a space nobody visits; it will sit full and idle for months. If a sump pit or floor drain sits below the unit, a gravity hose with a genuine continuous fall is the most reliable arrangement. In most crawl spaces there is nowhere lower to send water, so a condensate pump that lifts it out to a sump, a standpipe, or outside the foundation is standard practice. Add a remote humidity sensor so you can read the space from indoors and actually notice when something has stopped working. For the broader sizing and drainage logic that carries over from basements, our basement dehumidifier guide and the large-capacity guide go deeper.
When the Barrier Alone Is Genuinely Enough
Plenty of Canadian crawl spaces never need the appliance, and it is worth knowing what those look like before you commit to a four-figure purchase. If the soil is naturally dry, the site drains away from the house, gutters and downspouts discharge well clear of the foundation, there is no history of standing water underneath, and the space is small, then covering the earth removes the dominant source and the humidity settles into a workable range on its own. The barrier plus a decent hygrometer is the entire project.
The cases that do need both are equally recognisable: a crawl space you have sealed up as part of an encapsulation and which therefore has no other moisture removal; a space with a seasonally high water table; a house where humid summer air was previously being vented in and the surfaces are still cool; or a crawl space that has already grown mould and needs holding well below the threshold while the remediation is done. On that last point, be clear about what a dehumidifier can and cannot do — it prevents and controls, it does not clean established growth, which is the argument we lay out in full in our dehumidifier for mould guide.
The Pest Angle, and Its Limits
A damp crawl space is close to perfect habitat for the moisture-loving pests — camel crickets, silverfish, centipedes, springtails, sowbugs — and a sheltered undisturbed one is an obvious nesting site for rodents. Covering the ground and holding relative humidity in the 45 to 55 per cent band removes the main reason the moisture crowd is there at all, and the liner also cuts down the musty soil-air path from the crawl space into the rooms above, which is often what a homeowner is actually smelling upstairs.
Be honest about the limit, though. Drying a crawl space does not close a gap in the foundation, seal a utility penetration, or fix a rim joist a mouse can walk straight through. Humidity control and physical exclusion are two separate jobs, and a crawl space with a rodent problem needs both — our guides to getting rid of silverfish and to rodent exclusion cover the half this page does not. Do the water work first regardless; a dry crawl space makes every other pest measure work better, and it is the one improvement that also protects the structure above it.
Frequently Asked Questions
Do I need a crawl space dehumidifier or a vapour barrier first?+
The vapour barrier goes first, almost without exception. In a Canadian crawl space with an exposed earth floor, the single largest moisture source is evaporation out of the soil, and that source runs continuously, day and night, all year. A dehumidifier placed over bare soil is trying to out-pump the ground — it will hold the humidity down while it runs, at a real electricity cost, and the moment it stops the crawl space refills. A sheet of polyethylene laid over the soil, lapped and taped and run up the walls, cuts that evaporation off at the source for a materials cost measured in the low hundreds of dollars. Do that first, watch the humidity for a few weeks with a wireless sensor, and only then decide whether a dehumidifier is still needed. Very often the answer changes once the ground is covered.
Is a vapour barrier alone enough for a crawl space?+
Sometimes, and it is worth finding out before you spend four figures on an appliance. A ground barrier alone is often enough where the soil is naturally dry, the site drains away from the house, gutters and downspouts discharge well clear of the foundation, there is no history of standing water under the house, and the crawl space is small. In those conditions covering the earth removes the dominant moisture source and the space settles into a reasonable humidity range on its own. It is usually not enough where the crawl space is vented to humid summer air, where plumbing has leaked, where groundwater rises seasonally, or where you have sealed the space up as part of an encapsulation. The way to know is to measure: put a wireless hygrometer down there after the barrier goes in and read it for a month before buying anything else.
How thick should a crawl space vapour barrier be?+
Six-mil polyethylene is the common minimum and it is what most codes and inspectors will accept as a ground cover, but it is thin, it tears where it passes over gravel or debris, and it will not survive being crawled on. If anyone is ever going to enter that space — to service ductwork, chase a plumbing leak, or store anything — step up to a reinforced liner in the 10 to 20 mil range. The reinforcement is a woven scrim between the film layers, and it is the difference between a barrier that lasts twenty years and one that is shredded within two. The extra cost on a typical crawl space is modest against the labour of doing the job twice. Rake the ground clear of rock and construction debris before you lay anything, whatever thickness you choose.
Should crawl space vents be open or sealed in Canada?+
This is the most contested question in the whole subject and the honest answer is that it depends on your building and that you should confirm locally before you seal anything. The traditional approach vents a crawl space to the outside year-round. The problem is a physics one: on a humid Ontario summer day, warm outdoor air at high absolute humidity enters a crawl space whose surfaces are still cool, drops below its dewpoint, and condenses — so the vents that were meant to dry the space are actively wetting it. That is why sealed and encapsulated crawl spaces have become common. But sealing changes how the space behaves and it is not a decision to take casually: if there is any atmospherically vented combustion appliance down there, if radon is a consideration in your area, or if your building inspector requires venting, get a qualified professional and your local building official involved before you close a single vent.
What size dehumidifier does a crawl space need?+
Size it for the moisture load, not the floor area, because a crawl space breaks the usual charts in both directions. A sealed, barriered crawl space has a very low load and a modest unit will hold it easily. An open, vented crawl space over bare soil has an effectively unlimited load and no size of dehumidifier wins, which is precisely why the barrier comes first. For an encapsulated Canadian crawl space, the specification that matters far more than raw pint rating is the minimum operating temperature: crawl spaces run cool, and a standard room dehumidifier whose coil frosts below about 5°C is the wrong machine. Purpose-built crawl space units are designed for low-temperature operation, for being left unattended, and for draining continuously rather than into a bucket nobody will ever empty.
Where does a crawl space dehumidifier drain to?+
Never into a bucket — that is the one guaranteed failure. A crawl space is the least-visited part of a house, so the unit has to be hands-free or it will sit stalled on a full tank for months. If there is a sump pit or a floor drain below the unit, gravity drainage through a hose is the simplest and most reliable answer, provided the hose falls continuously with no low spot that can hold water. Most crawl spaces have nowhere lower to send it, which is why a condensate pump is standard practice here: it collects the water and pushes it up and out to a sump, a laundry standpipe, or outside the foundation. Plan the drainage route before you choose the machine, because it often decides which machine you can use.
Will a dry crawl space keep pests out?+
It removes the main reason many of them are there, which is not the same as an exclusion job but is a bigger factor than most homeowners expect. A damp crawl space is close to ideal habitat for the moisture-loving crowd — camel crickets, silverfish, centipedes, springtails, sowbugs — and it also draws rodents looking for a sheltered, undisturbed place to nest. Covering the ground and holding relative humidity in the 45 to 55 per cent range makes the space markedly less attractive, and a sealed liner also cuts down the soil-gas and musty-air path from the crawl space into the rooms above. It does not, however, close gaps in the foundation, seal utility penetrations, or fix a rim joist a mouse can walk through. Humidity control and physical exclusion are two different jobs and a serious crawl space needs both.
What does a crawl space vapour barrier and dehumidifier cost in Canada?+
Treat these as two very different price tiers, which is part of why the order of operations matters so much. A do-it-yourself ground barrier for a typical crawl space is a materials purchase: reinforced sheeting, seam tape, fasteners and a termination strip generally land in the low-hundreds range for a small to mid-sized space, and the work is unpleasant rather than skilled. A purpose-built low-temperature crawl space dehumidifier is a different order of magnitude — a serious unit runs into four figures once you add a condensate pump and the drainage run, plus the electricity to keep it going. Spending a few hundred on the barrier first, then measuring for a month, is the cheapest way to find out whether the four-figure purchase is necessary at all.
Related Reading
- Best Dehumidifier for a Cold Garage Canada — The Temperature Line
- Best Dehumidifier for a Canadian Basement — Sizing, Drainage & the Pest Angle
- Best Large-Capacity Dehumidifier Canada — For the Wettest Spaces
- Best Dehumidifier for Mold Canada — The 60% Humidity Threshold
- How to Get Rid of Silverfish in Canada — A Humidity Problem in Disguise
- Rodent Exclusion Kit Canada — Sealing the Gaps Drying Cannot Close