Sizing a Dehumidifier for a Sealed Crawlspace
Oversized short-cycles, undersized never reaches target, and a bucket drain guarantees failure. What matters when specifying humidity control.
The active part of a sealed crawlspace
Encapsulation stops moisture entering from the soil and the outdoor air. It does not remove moisture already present, moisture arriving with normal air exchange, or moisture released by damp soil and concrete during the months after sealing.
Something has to hold the space at target, and the target is relative humidity in the 50 to 55 percent range. That keeps wood moisture content comfortably below the roughly 19 percent threshold where decay and mould establish.
Getting the size wrong, in both directions
Undersized: the unit runs continuously, never reaches target, and wears out early. Homeowners interpret constant running as the unit working hard. It is actually the unit losing.
Oversized: the unit short-cycles. It satisfies the humidistat quickly, shuts off, and the space rebounds. Short cycling is inefficient, hard on the compressor, and — counterintuitively — gives poorer humidity control than a correctly sized unit running longer cycles.
What sizing actually depends on
- Volume of the space, not just floor area. A 1.5-metre crawlspace holds twice the air of a 0.75-metre one over the same footprint.
- Envelope condition after sealing. A well-sealed space has a modest, predictable load. A space with incomplete air sealing has an open-ended one.
- Expected moisture load, including whether the soil and any concrete are still giving up moisture from before the work.
- Operating temperature. This is the detail most often missed.
The temperature problem with consumer units
A crawlspace runs cooler than a basement, and many consumer dehumidifiers perform poorly at lower temperatures — some barely function below about 18 degrees, and capacity ratings are typically published at warmer conditions.
Units built for crawlspace service are rated to operate at the temperatures these spaces actually see, and they are built to run against a modest duct run if the unit is mounted somewhere more accessible.
That is the real argument for purpose-built equipment: not brand prestige, but rated performance at the temperature where it has to work.
The drain decides whether it still works in five years
A dehumidifier with a collection bucket will be emptied diligently for about a month. After that it is an expensive ornament.
Every unit we install gets a permanent drain:
- Gravity to a sump or suitable drain where there is enough fall, which is the simplest and most reliable option
- A condensate pump where there is not
And in our climate, where that condensate goes in winter matters. A line routed somewhere it can freeze will back up. We detail the route so it stays inside the conditioned space or drains to a point that cannot ice.
Controls worth having
A dial marked low–medium–high tells you nothing. We set control to a humidity target with a sensor in the space.
Remote monitoring is worth considering, and here is why: a crawlspace is the one part of a house nobody visits. If the dehumidifier fails in May, the first sign is often a smell or visible mould in August. A reading you can check from your phone closes that gap entirely.
Winter operation: less than you would think
Winnipeg winter air is extremely dry. Once a crawlspace is sealed and insulated, winter humidity is rarely the problem — condensation on cold surfaces is, and that is solved by insulation and air sealing rather than dehumidification.
Expect the unit to earn its keep from roughly May through September, cycling rather than running flat out. If it is running constantly in summer, that is a signal the envelope is leaking, not that the unit is wrong.
When you may not need one at all
Two situations:
The space holds target passively. After sealing and insulating, some crawlspaces sit comfortably at 50 to 55 percent through summer on their own. We would rather monitor for a season and install a unit if the numbers call for it than sell equipment the space does not need.
Supply air from the house works instead. Delivering a small amount of conditioned air from the home’s heating and cooling system, with a return path, holds humidity adequately in some sealed crawlspaces. Whether it is appropriate depends on your mechanical system, the size of the space and the code requirements that apply — it needs assessing rather than assuming.
And the condition that no equipment solves
If the space is still taking liquid water, no dehumidifier will help. Drainage comes first. A dehumidifier fighting an active water source is a running meter attached to an unsolved problem.