Why the vents decide everything
Put a dehumidifier in a crawl space with open vents and it pulls water from air that is replaced as quickly as it is dried. The machine runs without rest, the power bill climbs and the joists stay damp.
Close the vents, cover the soil and the same machine has a fixed volume of air to deal with. It reaches its setting and cycles off. That is why a dehumidifier is normally one part of encapsulation, not a substitute for it.
A crawl space unit is not a basement unit
The portable dehumidifier sold for a finished room is built for warm, clean air and has a bucket to empty. Under a floor it is cooler and dustier, and nobody is there to empty anything. Units made for crawl spaces work at lower temperatures, move more air, drain by themselves and are low enough to fit under the joists.
What an installer has to decide
- Size
- By the volume of the space and how wet it is, not by the floor area of the house alone. Ask how the size was chosen.
- Position
- Where air can circulate around the whole space, and where a person can reach the filter without crawling the full length of the house.
- Drain
- By gravity to a sump basin or to the outside, or through a small condensate pump. A drain hose left lying on the liner is not a drain.
- Power
- A proper outlet near the unit, not an extension cord through the access door. New wiring is electrical work with its own permit.
- Controls
- A humidity setting you can understand, and ideally a display or remote sensor so you can read the level without going under the house.
When a dehumidifier is the wrong fix
If water is running in, a dehumidifier will not keep up, and it should not be asked to. Liquid water is a drainage problem. Deal with it through yard drainage or a sump pump first, then dry the air.
Ask before one is installed
- What humidity level will it be set to hold, and how will you know it is holding it?
- How often does the filter need changing, and can you do that yourself?
- Where exactly does the water go, in summer and in a hard freeze?
- Who do you call if it stops, and is that the installer or the manufacturer?