Dry air under the floor

Crawl space dehumidifiers

A dehumidifier is the working part of a dry crawl space, and the part that most often gets bought by itself in the hope that it will be enough. Sometimes it is. More often it is being asked to dry air that is replaced from outdoors as quickly as the machine can treat it.

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What it can do alone, and what it cannot

A dehumidifier takes water vapor out of the air around it. In a crawl space with a sound ground cover and vents that are shut, that air stays put, so the machine reaches its setting and rests.

With open vents and bare soil, new moisture arrives constantly from two directions. The unit runs without pause, its bucket or pump works overtime and the wood stays damp anyway. If a company offers a dehumidifier and nothing else, ask what is being done about the vents and the ground. The full picture is on the encapsulation page.

Why a household unit is the wrong tool

The portable dehumidifier sold for bedrooms and basements is built to stand upright in a warm room and be emptied by hand. Under a house it sits in cool, dusty air, low to the ground, with nobody looking at it for months. Units made for crawl spaces are lower, cope with cooler temperatures, have filters meant for dirty air and are designed to drain by themselves.

Size is stated as pints of water removed per day at a given temperature and humidity. The right size depends on the area and height of the space and how well it has been sealed. Ask how the company arrived at the model it quoted.

What it needs to keep running

A drain. The collected water has to leave. It can fall by gravity to a sump basin or the outdoors, or be lifted by a small condensate pump. That pump is the commonest failure point, so ask where it is and what happens when it stops.
Power. An outlet under the house, on a circuit that is not shared with something that trips it. Fitting one is an electrician's job.
A filter change. Usually once or twice a year. Find out whether you can reach it or whether someone has to crawl in.
A humidity reading you can see. A remote display indoors, or a gauge near the access door, so you know it is working without going underneath.

Aim to keep the air below 60 percent relative humidity. That is the figure the Environmental Protection Agency's mold and moisture guide gives for indoor air.

Ask about the unit

Which model is it, and what capacity?
Where does it drain, by gravity or by pump?
Where will it sit, and can it be reached without disturbing the liner?
Is there a display I can read from inside the house?
Who services it, how often, and is that included for any period?
What does the manufacturer's warranty cover?

Dehumidifier questions

Will I hear it?

A faint hum through the floor directly above is possible. Ask for it to be placed away from bedrooms and set on a pad or hung, not laid on the joists.

What does it cost to run?

That depends on the unit and how hard it has to work. In a well-sealed space it cycles on and off. In a leaky one it never stops, which is the expensive case.

Can my air conditioning do the job instead?

Some sealed crawl spaces are dried with a small supply of conditioned air from the house system. Whether that suits your house is a question for the waterproofing company and your heating and cooling contractor together.

Is a fan in the vents an alternative?

Vent fans move outdoor air through the space. On a humid day that brings moisture in. They are not a substitute for closing the space and drying it.