Do I need a dehumidifier in my basement?

Why basements are damp

A basement is surrounded by cool, moist soil. Its walls and floor stay cool all year. That causes dampness in three ways:

• Moisture moves through the concrete from the soil as vapour, even when no liquid water is visible
• Humid summer air condenses on cool walls, floors, pipes and ducts
• Activities add moisture: laundry, showers, a sump pit, stored firewood

In Ontario the problem peaks from late spring to early autumn, when outdoor air is warm and humid.

Signs you need one

• A musty smell
• Condensation on cold water pipes, the toilet tank, windows or the floor
• Damp or clammy air
• Mold or mildew on walls, joists or stored things
• White, powdery deposits on concrete
• Rust on tools, appliances and metal shelving
• Cardboard boxes that feel soft
• Wood that swells, and doors that stick in summer
• Paint peeling from basement walls

Measure it

A hygrometer, a small humidity gauge, costs little and removes the guesswork. Put it in the basement away from the dehumidifier and check it over a few days.

• Below about 50 percent: fine
• 50 to 60 percent: borderline. Mold can grow on cool surfaces, where the humidity at the surface is higher than in the room.
• Above 60 percent: too damp. Mold, dust mites and musty smells follow.

Health Canada recommends keeping indoor relative humidity between 30 and 50 percent.

Choosing a unit

Capacity. Dehumidifiers are rated in litres or pints of water removed per day. For a typical damp basement, choose a larger unit over a smaller one. An undersized unit runs constantly and never reaches the set point. A larger one cycles off. Very damp or large basements may need two units or a high-capacity model.

Low-temperature operation. Basements are cool. Many refrigerant units ice up below about 18 degrees Celsius unless they have automatic defrost. Check the stated operating range.

Drainage. This matters more than people expect. A unit that relies on its bucket stops when the bucket is full, often within a day, and then does nothing until someone empties it. Look for:

• A hose connection for continuous gravity drainage to a floor drain or sump pit
• A built-in pump if the water has to go up to a laundry tub or out through a wall

Energy use. Look for an ENERGY STAR certified model. A dehumidifier can run for many hours a day through the summer.

Humidistat. A built-in control lets you set a target and leave it.

Noise, if the basement is living space.

Recalls. A large number of dehumidifiers have been recalled in Canada over the years for overheating and fire risk. Check the model against Health Canada’s recall listings, particularly if it is an older or second-hand unit.

Setting it up

• Place it centrally, with clear space around the air intake and outlet
• Set it to 45 to 50 percent
• Run the drain hose with a continuous downward slope to the drain, and check that it does not kink
• Close the basement windows. Open windows in humid weather bring in more moisture than the unit can remove.
• Keep the door to the rest of the house closed if only the basement is damp
• Clean the filter regularly
• Clean the bucket and coils now and then to prevent slime and odour

When to run it

Usually from about May to October in southern Ontario, or whenever the hygrometer reads above 50 percent. In winter, basements are generally dry enough, and the unit can be switched off. If humidity is high in winter, look for a source such as a leak, an unvented dryer or a wet crawl space.

What a dehumidifier will not do

• It will not fix water coming in. Leaks through cracks, the floor joint or windows need drainage and repair. A dehumidifier fighting liquid water is fighting a losing battle.
• It will not remove existing mold. It stops growth from continuing, but what is there has to be cleaned off or removed.
• It will not dry a flooded basement in any useful time. Household units are far smaller than restoration equipment.
• It will not cure a bad smell whose source is wet carpet, moldy drywall or a dry floor drain trap.

Reduce the load first

The less moisture arrives, the less the unit has to remove.

• Extend downspouts and slope the ground away from the foundation
• Fit a sealed lid on the sump pit
• Vent the clothes dryer outdoors and check the duct is intact
• Do not hang laundry to dry in the basement
• Run the bathroom fan after basement showers
• Insulate cold water pipes to stop them dripping
• Fix plumbing leaks
• Keep things off the floor and away from the walls
• Avoid carpet directly on a basement slab

Other ways to control basement humidity

• Central air conditioning dehumidifies as it cools, but basements often need more because they are cool already and the system does not run long enough.
• A whole-house dehumidifier connected to the ductwork treats the whole home.
• A heat recovery ventilator exchanges indoor and outdoor air. It helps in winter. In humid summer weather it can bring moisture in, so it is not a substitute.
• Insulating basement walls properly, with materials that tolerate moisture, raises surface temperatures and reduces condensation.

After a flood or leak

A household dehumidifier is useful once the bulk drying is done, to keep humidity down while the last moisture leaves concrete and framing. For the drying itself, restoration-grade dehumidifiers and air movers are needed. See how structural drying works.

More: mold removal and remediation.