Dehumidifiers and a damp basement

Keep indoor humidity below 60 percent, and ideally between 30 and 50. Above that, mold can start to grow. A basement that feels damp and smells musty is usually somewhere between 50 and 75 percent. A dehumidifier brings it back down.

Measuring it

A humidity meter, called a hygrometer, costs about $10 to $50 at a hardware store in 2026. If your dehumidifier doesn't show the humidity, hang a hygrometer near it. Water beading on the windows, walls or cold pipes is another sign the air is too damp.

Picking the size

Dehumidifiers are rated by how many pints of water they pull out of the air a day. The size you need depends on the room and how damp it is without one:

If you're between two sizes, go up. If you're replacing an old unit, the amount of water you empty from it each day is a good guide. An ENERGY STAR model removes the same water as a standard one with 20 percent less electricity.

Setting it up

• Set the humidity. ENERGY STAR models have a built-in humidistat. Set it to the level you want, and the unit turns itself on and off to hold it.
• Give it room. If the air doesn't come out the top, keep the unit away from walls and furniture.
• Close the doors and windows while it runs.
• Keep it out of the dust. Sawdust and dirt clog the coils.
• Run a drain hose. A hose from the unit to a floor drain or the sump means you never empty the bucket. Put the unit close enough to the drain that the hose isn't a trip hazard, keep the hose away from cords, and plug the unit into a grounded outlet.
• Watch for frost in a cold basement. Below about 65°F, frost can build up on the coils and stop the unit pulling out water. If it does, turn the unit off and let it thaw.

Stopping the water at the source

Better drainage outside lowers the humidity in the basement. Extend the downspouts so they empty well away from the foundation, and keep the gutters and downspouts clear. Gutter cleaning means a ladder, so have it done. Slope the soil so it falls away from the house, and don't overwater the beds along the foundation. Inside, make sure the clothes dryer vents outdoors, and fix any dripping outdoor faucet.

If water does get in, dry the wet area and anything wet within 24 to 48 hours. In most cases that's soon enough to stop mold.

An annotated example

What to do next

Put a hygrometer in the basement for a week in summer and write down the readings. If they stay above 50 percent, use the chart to pick a size. Small mold cleanup covers mold that's already there.