Damp Basement? Aim for This Humidity and Pick the Right Size

Keep indoor humidity below 60%, and ideally between 30% and 50%. That is the target the U.S. Environmental Protection Agency (EPA) gives for avoiding mold, and it is the number to check before you buy a dehumidifier. You can measure it with a small humidity meter, which the EPA says costs about $10 to $50 at many hardware stores.

A dehumidifier treats the air, but the EPA is clear that the real fix is stopping the water from getting in.

If you are looking at a used dehumidifier for a damp basement, this guide covers what the federal pages say about humidity, how to pick a size and what to check on a secondhand unit.

Start with the water, not the machine

The EPA's mold guide puts it in one line: the key to mold control is moisture control. Its main points:

• Fix the source. Repair plumbing leaks and other water problems as soon as possible, because cleaning up mold without fixing the source usually means it comes back.
• Dry wet things within 24 to 48 hours. The EPA says that if materials are dry within that window after a leak or spill, mold usually will not grow.
• Make sure the ground slopes away from the foundation so water does not collect there.
• Use air conditioners or dehumidifiers when needed to reduce humidity.
• Ventilate: run bathroom fans, use exhaust fans when you cook or run the dishwasher and vent clothes dryers outdoors where you can. (Our used dryer guide covers the venting.)

On cleaning mold, the EPA says to scrub it off hard surfaces with detergent and water, dry completely, and not to paint or caulk over moldy surfaces. If the moldy area is more than about 10 square feet, or the water came from sewage, it advises consulting a professional. I am not a mold expert, so for anything bigger than a small patch, follow that advice.

What size dehumidifier do you need?

ENERGY STAR measures dehumidifier capacity in pints removed per 24 hours. Its suggested minimums for portable units:

ENERGY STAR's advice is to choose a larger unit when in doubt. If you are replacing one, check how much water you empty each day to see what you really need. Whole-home units are sized differently, and ENERGY STAR recommends hiring an installer.

On efficiency, the page says certified dehumidifiers use 20% less energy than a similarly sized conventional unit while removing the same amount of moisture.

Checking a used dehumidifier

The agency pages do not have a used-unit checklist, so this one is mine, built from ENERGY STAR's maintenance advice.

1. Plug it in and run it for a while. The fan should spin smoothly and the compressor should start.
2. Look at the coils and grills. ENERGY STAR says to keep units away from dust and dirt sources that can clog coils. Heavy dust or woodworking residue is a bad sign.
3. Check the bucket. A cracked bucket or float switch means leaks or a unit that will not stop.
4. Check for a drain hose connection, which saves emptying the bucket. ENERGY STAR notes you can route a hose to a drain and should keep it away from electrical cords.
5. Look for frost on the coils. ENERGY STAR says that if frost forms, you switch the unit off and let it defrost before restarting. A unit that frosts constantly may have a fault.
6. Check the capacity label against the table above.
7. Run a recall check using our five-minute guide.

Using it well once you have it

• Leave room around the unit so air can circulate.
• Empty the bucket regularly or use the hose.
• Check the humidity with a meter and aim for the 30% to 50% range.
• Keep it away from dust.

If you are replacing an old one, note that in Fairfax County, county trash customers can schedule dehumidifiers for pickup in the Freon category, as we explain in the DC and Fairfax furniture and appliance rules.

Winter and the electricity bill

Running a dehumidifier costs electricity, and the Energy Information Administration expects homes that heat with electricity to spend about 4% more this winter. See who pays more this winter. A certified model that uses 20% less energy helps, but fixing the water source helps more.

For appliances in stock, browse Buy Now, and for how pickup works in the DC area, read the help page.

The bottom line

Aim for 30% to 50% humidity, keep it below 60%, and fix the water source first. Size the unit by the dampness of the space and check a used one for dust, frost, leaks and recalls.

If you know someone with a basement that smells damp, send this their way.