Key Takeaways

• Find the moisture source first. Tape a square of plastic to a damp wall for 24 to 48 hours. Water behind it means seepage from outside, and water on the room side means condensation.

• Fix outside water before inside air. Clean the gutters, extend downspouts 6 to 10 feet from the house, and make sure the soil slopes away from the foundation.

• Keep basement humidity under 50%. Run a dehumidifier with a drain hose, keep windows shut on humid days, and use a hygrometer to confirm it's working.

• Remove what mold feeds on. Toss moldy porous items, swap cardboard for plastic bins, and scrub small hard-surface patches with detergent, not bleach.

• Clean the air. Skip ozone machines and fresheners, and use an air purifier with both True HEPA filtration and activated carbon. 

You've lit the candle. You've cracked the window. You've sprayed whatever was under the kitchen sink. Two days later, the basement smells exactly the same. That's because a musty basement smell isn't really a smell problem. It's a moisture problem that happens to have a smell.

The odor comes from microbial volatile organic compounds, or MVOCs: gases that mold and bacteria give off as they feed on damp material. They're common, even in homes that look fine. 

When researchers sampled 23 U.S. homes whose owners reported no mold problems, about half of the basements still had musty odors. The microbial gases measured in those basements usually turned up on the main floor at similar levels. So the smell doesn't stay downstairs.

It's also worth taking seriously. In its dampness and mold guidelines, the World Health Organization treated mold odour as one sign of a damp building, and it found sufficient evidence that people in damp or moldy buildings have higher rates of respiratory symptoms, respiratory infections, and asthma flares.

Here are some tips on the best ways to get rid of a musty basement smell. 

1. Determine Where The Basement Moisture is Coming From

Basement moisture has two main sources, and they need different fixes. 

A simple test is to follow your nose. 

If the basement odor spikes after rain, water is seeping through the walls and floor from the soil outside. If it spikes during humid stretches, moisture is condensing out of the air.

Many basements experience both. 

Here’s a quick DIY way of determining which one you’re dealing with. 

Start by getting a plastic sheet and some tape. Tape a roughly one-foot square of clear plastic flat against the damp part of the wall, and seal all four edges so no air gets behind it. Check it after 24 to 48 hours. 

What you’re doing here is a simplified version of a standard concrete moisture test, ASTM D4263.

• Moisture between the plastic and the wall means seepage: water is moving through the concrete from outside.

• Moisture on the room side of the plastic means condensation: humid basement air is hitting a cool surface.

Another helpful clue is visual. White, chalky powder on the walls, usually near the floor, is efflorescence. It's mineral salt left behind when water moves through concrete and evaporates at the surface, so it points to seepage.

2. Move Rainwater Away From Your Foundation

Most seepage comes from rain and snowmelt collecting against the foundation, so the cheapest fixes are in the yard, not the basement. 

Work through University of Georgia Extension's moisture checklist in this order:

• Clear the gutters and make sure every downspout carries water 6 to 10 feet from the house. A $10 plastic extension often solves a corner that's always damp.

• Check the grading. Soil should drop about 6 inches over the first 6 to 10 feet from the foundation. Settled soil or mulch beds piled against the wall send water straight toward the house. Rake in fill dirt to restore the slope.

• Ease up on foundation plantings. Beds that get watered daily keep the soil against your wall wet.

Then look inside for the spot where water is getting in. Cracks and pipe penetrations are the most common entry points in poured foundations. 

If the wall still gets wet after heavy rain once the outside fixes are done, that's a job for a waterproofing contractor. They typically install an interior perimeter drain that feeds a sump pump, or excavate and seal the foundation from outside. It costs more, but no dehumidifier or air purifier can keep up with water coming through the wall.

3. Keep Humid Air Off Cold Surfaces

This one surprises people. Summer is often when basement walls get wettest.

Building scientist Joseph Lstiburek explains why: warm, humid air touches concrete that's cooler than the air's dew point, and the water condenses right on the wall. The same thing happens on cold water pipes and on the rim joist, the wood framing where the house sits on the foundation.

Here's how to cut it down:

• Keep basement windows shut on humid days. Opening them in July lets more wet air reach those cold surfaces.

• Insulate cold water pipes. Foam pipe sleeves cost a few dollars at any hardware store and keep pipes from dripping onto the floor.

• Vent the dryer outside. The EPA flags improperly vented dryers as a source of indoor humidity. Check that the duct is connected and runs all the way outdoors.

4. Run a Dehumidifier to Keep Humidity Under 50%

The EPA recommends keeping indoor humidity below 60%, ideally 30 to 50%. Most basements can't stay in that range without help, especially in summer.

That’s where a dehumidifier can come into play. To get the most out of it, put it in the middle of the basement floor, not jammed into a corner. It’s also helpful to keep the doors to the rest of the basement open so the dehumidifier is drying out as much as possible. 

A hygrometer is worth having here. Setting the dehumidifier to 50% doesn't mean the room gets there, and steady readings under 50% are how you'll know the moisture is handled. More on humidity targets in our guide to ideal humidity levels.

5. Clear Out What's Feeding the Musty Basement Smell

To grow, mold needs food and moisture, two things basements are in no short supply of. 

Cardboard boxes, old rugs, stacks of paper, and stored clothes all count. In the 23-home study, basements held far more of this material than upstairs rooms. The researchers suggested that the amount of slowly colonized stuff helps explain the higher levels of musty gases.

Here's where to start:

• Throw out anything visibly moldy that's porous. The EPA notes that materials like carpet and ceiling tiles may need to go, because mold grows into them and can't be fully cleaned out.

• Swap cardboard for lidded plastic bins. Cardboard soaks up moisture from the air and from the floor.

• Get storage off the floor with wire or metal shelving.

• Keep everything a few inches from exterior walls, which are the coldest and dampest surfaces. HUD recommends the same for boxes and furniture.

6. Scrub Hard-Surface Mold With Detergent, Not Bleach

For small patches on concrete, metal, or other sealed surfaces, you’ll likely be able to handle the cleanup yourself with detergent or dish soap and warm water. 

Be sure to protect yourself by wearing gloves and an N-95 respirator, since scrubbing releases mold spores. When you’re done, be sure to dry the surface completely, bag the rags and debris and take them outside. 

Skip the bleach. Bleach usually isn't necessary unless sewage is involved, and notes that it can trigger asthma. If you use it anyway, never mix it with ammonia or other cleaners.

Killing mold isn't the point anyway, as dead mold can still cause allergic reactions via airborne spores and fragments. The goal is to physically remove it.

7. Know When to Call a Mold Professional

Sometimes a musty smell in the basement isn’t a quick fix, especially for severe mold infestations or for people with mold allergies. 

Call a professional if:

• The mold covers more than about 10 square feet, roughly a 3-foot by 3-foot patch. 

• The smell lingers after you've fixed the moisture. If humidity stays under 50% and the walls are dry but the basement still smells musty, something is likely growing out of sight, behind finished walls or under flooring. The gases behind that smell can seep through drywall and vapor barriers, so you'll smell growth you can't see.

• There's mold near your furnace or ductwork. The EPA recommends checking its guidance on air duct cleaning before you run a system that may be spreading spores through the house.

• The basement flooded. Water can easily seep into corners and crevices you can’t always see, or effectively dry out. 

• Anyone in the house has asthma, allergies, or a weakened immune system. They should stay out of the space during cleanup.

If you’re unsure, call the pros. You don’t want to DIY a mold inspection.

8. Skip Ozone Machines and Air Fresheners

Plug-ins, sprays, and candles just layer a scent over the problem. The musty gases are still there, and so is whatever's producing them.

Ozone generators tend to be worse. They're often sold specifically for odors, but the EPA says that at concentrations within public health standards, ozone isn't effective at removing many odor-causing chemicals.

It can also make the air worse. The California Air Resources Board notes that ozone reacts with chemicals already in the air to form new pollutants, including formaldehyde.

9. Use HEPA Filters and Activated Carbon to Clear the Air That's Left

Once the water and humidity are under control, an air purifier can help clear the unwanted particles in the air. Look for one with both of these filters, because they do different jobs:

• HEPA captures airborne particles, including mold spores, but it doesn’t touch gases and vapors. 

• Activated carbon traps gases and odors on its surface. The musty smell itself is a gas, so a purifier without carbon won't fix it.

Keep in mind that an air purifier won't dry a wall or stop mold that's still growing. What it does is lower the spore count in the air and the odor you're breathing while the other fixes take hold.

Here's how to get the most from one in a musty basement:

• Run it during cleanup. Scrubbing kicks spores into the air, and that's when a purifier does some of its most useful work.

• Place it in the open, not in a corner, with a few inches of clearance on every side.

• Size it to the space. An undersized unit will run constantly and still fall behind.

• Change filters on schedule. A filter that's been pulling musty gases and spores out of a damp basement for months will run out sooner than one upstairs.

The iAdaptAir pairs True HEPA filtration, which captures particles down to 0.05 microns, with activated carbon for the musty gases. UV-C light helps keep the filter itself clean, and the bipolar ionization is CARB-certified ozone-free, so it's safe to run around the clock. Auto Mode adjusts fan speed to real-time particle readings, and the filter indicator flashes when it's time for a replacement.

Not sure which fits your layout? See how to size a purifier for a finished basement or browse our mold concern page.