Musty smell in the basement: causes and how to remove it

Step by step

1. 1

   Measure the humidity

   Put a humidity meter in the basement for a day. Aim for below 60%, ideally 30 to 50%.

   Why: Basements are cool and below grade, so humidity often runs high enough for mold even with no visible leak.

2. 2

   Find where water gets in

   Look for damp spots, water stains, condensation on pipes and walls, and leaks around windows, the water heater and plumbing. Check that gutters and the ground outside direct water away from the foundation.

   Why: Mold needs moisture. Until the source is fixed, the smell will come back.

3. 3

   Run a dehumidifier

   Run a dehumidifier sized for the space, empty or drain it regularly, and add fans to move air.

   Why: Lowering humidity stops mold growth and helps everything dry.

4. 4

   Clear out what's moldy

   Wearing an N-95, gloves and non-vented goggles, go through stored boxes, books, rugs and fabrics. Throw out cardboard, carpet, padding, ceiling tiles or drywall that is moldy or stays damp.

   Why: Absorbent materials hold mold and the musty smell deep inside, where cleaning can't reach.

5. 5

   Clean hard surfaces

   Scrub mold off concrete, block, wood and other hard surfaces with detergent and water, then dry them completely.

   Why: The mold has to be physically removed. Dead mold still causes allergic reactions.

6. 6

   Keep things up and dry

   Store items in plastic bins off the floor and away from outside walls, and dry any spills or leaks within 24 to 48 hours.

   Why: Mold usually won't grow on materials dried within that window, and bins don't feed mold the way cardboard does.

Why basements smell musty

Basements sit below ground level and stay cool. Ground moisture seeps through foundation walls and slabs, warm humid air condenses on cool surfaces, and leaks from plumbing or outside drainage add more. That's exactly what mold needs, and as it grows it releases the musty compounds you smell. Cardboard boxes, carpet, drywall and stored fabric soak up both moisture and odor.

EPA's rule applies here: cleanup isn't finished until the moisture is fixed and the mold and moldy smell are gone. A smell with no visible mold suggests hidden growth behind drywall or paneling, under carpet or inside ductwork. See mildew smell for the general science.

When the smell isn't mold

Not every basement smell is musty. A sewage or rotten-egg smell often comes from a floor drain whose trap has dried out; sewer gas is heavier than air and collects in the lowest part of the house. Pour water into floor drains regularly. A fuel-oil smell points to a tank or spill and needs ventilation and cleanup, with professionals for spills over a gallon. A smell that grows over days may mean an animal living or dead in a window well or crawl space.

If there's carpet down there, see mildew smell in carpet.

By situation

Musty smell only in summer

Warm, humid air condenses on cool basement walls and pipes. Run the dehumidifier more in summer and keep an eye on the humidity meter.

After the basement flooded

Dry it within 24 to 48 hours if you can, remove soaked carpet, padding and drywall, and get professional help for large areas. EPA points to larger-scale guidance for mold over about 10 square feet or major water damage.

Finished basement with carpet and drywall

Hidden mold behind drywall or under carpet is common. Check the backs of baseboards and lift a carpet corner. Materials that stay moldy need replacing.

Musty smell from stored boxes

Cardboard soaks up moisture and grows mildew. Move contents into plastic bins and throw the boxes out.

What not to do

• Don't cover the smell with air fresheners. It hides a mold problem that keeps growing.
• Don't rely on bleach as routine mold cleanup. EPA recommends detergent and water and doesn't recommend routine biocides. Some older extension guides suggest bleach scrubs, so sources differ. Never mix bleach with ammonia.
• Don't store cardboard or fabric directly on the floor. They wick up moisture and feed mold.
• Don't tackle large mold areas yourself without protection. Areas over about 10 square feet, or after major water damage, call for larger-scale guidance or a professional.

Other methods that work

Professional waterproofing or remediation

For ongoing seepage or a large mold problem, a waterproofing contractor or remediation professional can address the source.

Bleach-detergent scrub

A UF/IFAS extension guide describes a strong bleach-and-detergent scrub for basement surfaces.

Caution: EPA doesn't recommend routine biocides for mold cleanup. If you use bleach, ventilate well and never mix it with ammonia or acids.

Stain still there?

The smell returns after cleaning

Humidity is still high or water is still getting in. Check the dehumidifier, gutters and grading, and look for hidden mold.

It smells like sewage, not mildew

Refill dry floor-drain traps with water and check for plumbing leaks. Leave the house and call the fire department if you suspect a strong gas buildup.

Safety

• Wear an N-95 respirator, gloves and goggles without vent holes when cleaning mold.
• Ventilate when using any cleaner, and never mix bleach with ammonia or acids.
• Leave and call for help if you smell a strong sewer or fuel odor or feel sick in the basement.

Never mix cleaning products; see chemicals you must never combine.

Questions

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