Short answer: A musty odor is a moisture problem. The EPA recommends keeping indoor humidity below 60%, ideally between 30% and 50%, and drying anything wet within 24 to 48 hours. Check humidity with an inexpensive hygrometer, look for damp carpet, boxes, wall coverings and leaks, and fix water coming in from outside. Small areas of mold (under about 10 square feet) can usually be cleaned yourself.
Quick diagnosis
| What you notice | Most likely cause | What to check | Call a pro? |
|---|---|---|---|
| Smell worse in summer | Humid air condensing on cool walls and floors | Measure humidity; run a dehumidifier | No |
| Smell after rain | Water coming in from outside | See water in basement after rain | If it continues |
| Smell near carpet or stored boxes | Damp materials growing mold | Lift carpet; check boxes | If large |
| Visible spots on walls or wood | Mold | Measure the area | If over about 10 sq ft |
| Smell like sewage, not just musty | Dry floor drain trap or sewer issue | See sewer smell in basement | Often |
Measure and lower the humidity
A basic hygrometer costs about $10 to $50 (EPA). Aim for 30% to 50%. Run a dehumidifier, keep it draining, and vent the clothes dryer outside. The University of Minnesota Extension cautions that a dehumidifier helps with humidity and odor but does not fix water coming through the walls.
Find damp materials
- Carpet and pads on concrete floors.
- Cardboard boxes against walls or on the floor.
- Drywall, paneling and insulation against foundation walls.
- Areas under windows, around the water heater, laundry and pipes.
Cleaning small mold areas
The EPA says to fix the water problem first, scrub hard surfaces with detergent and water and dry them completely. Porous materials like carpet and ceiling tiles may need to be thrown away. Do not paint over mold.
When to call a professional
- Mold covers more than about 10 square feet or comes from sewage or flood water.
- Water keeps coming in from outside.
- You suspect mold inside the HVAC system or ducts (EPA advises not running the system).
- Anyone in the house has health concerns related to mold.
What it usually costs
A hygrometer costs about $10 to $50 (EPA). Waterproofing costs depend on the cause: Bob Vila reports professional crack repair at about $250 to $800 and interior drainage systems at $5,000 to $18,000.
Frequently asked questions
What humidity should a basement be?
The EPA recommends keeping indoor humidity below 60%, ideally 30% to 50%.
Is a musty smell always mold?
It means conditions for mold are present. It often comes from damp materials even when you cannot see mold.
Related problems
- Water in basement after rain
- Sewer smell in basement
- Mold on bathroom ceiling
- Sump pump running constantly
Sources
- U.S. EPA: A brief guide to mold, moisture and your home
- U.S. EPA: Mold cleanup in your home
- University of Minnesota Extension: Moisture in basements, causes and solutions
- Bob Vila: How to stop water leaking into the basement after heavy rain
This guide is general information, not a substitute for an inspection by a licensed professional. See our disclaimer.