Last updated: October 9, 2026
Mold grows where moisture remains. A roof, plumbing, window, or foundation leak; floodwater; condensation; or persistently humid indoor air can provide the conditions for growth. Cleaning visible mold without correcting the moisture source usually allows the problem to return.
Start with the moisture source
Look for an active leak, wet building material, condensation, poor bathroom or kitchen exhaust, or a room that stays damp. Repair the leak and dry wet materials promptly. EPA says water-damaged areas and items should generally be dried within 24–48 hours to help prevent mold growth. If the water came from sewage or another contaminated source, do not treat the cleanup like an ordinary clean-water spill.
EPA recommends keeping indoor humidity below 60% when possible, ideally between 30% and 50%, and using ventilation or a dehumidifier where needed. See EPA’s guide to mold and moisture.
When small cleanup may be manageable
EPA says a moldy area smaller than about 10 square feet can, in many cases, be handled by a homeowner following its cleanup guidance. That is a general guide, not a guarantee that every small job is safe: the water source, material, hidden damage, and people in the home matter. Read EPA’s mold cleanup guidance before starting.
For a small area on a hard surface, CDC says mold can be removed with household products, soap and water, or an appropriately diluted bleach solution. Follow the product label, ventilate the space, and wear protective gloves and eye protection. Never mix bleach with ammonia or other cleaners. Porous materials such as drywall, ceiling tile, carpet, or insulation may need removal if moldy or water-damaged; do not simply paint over them.
When to call a qualified professional
- The affected area is larger than about 10 square feet, or growth is widespread or recurring.
- There has been substantial water damage, a flood, sewage backup, or moisture inside a wall, floor, or ceiling.
- You suspect mold in or near HVAC equipment or ductwork; do not run a system that may spread contamination without professional advice.
- The source is unclear, the material cannot be dried, or the repair involves structural or electrical work.
- People in the home have asthma, significant allergies, chronic lung disease, or weakened immune systems and may be exposed.
CDC says damp or moldy environments may cause symptoms in some people, including stuffy nose, sore throat, cough, wheezing, eye irritation, or skin rash; people with asthma or mold allergies may have more severe reactions. Health effects vary. For symptoms or personal health concerns, contact a healthcare professional. See CDC’s mold guidance.
Prevent the problem from returning
- Repair roof, window, plumbing, or foundation leaks promptly.
- Use kitchen and bathroom exhaust fans that vent outdoors.
- Measure humidity and use ventilation or a dehumidifier if levels remain high.
- Dry water-damaged materials quickly and replace items that cannot be dried or cleaned appropriately.
- Check behind stored items and in damp areas for recurring moisture, not just visible spots.
CDC does not recommend routine mold testing for homes: if mold is visible or smelled, the priority is to remove it safely and fix the moisture problem rather than identify the species. See CDC’s testing and cleanup guidance.
Related guide
A persistent musty odor can have several causes. See our practical guide to finding the source of a musty smell.
Sources and limitations
Sources: U.S. EPA and CDC. Updated October 9, 2026. This article is general home-maintenance information, not a mold assessment, remediation plan, or medical diagnosis. Follow local requirements and professional advice for complex or contaminated water damage.
