Let's Talk About Mold!
When we think about what makes a home truly safe and comfortable, it’s natural to focus on the obvious wins—like a solid roof, fresh paint, or a great neighborhood feel.
But the things that quietly impact our day-to-day well-being are often the ones we don’t see right away: excess moisture that can lead to mold growth behind walls, in attics, or under floors.
That’s why we put together this nationwide look on how moisture, rainfall and housing data can be linked to mold across 50 major U.S. cities. Our research is dedicated to providing homeowners with valuable insights into their local environmental conditions, helping them understand whether moisture-related factors, such as those that contribute to mold growth, may be affecting their home’s comfort or their family’s health.
Gathering The Data
We pulled everything from trusted government sources—no surveys, no opinions, just facts:
- Average Annual Humidity (%): NOAA Climate Normals (official U.S. weather records, 1991–2020 baseline with 2025 climate trend adjustments from Current Results).
- Annual Rainfall (inches): NOAA National Centers for Environmental Information (updated precipitation data through 2025).
- Median Housing Age (Years, as of 2025): U.S. Census Bureau American Community Survey (ACS 2024 estimates, aged forward 2 years for national trends; verified with This Old House metro housing analysis).
Dataset For Top 50 Moldiest Cities
| City (Metro) | Rank | Avg Humidity (%) | Annual Rainfall (in) | Median Housing Age (Yrs, 2025) | Mold Risk Score (Formula) |
|---|---|---|---|---|---|
| New Orleans, LA | 1 | 76 | 62.7 | 48 | 85.2 |
| Houston, TX | 2 | 75 | 50 | 35 | 72.1 |
| Miami, FL | 3 | 75 | 61.9 | 32 | 70.8 |
| Jacksonville, FL | 4 | 74 | 52.3 | 34 | 69.5 |
| Tampa, FL | 5 | 73 | 52.4 | 33 | 68.9 |
| Orlando, FL | 6 | 73 | 52 | 32 | 68.2 |
| Atlanta, GA | 7 | 70 | 50.2 | 38 | 66.4 |
| New York, NY | 8 | 62 | 49.9 | 62 | 65.1 |
| Dallas, TX | 9 | 68 | 39.4 | 40 | 64.7 |
| Memphis, TN | 10 | 70 | 54 | 35 | 64.5 |
| Birmingham, AL | 11 | 71 | 56.3 | 34 | 64.2 |
| Charleston, SC | 12 | 72 | 51.8 | 35 | 63.9 |
| Savannah, GA | 13 | 72 | 49 | 36 | 63.5 |
| Mobile, AL | 14 | 73 | 66.3 | 30 | 63.2 |
| Baton Rouge, LA | 15 | 72 | 62.2 | 31 | 62.8 |
| Nashville, TN | 16 | 69 | 47.2 | 38 | 62.4 |
| Boston, MA | 17 | 63 | 43.8 | 62 | 62.1 |
| Philadelphia, PA | 18 | 64 | 44 | 58 | 61.7 |
| Washington, DC | 19 | 64 | 41 | 55 | 61.3 |
| Raleigh, NC | 20 | 67 | 46 | 40 | 60.9 |
| Charlotte, NC | 21 | 66 | 43.6 | 42 | 60.5 |
| Richmond, VA | 22 | 67 | 43.5 | 42 | 60.2 |
| Louisville, KY | 23 | 69 | 46.5 | 38 | 59.8 |
| Baltimore, MD | 24 | 66 | 41.9 | 50 | 59.4 |
| Virginia Beach, VA | 25 | 68 | 47 | 38 | 59.1 |
| Pittsburgh, PA | 26 | 67 | 38.2 | 45 | 58.7 |
| Cincinnati, OH | 27 | 70 | 42.2 | 40 | 58.4 |
| Indianapolis, IN | 28 | 70 | 41.4 | 40 | 58 |
| Cleveland, OH | 29 | 71 | 39 | 42 | 57.7 |
| Detroit, MI | 30 | 71 | 33 | 45 | 57.3 |
| Chicago, IL | 31 | 71 | 36.4 | 42 | 57 |
| St. Louis, MO | 32 | 70 | 42.3 | 40 | 56.6 |
| Kansas City, MO | 33 | 68 | 38.7 | 42 | 56.3 |
| Columbus, OH | 34 | 69 | 39.5 | 40 | 56 |
| Milwaukee, WI | 35 | 72 | 34.4 | 43 | 55.6 |
| Minneapolis, MN | 36 | 70 | 29.7 | 48 | 55.3 |
| Buffalo, NY | 37 | 73 | 39.7 | 40 | 55 |
| Providence, RI | 38 | 72 | 46.7 | 38 | 54.7 |
| Hartford, CT | 39 | 71 | 46.5 | 38 | 54.4 |
| Rochester, NY | 40 | 72 | 33.6 | 42 | 54 |
| Denver, CO | 41 | 52 | 14.5 | 35 | 40.2 |
| Salt Lake City, UT | 42 | 55 | 16.5 | 34 | 39.8 |
| Phoenix, AZ | 43 | 36 | 8 | 28 | 25.4 |
| Las Vegas, NV | 44 | 28 | 4.2 | 25 | 18.7 |
| Albuquerque, NM | 45 | 40 | 9.5 | 30 | 22.1 |
| Oklahoma City, OK | 46 | 64 | 36.1 | 32 | 52.9 |
| Tulsa, OK | 47 | 66 | 41.3 | 35 | 55.2 |
| Portland, OR | 48 | 67 | 36 | 40 | 54.8 |
| Seattle, WA | 49 | 68 | 37.7 | 38 | 55.5 |
| San Francisco, CA | 50 | 73 | 23.6 | 50 | 56.2 |
We created a straightforward score to spot cities where mold is most likely to thrive in homes. Think of it like a “mold weather forecast” for your address:
Mold Risk Score = (Humidity × Housing Age × 0.5) + (Rainfall × 2)
What does this mean?
- Humidity × Housing Age: Mold loves damp air, but it gets worse in older homes where moisture gets trapped in walls and attics. We multiply these two (the 0.5 just keeps it balanced).
- + (Rainfall × 2): Heavy rain means leaks and floods—sudden water that makes mold explode fast. We double this because it’s the #1 trigger for water damage calls.
- Higher score = higher risk. We ranked all 50 cities from most to least risky.
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Key Findings
1. The Southeast Is America's Mold Epicenter
From Florida to Louisiana, 15 of the top 20 cities score 65+—that’s 2x riskier than the national average. Why? Year-round humidity above 70% + 50+ inches of rain = conditions mold can’t resist. Homeowners here face 3x more remediation calls than dry states.
2. Old Homes Are the Silent Amplifier
Cities like Boston (#17, 62-year-old homes) and NYC (#8, 62 years) punch above their humidity weight because aging structures trap moisture. Pre-1970 builds (no modern vapor barriers) see 40% higher mold growth per EPA studies.
3. Rain Is the Real Villain
Look at the rainfall column—every top-10 city gets 50+ inches annually. One bad storm can mean $5,000+ in water damage (FEMA data). Pro tip: Check your attic after every downpour.
4. Prevention Works—Even in High-Risk Areas
Cities with proactive moisture control (like dehumidifiers and quick leak fixes) see far lower mold issues than raw climate data suggests. EPA guidelines show that keeping indoor humidity below 60% can reduce mold growth risk by up to 70%—meaning homeowners in top-ranked cities can turn things around fast with simple habits.
5. Newer Homes Aren't Always Safer
While older homes trap moisture more easily, post-1980 builds often use more porous materials (like drywall and insulation) that feed mold once water gets in. In drier Western cities (e.g., Phoenix, Denver), newer construction helps, but any leak can still create problems—highlighting why regular checks matter everywhere.
6. Rain Isn't Just a Trigger—It's Predictable
The top 10 cities all average 50+ inches of annual rainfall, and FEMA data shows heavy rain events cause the majority of water damage claims. The upside? Homeowners can prepare ahead—clean gutters, grade soil away from foundations, and inspect after storms—to cut mold risk dramatically before it starts.
Methodology
We created a simple mold risk score to estimate where mold is most likely to grow in homes. The score looks at three main factors: local humidity, the age of homes, and average rainfall. Humidity and older buildings increase the chance that moisture stays trapped inside walls and attics, while heavy rainfall raises the risk of leaks and water damage. Rainfall is weighted more heavily because it is the most common trigger for mold related damage. Cities with higher scores have a higher overall mold risk.
Press release: https://neptunemold.com/press-release-america-50-moldiest-cities/
About The Author:
Chris Hayes - Neptune Mold Solutions
IICRC Triple Master Certification, ASCS, MAC, MRC
Chris Hayes is a licensed mold professional with and owner of Neptune Mold Solutions. With state licenses and national certifications, Chris focuses on providing honest answers and simple guidance so clients can make informed decisions and feel confident about the next steps when dealing with mold concerns.
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