Drought Summer, Wet Basement: Maine Mold in August
We have taken a run of calls this month that all start the same way. Somebody goes down the basement stairs for a tote of school clothes, gets hit with that heavy, wet-cardboard smell, and finds gray fuzz on the back of a bookcase. Then comes the part that throws them: “It has not rained all summer. How is anything down there wet?”
Fair question. Maine is having a dry year. Statewide precipitation this past July came in at 2.06 inches, the fourth driest July in a record going back to 1895, according to NOAA’s National Centers for Environmental Information. The U.S. Drought Monitor map valid August 11 put 82 percent of the state in abnormally dry conditions or worse, with about 5 percent in severe drought, concentrated along the coast and Midcoast.
None of that keeps a basement dry, because summer basement moisture in Maine usually has nothing to do with rain.
Your basement is a condenser, and August is when it runs hardest
Below-grade walls and slabs sit against soil that stays cool all summer. We routinely read foundation surfaces in the mid to high 50s in August while the air upstairs is in the mid 70s. When outdoor air carrying a 65 or 70 degree dew point gets into that space, it hits a surface colder than its dew point and gives up its water. That is condensation, not a leak. University of Minnesota Extension describes the same mechanism in its basement moisture guidance, and notes that homeowners often mistake the resulting water for wall seepage.
Here is the part that catches people. The most common way humid air gets in is that somebody opens the basement windows to air it out. In July and August that is the exact wrong move. You are feeding the problem.
Signs we look for on a first walk-through, all of which you can check yourself:
- Sweat on the cold water supply line and on the outside of the pressure tank.
- A damp ring on the slab under anything stored flat, like a tote or a paint can.
- Cardboard gone soft and wavy at the bottom edge but dry at the top.
- Fuzz starting on the paper facing of fiberglass insulation and on the back of furniture, meaning the side with no airflow.
- A dehumidifier running and still not holding the room under 60 percent, usually because a window is open or the unit is too small for the space.
Growth in that pattern is a humidity problem. Growth that runs in a vertical stripe down one wall, or fans out from a corner of the slab, is a bulk water problem, and a dehumidifier will not touch it.
What to do this week if you already have it
Close the basement windows and the bulkhead. Put a cheap hygrometer on a shelf at waist height and check it twice a day. EPA’s guide to mold, moisture and your home says to keep indoor relative humidity below 60 percent and ideally between 30 and 50 percent. CDC puts the target at no higher than 50 percent. If your dehumidifier cannot hold that with everything shut, it is undersized or its coils are loaded with dust.
Get wet material dry fast. EPA’s window is 24 to 48 hours. After that you are no longer preventing growth, you are cleaning it up. Cardboard, soaked carpet padding and water-damaged upholstery in a basement are usually not worth saving.
You do not need to call anybody for a small patch. EPA’s threshold is about 10 square feet, roughly 3 by 3 feet. Under that, on a hard surface, soap and water or a diluted household bleach solution handles it, with gloves and eye protection and the moisture source corrected. Never mix bleach with ammonia or another cleaner. We will tell you that on the phone, and we would rather say it than sell you a job you do not need.
Call somebody when the area is larger than that, when it is inside a wall cavity or under a finished floor, or when the source is still not obvious after you have looked. Those are the jobs where containment, negative air and proper disposal actually matter. Before you collect quotes, read how to read a mold remediation scope in Maine and our published remediation cost ranges so you have a number to check them against.
On health, and on landlords
CDC’s position is that exposure to damp and moldy environments may cause a variety of health effects, or none at all. Some people get a stuffy nose, sore throat, cough, wheeze, burning eyes or a rash. People with asthma or a mold allergy can react more severely, and people who are immune compromised or have chronic lung disease can develop lung infections. That is the honest range, and a blog post cannot tell you why you feel the way you feel. CDC also does not recommend testing to decide whether to clean, because you remove visible growth regardless of species.
If you own rental property, the calendar matters. Maine turnover clusters around September 1, and a unit closed up and unconditioned since June is exactly the setup above. Walk the basements and ground floor units before the tenant does. Photograph what you find with a date, write down the hygrometer reading, and keep the record. A documented moisture correction is a far better position than a text from a tenant in October.
Focused Environmental Services works out of Gorham and covers greater Portland and southern Maine. Tyler Chandler, our Lead Environmental Specialist, grew up in the family inspection business. If you cannot tell whether what you are looking at is condensation or a leak, that is a five minute phone call: 207-464-8202. You can also read how we approach mold remediation, review our environmental testing options, or request a free assessment.
Dry summer, wet basement. It sounds backward until you remember the water was never coming from the sky.