Cold Nights, Musty House: Allergies or Mold?

The National Weather Service in Caribou posted the season’s first frost advisory for Aroostook and northern Piscataquis counties overnight into September 9, with temperatures down to 32. Portland did not frost, but the Gray office’s climate summary for the same night shows a low of 51, a high of 67, and just 0.21 inches of rain for the month so far against a normal of one inch. Cold nights, mild days, a dry summer behind us. This is the week the phone starts ringing with the same sentence: the house smells musty again and everybody is sneezing, and I cannot tell if it is allergies or mold.

Why September fools people

Outdoors, ragweed is in full swing and leaf litter is starting to break down, so pollen and outdoor mold spores both run high. Indoors, three things change in the same ten days. The heat fires for the first time since April and pushes a summer’s worth of dust into the living space. Windows that were open all summer get shut, and the humidity stays in. And the first cold nights chill exterior walls, window glass, closet corners and uninsulated basement walls below the dew point of the still-humid indoor air, so water condenses where you cannot see it.

The EPA’s Brief Guide to Mold, Moisture and Your Home puts the target at indoor relative humidity below 60 percent, ideally 30 to 50, and specifically flags condensation on the inside of window glass as a warning sign. The Maine Indoor Air Quality Council’s mold guidance says the same thing in five words: no excess moisture, no excess mold growth.

What I see on a September call

Most houses I walk through this month tell the same story. The dehumidifier in the basement got unplugged in August when the drought made everything feel dry. Then the nights dropped into the forties, the slab and the block walls went cold, and the first morning with the windows shut put a film of water on the north wall behind the shelving. Run a pinless moisture meter across that wall and it reads wet in a band about two feet up from the floor and dry above. Pull the shelving back and there is a gray-green haze on the paper face of the drywall that was not there in July. Upstairs, the bedroom window has a wet sill by seven in the morning.

None of that is a leak. It is condensation, and it returns every cold night until the humidity comes down.

Sorting allergies from mold, honestly

I am not a physician and neither is anyone who shows up with a moisture meter. What the CDC says is that exposure to damp and moldy environments may cause a variety of health effects, or none at all, and that for sensitive people the symptoms are a stuffy nose, sore throat, coughing or wheezing, burning eyes or a skin rash. Ragweed produces the same list, so symptoms alone will not tell you which one you have.

The pattern will help. Seasonal allergies follow the outdoor count and usually ease indoors with the windows shut. A mold problem follows the building: worse in one room, worse first thing in the morning after a night in the house, better after a weekend away. Add a musty odor that returns after you air the place out, or staining on a wall, sill, or closet corner, and you have enough to act on.

You do not need a lab test to get there. The CDC does not recommend home mold testing, and the Indoor Air Quality Council says testing is useful mainly when you cannot see or smell mold but suspect it. If you can see it or smell it, the next step is finding the water, not naming the species.

What to do this week

Buy a humidity meter; the EPA guide recommends one and they cost twenty to fifty dollars. Put it in the basement and read it in the morning, when it will be highest. Above 60 percent, plug the dehumidifier back in and set it to 50. The Council’s basement guidance is 50 percent in summer and 30 in winter.

Check the cold surfaces at dawn: window glass, the sill, exterior-wall closets, behind headboards and furniture pushed against outside walls, and the basement walls behind stored boxes.

Clean what is small. Under about ten square feet, roughly a three-by-three patch, both the EPA and the Council say a homeowner can handle it with soap and water and an N95, after the moisture source is fixed. Bleach is not required, and the Council notes that anyone you hire to apply bleach or any other chemical against mold in Maine must hold a Maine pesticide applicator license. Tyler Chandler at FES carries a Maine Board of Pesticides Control commercial applicator license for that reason.

Call for help when it is bigger, when it keeps returning after you have dried it, when it is inside a wall or under flooring, or when someone in the house has asthma or a compromised immune system and you want it gone without spreading it. We covered what the cleanup itself involves in our mold remediation overview and priced it out in what mold remediation costs in Maine.

If you rent

Since July 29, mold in a rental is also a habitability matter under Maine law. Our post on the new rental mold law lays out what changed. Document it with dated photos and the humidity reading, and put the report to your landlord in writing.

Where FES fits

If you want a second set of eyes, FES does a free on-site assessment in Gorham and the greater Portland area with no obligation. We will tell you if it is a dehumidifier problem you can solve yourself, and we will say so plainly when remediation is not warranted. Call 207-464-8202. If the meter reads 65 and the window is wet, do not wait for the smell to get worse.