You've found a damp patch on the wall. Maybe there are black spots starting to creep in from the corner. The first question everyone asks is: is the house falling apart?
Almost always, no. But a damp patch can come from two very different places, and the fix for one does nothing for the other. Water either comes out of the air in the room (condensation) or through the wall itself (a leak, or rain getting in from outside). Spend money on the wrong fix and the patch comes straight back.
Here's a test that costs nothing and tells you which one you have.
You'll need
- Aluminum foil
- Tape
- A dry cloth
- 48 hours of patience
The foil test, step by step
- Dry the damp patch with a cloth.
- Cut a square of foil about 12 in (30 cm) across.
- Tape it tightly over the damp patch, sealing all four edges so no air can get behind it.
- Leave it for 48 hours. Don't touch it.
- Peel it off carefully and look.
Reading the result
Water on the room side of the foil (the side facing you): the water came from the air. That's condensation, and it's the most common cause of mold in homes by far.
Water under the foil, between the foil and the wall: the water came through the wall. Look for a leak, a blocked gutter or damaged outside walls.
Both sides wet: you probably have both. Find and fix the water source first, then work on the condensation.
If it's condensation, here's the good news
Condensation is caused by everyday habits, and none of them costs money to change. A few to start with today:
- Open windows wide for 5 to 10 minutes in the morning, after cooking and after showers. Short blasts work better than a window left open a crack all day.
- Wipe windows dry every morning and wring the cloth into the sink, not back into the room.
- Move furniture 2 in (5 cm) away from outside walls, so warm air can reach the cold wall behind it.
- Don't dry laundry on radiators. One load dried indoors puts around 4 pints (2 liters) of water into your air.
Do these for a couple of weeks and most people see the difference on their windows.
This is general home-care guidance, not medical advice. Large or recurring mold problems need a professional, and if anyone in your home has symptoms you think are linked to mold, see a doctor.
The full fix is in Mold and Moisture: See the book →
It covers safe mold removal, ventilation, renters' rights and a seasonal checklist that keeps your home dry all year.