Three causes, three fixes
- Condensation damp: humid indoor air condensing on a cool wall. Widespread grey spotting, worst behind furniture, in corners and on external walls, with a musty smell and sweating windows. Fix: dry the air.
- A leak: plumbing, AC condensate drain or roof. One stain that grows, wet to the touch, worse after showers or rain. Fix: repair the leak, then dry out.
- Rising damp: ground moisture climbing the wall. A horizontal band up to about a metre above the floor with white salts and crumbling paint, ground floors only. Fix: waterproofing and drainage.
The foil test (48 hours)
Kitchen foil taped to a dried patch of wall for 48 hours separates condensation from water coming through the wall. The steps, and what to do with the result, are below.
When dehumidifying is the answer
- When the foil says condensation, and after every leak repair, to pull stored water out of the wall before repainting.
- A whole-home ducted dehumidifier holds 45–55% so the wall never reaches its dew point again. Repainting a condensing wall without it fails within a season.
Step by step
How to find the cause of a damp wall and fix it
- 1
Dry and cover a test patch
Dry a 30 cm square of the wall and tape kitchen foil tightly over it, all four edges sealed.
- 2
Wait 48 hours
Leave the room in normal use: AC, showers and cooking as usual.
- 3
Read the foil
Moisture on the room side means condensation. Moisture on the wall side means water is coming through the wall: a leak or rising damp.
- 4
Fix the cause
Repair a leak, or call a waterproofing contractor for rising damp. For condensation, the fix is drier air.
- 5
Dry the wall before repainting
Keep dehumidifying until the wall is dry. Paint on a wall that still condenses peels within a season.
- 6
Hold the room at 45 to 55%
A whole-home ducted dehumidifier keeps the air below the wall's dew point, so the damp does not return.
