What It Tells You
So what does an aluminium foil taped to the walls tell us? It can help show whether the moisture is coming from the room or from inside the wall. If the room-facing side of the foil is damp, the issue is often condensation. That means humid indoor air is hitting a cooler surface and turning into water.
If the wall-facing side of the foil is damp, the warning is more serious. That suggests moisture may be coming through the wall itself, possibly from a leak, seepage, cracks, gutters, drainage problems, or hidden plumbing. If both sides are wet, you may be dealing with more than one issue at once.
This simple trick is a quick way to decide what to do next. Condensation usually means improving ventilation, lowering humidity, and using fans or a dehumidifier. Moisture from the wall means you should investigate the source quickly before it can do more damage to the wall and probably the entire structure of your house.