What To Look For
While the foil sits there, you are putting the wall to a small test. The airflow is blocked, and evaporation slows down. The patch can no longer dry the same way as the rest of the wall. Moisture at that part of the wall behaves differently, and that’s what you are looking for when the waiting time is over.
When you finally remove the foil about 48 hours later, do it carefully. Do not wipe anything yet. Look at the side of the foil facing the room. Then check the side that was pressed against the wall. Also, don’t forget to examine the wall itself before it dries again.
The foil will not solve the problem, but it may help you understand which problem you actually have. Click on the next page to understand the results of this tiny experiment.