Wet spots near the exhaust duct

Hello. Since there was no problem initially after installing the hood, I can make an assumption that the neighbors managed to get something with ventilation. For example, one client had similar symptoms with an extract in the kitchen, like yours. It turned out that the neighbors removed the “obstruction” ventilation duct, due to which warm air immediately began to enter the ventilation, as a result of which condensation began to form on the walls. Which in your case we are observing.

First of all, I advise you to call a specialist from the Criminal Code, who should find out the cause of the problem, and not deal with the symptoms. As a temporary solution, I can recommend replacing the corrugation with a plastic box and mounting a check valve in it, which will close when the hood is working. In the inoperative state of the hood, the valve will open, which should facilitate normal ventilation and reduce condensation.