So we purchased our house brand new about 5 years ago. It came with all new GE stainless steel appliances including an OTR Microwave that is direct vented out the back to an exterior vent on the side of the house. We noticed that during the summer months, that we would get dripping from the microwave during the daytime as well as a fogged up window on the microwave. Also in the winter, the microwave became a secondary icebox because it would get so cold. We called and somebody came out, took the microwave down and just put it back up, saying it was mounted wrong. Fast forward 5 years, cleaning up the sweat produced by the microwave and noticing that the bottom portions of the cabinets that butted up next to the microwave were showing some moisture issues, the Microwave fried itself, due to what I believe is the moisture in the machine. So I took down the microwave, only to find a bunch of mold growing both on the top of the microwave as well as the side cabinets. I removed the top cabinet to find more mold.
After cleaning this all up and ordering a new microwave, I can’t seem to find a reason as to why I’m still getting hot moist air coming in through the vent, even with the vent cap on the outside securely closed, and I don’t want to put the new microwave back up until I figure this thing out. The side of the house that the vent is on, is bricked. The ductwork is in the wall and unexposed anywhere.
You might have to remove the outside cap and the duct in between to sleuth further. My only guess is that the outside damper is not closing completely. In the case for more competent kitchen cooktop ventilation, a damper is used both at the exhaust and at the hood.