Many people have a microwave vent hood above their cooktop. It’s a great feature and it saves valuable countertop space. When these are installed, the vast majority are installed so that they recirculate the air back into the living space. Although the air goes through a charcoal filter, you still get plenty of cooking smells thrown back into your face. The answer is to ventilate the unit to the exterior.
Ventilating the microwave hood to the exterior is pretty straightforward as long as the unit is installed on an exterior wall. You will need to rotate the blower so that it redirects the air out of the top of the cabinet rather than through the front vent.
You will need to buy some ducting that will allow you to direct the stream of air up and out of the wall. You will also need to buy a vent cover for the exterior side of the wall.
The vent cover is simply a shroud with some mesh on it (to keep the critters out) attached to a short piece of ducting. Hold the ducting up to your stucco wall in the location where you want it, and draw a line around it. This is where you will cut. A small grinder with a masonry wheel will do a nice job of cutting stucco. Once you have the hole cut, liberally apply caulking around the hole and to the underside of the vent and push it into the hole. This will seal out the weather. Then it is a matter of connecting the ducting together and blowing the cooking smells outside.Cleaning Copper Plumbing Lines for Soldering » « Shower Valve Loose in Wall