Many microwaves and range hoods are vented to the exterior through the top of the cabinet they are mounted to. When this is done the area that is cut open to accommodate the venting should be sealed around the duct. If it is not warm air can bypass here and enter the attic space which can contribute to ice dams in the winter. There will also be a considerable amount of conditioned air lost through here which equates to dollars being tossed out the window. Typically batt insulation or expanding foam is used to seal up the voids. I would also recommend checking your ducting as I often find ducts that are disconnected or are leaking a good amount of air at the joints. If there are leaks in the joints it can cause a sticky grease residue to build up in the cabinet and coat the walls over time.