I’m replacing the microwave/recirculating fan above our stove with an actual range hood that will vent to the outside rather than blowing smoke and grease back into the kitchen (I don’t care what they say, those filters suck). The stove is on an interior wall, with a high angled ceiling. Because of an awkwardly placed wall stud, I can’t run the ductwork inside the wall cavity. Instead, I’ll be running it vertically from the hood, up through a cabinet, then out through the ceiling, through about three feet of attic, and then through the roof. The duct will be exposed in the kitchen for about four feet between the top of the cabinet and the ceiling; I’ll figure out some sort of cosmetic cover for it later.
Here’s my question: the roof vent cap has a gravity damper on it which closes when the fan is not running. In addition, the range hood comes with an additional damper that sits flush with the top of the hood. If I use this as is, I seem to be trapping a column of dead air that runs from the top of the hood to the roof (about nine feet long). The attic space gets quite cold in winter and quite hot in summer, and this seems like a nice fat pipe for heat to travel through in ways that I don’t care for. In addition to insulating around the ducting in the attic, and in the kitchen too if possible, it seems to me that it would be smarter to have the lower damper at the kitchen/attic interface rather than at the hood/duct interface: put it where the greatest thermal gradient is.
But perhaps I am unaware of something else that is a bigger concern, like condensation in the duct or grease precipitating out, or something. Any ideas as to what I might be missing?