A previous owner put a small addition on the house and the little room is not well heated. (I have forced air HVAC.) One wall of the small addition is shared with a larger room. On the side of the shared wall is a sheet metal plenum 8"x8" in cross section that brings HVAC to the larger room and a foyer. I want to open a vent from the plenum in the larger room into the small, colder room. Here is the problem.
The plenum is right next to a supporting beam, so I need to run a path for the air around the beam. There is no easy way to get into the crawl space above the two rooms in question. So, I need to run the path below the beam.
My question: Do I need to build a sheet metal duct or can I just make a path for the HVAC air using plasterboard?