Does a cold air return have to be through a metal duct? I want to put one in from a second story, down to the basement using an internal wall. Could the space in between the 2x4's be used, or do I have to insert a 3x14 metal duct?
#2 - Is it taboo to install supply ducts inside of outside walls? Ideally, I need to do this on the first floor to get to the second story from the basement. However, I noticed that the houses supply ducts are all in interior walls.