can rigid foam board be used for a short return air duct?
Currently, my furnace intake is wide open. There are no real returns in the house. There's a 24x24 hole in the wall that separates the laundry room (where the furnace sits) from the rest of the house, and that is where air is sucked through right now. The intake is facing the other way. I'm pretty sure at this point that not only is air being sucked through the 24x24 hole, but it's probably also being sucked through the laundry room exterior door, as well as the cold basement door, so I'd like to get a duct over from the living room duct to the intake to minimize this issue (I think that's the reason that my heat will run all day long, even calling for aux heat in 50 degree weather, but when the heat is turned off, the interior temp holds just fine for several hours on a day like today).
I see that metal is extremely expensive. Is there any problem with using rigid foam board to construct a duct to get me from point a to point b for the return only? I'm going to have to go vertical and run it across the ceiling and then drop it back down, and foam is much much cheaper than metal. I also can not find anyone in my area who has that fiberglass duct board.