We are currently finishing our basement and nearing the framing stage. Currently we have one supply return to the furnace, but would like to increase this to 3. Most of the returns in the house are run using thermopan on the floor joists and/or through the wall studs. Due to some obstructions we will not be able to do this for the basement returns. We are wondering what we can use for the return supply in tight spaces? Can we use 4 or 5" flexible ducting?