Correct Duct Sizing

**tonynace** · 03-26-2012, 12:11 PM

I am finishing an attic, turning it into two rooms, a master bedroom and master bath and need to run duct from my furnace in the basement up to the attic. The room dimensions will be 10 x 11 for the bathroom and 10 x 21 for the bedroom, both with 8' ceilings. I will also have a 7 x 8 walk-in closet off the MB. I've gotten a couple quotes to do this job, but they are slightly different on the duct size. Both want to supply the upstairs with an 8 x 12 rectangular duct, but on the return, one wants to use 8" round, and the other 10" round or another 8 x 12 rectangular. They will feed plenums in the attic which will then have flexible ducts to the registers. Since I am bringing the feed ducts up along a wall, I will have to build out the wall some to accommodate them, and I don't want to bring out the wall any further than absolutely necessary, as space is an issue in that location. First off, is round duct better for airflow than square or rectangular? If it is, can I go with 8" instead of the 10"? If round is really not any better, I will just go with the 8 x 12 as this will give a bigger duct area assuming the contractor does not want a whole lot more to do it that way. Also, does the walk-in closet need it's own register and feed? I plan on having the whole underside of the roof and walls insulated with spray on foam, so the whole attic will be a conditioned space, if that makes any difference in the equation. Do the plenums in the attic then have to be insulated?