I am having a new furnace and ac unit installed in my home, and was wondering about the return ducts. they used a cardboard like material with aluminum finish on the outsides of it, basically boxing in between 2 floor joists with this material and also using it between 2 wall studs the same way for the cold air returns. one of the things that bothers me is that they are not sealed up in the basement. I was under the impression that they were going to install sheetmetal ducting, so i'm alittle suprised by the material being used. should I be concerned with this and request sheetmetal returns, or is this routine practice? I appreciate any feedback.