pete_c said:Thank-you Del.
Yeah never seen insulation inside of a duct before. I have seen tin tacked inside of a space 16" between studs used for returns. In the old house the flex ducting in the attic was insulated such that it looked twice as round.
Ever seen this stuff? Not sure if this is what DEL was talking about, but this is used commonly here in FL. It's called duct board and it's rigid insulated panel, R-6 value, just like flex duct.
Usually it's just used for a few things like the main stack off the air handler, and as junctions between large trunk flex ducts into duct board boxes (often custom made) which then have smaller flex duct to the boots in the walls, and it's often used to build the return plenum, but I still see people use tin for return plenum and then insulation wrap it (my parents townhouse has this). They'll also use ductboard between floors as an alternative to tin.
Unless a customer requests otherwise, they generally always go flex otherwise because it's easier to work with. I'm actually planning to go rigid with ductboard when I redo my system. They'll use all ductboard for all trunk/distribution on supply/return and then come off that with flex to get to the boots in the walls. They do flex for the "last mile" like because it's A LOT more cost effective than getting rigid all the way to the boots in the walls.