I’m working with two materials. One is a foam for sound insulation, about 1” thick, with a flexible polymer foil backing. The other is a heavy dark grey sound insulator loaded with barium and other heavy metals, about a 1/4” thick. Putting rolls of it around ducting immediately entering and exiting the fan is easy, so long as the line of axis is straight, but to turn 45 or 90 degrees is a nightmare, as you can imagine all sorts of geometry comes into play when trying to join to cylinders as 45 or 90 degree and leave no gaps, especially when layering the stuff.