On Mar 20, 4:06*am, Kenneth wrote:

The ducts are not sealed, but they are (very modestly)
insulated with a shiny, "quilted" looking product to
something like R4.

One wants to remove the quilted stuff, and then foam over
the bare metal of the ducts.

The other wants to foam over the existing insulating
material.

The ducts aren't just insulated, they're muffled by the quilted
stuff. You want to keep it.

Because adhesion on the ducts would make (for instance)
foam removal destructive to ducting, I'd staple a layer of
paper (maybe Tyvek housewrap) over the existing ducts,
so the foam wouldn't penetrate and adhere. Tyvek isn't
going to chemically interact with the foam.