Best to remove the duct and SPF or foamboard the rim areas. Right now, the heating system is drying the rim area- through the air permeable fiberglass insulation, that will change dramatically after 3" SPF in front of it. If the steel studs are in front of the SPF, no worries; http://www2.dupont.com/Tyvek_Weathe...rior Insulation Building Science Bulletin.pdf
Gary
Gary