I'm finishing my basement. I plan to glue 1" XPS to the exterior concrete walls. Then I will build 2x4 walls against the XPS, fill with roxul and cover with drywall. All of the XPS seams will be taped or foamed sealed. I have read a lot about how important it is to make it air tight. My problem is that one of the concrete walls has ductwork and wiring running right up against it for about 10 feet at the ceiling. There is about 1/2" between the duct and the concrete. I can't move the duct. Do I need to get that section of the wall covered in XPS or can I leave it bare concrete? Maybe slide a 1/4 sheet in there? Does it matter? Is it ok for the XPS to be in contact with the ductwork? There will be a drywalled bulkhead built around the duct, so I'm not concerned about the XPS being exposed to flame, just heat.
I don't know if it matters, but the house is in Western Maryland. It's built into a hill so the back wall is below grade and the side walls are only partially below grade (bottom back corners). The wall in question is pretty much above grade except for the bottom couple of feet or so.

Thanks.