I did a little experiment with ducts and insulation and also used the wonderful 'Revit Lookup' tool to figure out the relationship between those two. As far as I understand the duct element is the host of the insulation element meaning that insulation has a HostElementId parameter which refers to the duct element.
The same as doors and windows, I do not know a direct way to retrieve the hosted element from the host (Wall --> doors). The only options I can think of are the following:
1- To use the 'delete' trick. Delete the host (which results in the hosted element been deleted) and iterate over the deleted elements and see which one is an insulation element. Then of course Rollback the delete and delete the insulation separately.
https://thebuildingcoder.typepad.com/blog/2009/06/host-reference.html
2- The other option is to iterate over all insulation elements and see which one's HostElementId refers to your duct element. Of course you can add proximity filters, etc to make this option more efficient and faster.
