I have a cabin in Minnesota that has two electrical panels in a bedroom. One is the "main" panel. The other is for off-peak power to the HWH and a couple of thermal storage heaters that store heat at night at a reduced electricity rate.
I plan on doing some repairs and improvements this summer.
 A single mast goes up through the roof from the double meter box to where the overhead cable is connected. The conduit was not anchored to the wall and is pulling against the roof board. There is a metal plate on top of the roof that fits over the conduit. It should have been tucked under the shingles above and tarred a little. Rain runs under the plate and down through the hole for the conduit. The edges of the hole have deteriorated over the years.
The poco said they would drop the overhead cable so I can disassemble the mast structure in order to repair the roof and redo the mounting of the mast.
 I also want to add some circuits for a couple more thermal storage heaters, an Ecosmart water heater below the kitchen sink just to heat the cool water in the hot water pipe while hot water arrives from the HWH, and some yard lighting and outlets;
 Here is the main reason I am writing.
In cold weather, the bedroom with the panels is always cooler than other rooms. The panels fit between adjacent 16" stud bays. This amounts to a thermal bridge to the outside about 36" x 36" in size.
As long as I am going to be adding circuits to the panels and working on the entrance service, now (or never) is the time to stop the heat lost through the panels. I know I can fill the stud bays with fiberglass or foam, cover them, then flush mount them. I would have to make an outer cover to hide all the wires going into the panels. I would like to avoid such a major intrusion into the room.
Is there some miracle material I could put between the panels and the siding to minimize the heat loss?
Thanks for your interest.