Both the body of our high efficiency gas furnace and the ductwork seem
to be quite well-insulated since they are at best slightly warm to the
touch.

However, the sheet metal supply plenum coming out of the top of the
furnace appears to be uninsulated and is relatively hot.

- Should this be insulated?
- If so what type of insulation?
(I assume some type of rigid board but I imagine there could be code
or safety issues given heat and proximity to furnace)