I noticed that while all the ductwork in my attic is insulated that the plenum and aquacoil unit off the air handler get very hot or cold to the touch depending on the air blowing. Shouldn't they be fully insulated since I'm in the North and would worry about ice dams from heat loss at this component. The air handler itself appears to be well insulated internally. My HVAC contractor said you don't insulate them as there is some problem doing so - that's what he told my builder at least when I asked for them to be insulated per plans calling for all ductwork to be insulated. What's the real story - should they or shouldn't they be insulated?

Thanks!