heat gain in branch pipes of AC system. What is acceptable?
 

My HVAC system is built with one main plenum that projects into the attic and all branch take offs come from that with insulated flex. I am curious how much heat gain would be typical or "acceptable" from plenum temp to vent temp? I am seeing about a 6-8*F delta T. I am getting ready to blow more insulation tomorrow because I realize I have some issues up there and looking to bury the HVAC branches to help with this issue. Just want an idea what is acceptable.

I also realize that having these lines outside the heated/cooled envelope is not ideal but this system was installed into an existing home so some things were not practical.

Be careful if putting more insulation in attic space to not choke off the air flow within the attic. The soffit and ridge vents need to stay connected via a 1" airspace along the roof sheathing. You can used foam venting panels to maintain this air space and allow the insulation to be blown in around it.

Have you thought of encapsulating the duct system in rigid foam?

While 5 to 8 is not unusual, its too much. The air moving through the duct, is moving at several hundred foot per minute. So to lose 5 to 8 degrees is a lot.