If necessary switch the wires to reduce the blower to its lowest speed.
How to insulate sheet metal plenum.
Do not use rigid foam insulation unless you know what the temperature rating is.
38 to 102mm insulation boards fibrous glass insulation boards are also available for application to the outside of sealed sheet metal ducts housings and plenums.
Plenum should be dry before external insulation is applied.
Press a continuous strip of foil tape to all longitudinal seams along straight runs of duct.
Ductwork is typically insulated with foil or vinyl backed fiberglass insulation specially made for the application.
You can fasten and secure the ends of the batts with duct tape but that may cause the insulation to get spoiled.
In this video this old house plumbing and heating expert richard trethewey helps a homeowner insulate his ductwork.
Check the speed of the blower motor on the furnace.
Use foil faced insulation batts to wrap up the pipes but see to it that the whole thing is properly covered and any previous layers are thoroughly overlapped.
Check for air leaks sealing them helps your system more than just the condensation problem.
Duct insulation may not pass through a fire wall or fire rated floor ceiling.
Insulation used to externally insulate hvac ducts.