Insulating the supply duct reduces duct loss or gain and prevents condensation. Use 11/2-lb density on ducts that deliver air velocities up to 1500 fpm.
Three-pound density or neoprene-coated insulation is recommended for ducts that handle air at velocities greater than 1500 fpm. Insulation can be 1/2 or 1 in. thick on the inside of the duct.
Where rooftop equipment uses the sandwich space for the return air system, a return air chamber should be connected to the air-inlet opening. Such an air chamber is shown in Fig. 15-24. This reduces air handling sound transmission through the thin ceiling panels. It should be sized not to exceed 1500-fpm return-air velocity. The duct can be fiberglass or a fiberglass-lined metal duct. A ceiling return-air grille should not be installed within 15 ft of the duct inlet.