mrfs- is spot on. Not that you didn't know that already. Proper sealing of ductwork is often half assed. Sloppy work often is the norm. Leaks will loose A/C or heat and will effect the flow in the rooms. You want all of your return air coming from the rooms, not attic. Be easier(less $$$'s) to cool return air from the rooms than a seriously hot attic. I don't even like any flex runs over 2ft. The last 2 ft to the register. What sometimes happens is the hard duct won't fit easily, but the flex will be jammed in tight spots. Squished, used to make 180 degree turns, etc. 5ft is OK as long as its straight and stretched out a bit. If possible i would put an elbow on top of register to make the turn and connect straight flex in between. And check the straps used to tie the flex on, lightly pull on flex to see if it comes off. I agree with the ductwork insulated on the outside. Sometimes we would insulate(inside) the first length of duct off of the unit, as stated above this was for noise. It works well. BTW- all these points made are alot of times easier said than done. Most generally its a PITA to retrofit a house after it's completely built. If possible with hold the last 25% until you have a punch list done, and the system runs for a few days to see if it performs as designed. Good Luck!