I recently got a new central a/c system with ducts included. PROBLEM: The ducts had interior insulation that gave off maple syrup odor. The contractor replaced the ducts with more interior insulation, unfortunately I get the maple odor at times and other odors. Why does anyone ever use interior insulation in A/C ducts?? Insulation is poisonous, carcinogenic? Isn't EXTERIOR insulation the only way to go?
I have never used interior duct insulation, I would think the foam or whatever material could off gass causing the smell you are getting. Metal ducts in my experiance are wrapped on the outside and flex ducting has the insulation outside the plastic. Since you are having smell issues I would say exterior is the way to go.
Putting insulation on the inside of ducts is a bad idea
There are two reasons to avoid insulation inside ducts -- you have found out one already, and the other lurks in the shadows, waiting to spring upon unsuspecting folks years down the road
First, of course, the insulation inside the ducts can emit gases or release fibers into the HVAC air. This is mostly a nuisance (odors, etal), but even when it isn't present, the larger hazard is lurking in the shadows: the insulation inside the ducts can trap moisture, organics, and other whatnot, creating a breeding ground for....GERMS! Yep, trapped moisture (often from inside-the-duct condensation due to humid air in the duct flowing into the insulation and condensing on cold surfaces) + organics (food) + spores/dormant bacteria = your ducts turning into a biology experiment, which is obviously not good for the macroscopic occupants of the building, and creates smells of its own (such as your whole house smelling like vomit, or dirty socks). Good luck cleaning them, too -- the inside-insulated approach makes that practically impossible due to the inherently porous nature of insulating materials.
Note: ducts should be inside the conditioned space to begin with to avoid problems with air and heat leakage. To do otherwise requires a big pot o' mastic, generously applied to every duct joint (even the boots!) to get a handle on these leaks, and that's before you wrap the ductwork in layers and layers of insulation to keep the heat (or cold) inside it.
Start with BSI-094 if you want more details.