I don’t know much about HVAC but my condo was built with insulated ducts in the space between my condo and the one above me. My thinking is that the architect and AC contractor must have decided it’s worth doing.

The builder for my community has a horrible reputation and can't expect they intended to do anything correctly. Also, my condos go back to the late 80s and early 90s. After that codes have changed as far as insulating duct work.

I added a picture to give you an idea of the quality of work they did. This is one of my HVAC returns. As you can see, they eye balled and cut holes in the intake air box and intake duct guaranteeing that the connector between them could never fit. Never connected the intake box to anything. The chimney is on the other side of the duct and the whole thing is in a cavity that goes up to the attic with nothing but a sheet metal fire stop at the top. And some openings on the sides at the top. Below is just wide open to the space between my floors. The grate was screwed right into the drywall with no anchors so those screw holes can't even hold the grate up.

Normally ducts are insulated when passing through "unconditioned" spaces. Between interior floors would be considered "within the envelope" and wouldn't normally be insulated. You could and should check that they're properly sealed at joints where visible and possible.

Your points for insulating and not insulating both carry good merit, but at the end of the day don't forget to consider how your condo feels now. Is your place too hot or too cold seasonally?

Twintech Heating & Cooling

https://twintechheating.ca/

Uncle Elmer is correct. It's generally considered unnecessary to insulate ductwork inside the conditioned space ("envelope").

Gives me a lot to think about. The space between floors should technically be inside the conditioned space, but there is so much that is open that goes straight to the attic that I don't know if it should really be considered conditioned space. I've seen the ducts sweating between floors when I have looked. Plus a hole in the ceiling where a conduit goes through in the furnace room, in the winter I can literally feel winter air coming through it so I know it is not well conditioned. There's walls, ductwork, pipes, electric wires all in the way to get to where I would need to seal the space between floors and the vertical section that houses intake/supply duct work for the furnace and chimney pipe. I also think there is air getting in from other places to the between space. NJ has just been a horrible experience for contractors, builders, mechanics or any other kind of vendor for that matter.

If it's open to the attic then, yes, insulate the ducts.

Made a ton of discovery since my original post. Situation is worse than I originally thought. In the process of sealing attic, walls, windows, doors, chases. Among a ton of other work.

Will insulate the ductwork between the floors just to be on the safe side since there is no way to get all the air leaks the builders left. Plus I've seen some newer engineering science posts that recommend it.

Will also replace the ductwork and properly seal it after I get a proper redesign with manual J and D calculations.

[Just venting about what I found as I did things, like took the grate of the return boxes, looked into the walls. Looked under attic insulation. So, you can probably skip this if you get this far.]

Basically, ballpark guess, the space between floors, behind walls, and even straight to the attic, was getting half the conditioned HVAC air and supplying half the return air.

I did a very very detailed heat load calculation, with the assumption that my place was properly sealed and insulated. Turns out the Furnace is a little over double what it should be and the A/C is double. Which has been inadequate in heat waves and cold spells. Now I know why. My first floor air return is even worse than that one. Ugh!!! And that is just the beginning of crap to deal with...

Hovnanian instructions to contractors (based on one of them who was involved when he was a young adult)

1) We will give you $6000 for each condo you build.

That was pretty much it. No real oversight. No inspection from Jersey City code enforcement. Drilled every single hole with a hammer through drywall, and the cinder block fire wall to my neighbor. Attached HVAC duct? Optional. Sealed almost none of it. Did find they used actual duct tape to hold together my dryer vent pipe. Use flex duct to go around fire rated chase? Better yet, use solid ductwork and some angles to get around the edge of it? Nope, hammer time. Smash the whole 2 feet or so of the side of the fire rated chase and put a straight pipe across. Even found electric wires connected by wire nuts behind the wall!! Out of 3 bathroom fans, only 2 were facing toward the vent pipe, only one was connected to a vent pipe. Baffles were blocked from opening on 2 unconnected ones.

I'm sure the hammer they used to cut the holes through the 2 layers of sheet rock with RC! between for a 1 hour fire rated ceiling, allowed enough air to go around the fan so it was not as restricted as it may seem.