ᐅ Duct embedded in screed

Hello everyone,
we have decided to install a slot ventilation system in the kitchen and need a flat duct for it.
The duct has the following dimensions: 220/90 mm (9/4 inches) with a length of 1800 mm (71 inches) (1000 mm (39 inches) under the kitchen island and 800 mm (31 inches) under the tiles) and is to be installed on the raw subfloor.
The floor construction above this is 15 cm (6 inches) thick, including tiles (underfloor heating).
The architect has now told me that this would only be possible if the height of the duct were 55 mm (2 inches) instead of 90 mm (4 inches).
Is this statement correct, and do we have to accept it?
Regards and thanks

We installed a future flat duct for the chimney, and according to the screed installer, 60cm (24 inches) height was the maximum possible with a floor build-up of 18cm (7 inches) (without hardwood flooring!). The flat duct is only allowed within the insulation and must still be able to be insulated on top. But you can simply use a wider one, right? That should be available as well?

Why is it necessary to insulate the duct?
Is this mainly to reduce noise transmission or to prevent thermal bridges?
Regards

Yes, because of thermal bridges. In our case, it was for the future installation of a fireplace. I’m not sure how it works with the exhaust air.

That should be sufficient if the duct is insulated only from below, right?

I have no idea. I only know that here, the water pipes were insulated. This was specified to us by the chimney installer and the screeding contractor.