ᐅ Floor construction with ventilation system on the upper floor

Hello,

for our planned single-family home (urban villa, 2 stories without a basement), which will be built by a developer, we want to install a mechanical ventilation system with heat recovery. The ventilation installer would like to use oval ducts measuring 115x51mm (4.5x2 inches) in diameter. These ventilation ducts are intended to be installed within the floor structure of the upper floor, between the concrete slab and the screed. The floor structure currently looks like this:
- 1.0 cm (0.4 inches) surface covering
- 6.5 cm (2.6 inches) cement screed CT-C25-F4 with fibers S65 H45
- 10.0 cm (4 inches) impact sound insulation
- 18.0 cm (7 inches) reinforced concrete slab C20/25

Now the construction manager has expressed concerns and says the floor structure needs to be raised by 4 cm (1.6 inches), which would increase costs by around 1200 euros.

If I understand correctly, the ventilation ducts (5.1 cm (2 inches) in height) would have sufficient insulation both above and below (within the 10.0 cm (4 inches) impact sound insulation). I have illustrated this in the attached image. Is there something fundamentally wrong here?

Thank you for your advice.