The insulation is next to/in between and then compensated with filling. Insulation on the flat duct also doesn’t help if it compresses under the load. Zehnder had pointed exactly that out in a planning with the flat system, or rather they didn’t want to assume liability for the slab’s statics. Literally: “Duct runs should lie a max of 30cm side by side and maintain 20cm to the next run.” That referred to a run with 3 flat ducts side by side. I’m telling you, their planning is careless. At least there was a note that this is actually not okay. The screed layer builder just shrugs, he can’t contribute much to the statics of the ducts. can you move the ventilation discussion?