floor preparation for open cavity after breaking through

I have just broken through from my dining room to my sun room extension. Both the new and existing floors are concrete slabs, the wall that we removed was a brick and block cavity with an 80mm cavity. The floor in the dining room is engineered oak, as it will be in the sun room.So at the floor level I now have an 80mm gap/void.. to make matters slightly worse the sun room slab is 10mm higher than the existing house. What do I do with this void? I can put up with the 10mm higher floor but wondering what to do with the gap. I thought about filling it with concrete, or maybe expanding foam? tbh I am not even sure how deep it goes down.

We're it mine I'd raise the other floor 10mm and feather it down to nothing over a metre or so, then floor straight over the top. It'll bend and cope with it. Avoid laying board joints at the transition angles

Fill the cavity with insulation board, gunfoamed into place. You want to avoid cavity drafts reaching the underside of your flooring really..

The only problem with that is I would have to rip up the engineered floor in my dining room to raise it 10 mil and being an engineered floor that is glued together it will be quite destructive. I'm not overly bothered about the ten Mill height difference more concerned about what to do with the gap. I'll clean it out, pack it with cavity bat and foam it.

I tripped over a 12mm step in a badly laid floor last Wednesday. Are you going to sell the house shortly?
Cjards way is correct, foam will collapse.
Frank

i'm going to fit a bit more insulation under the first 6 inch or so of the existing floor to raise 4mm and then let the other floor bend down to meet it over a distance (I should have a good 300-400 mm to play with) then use a straight threshold strip with the 10mm gap... insulation foamed into place! thanks again everyone. The end of any wood will not be resting on the foam.. i'll make sure there aren't any tiddlers in this part. If need be I'll make up any gap in height with either ply or foam and/or insulation.