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.