We built a conventional Timber Framed roof and loft inside and on the exterior ICF wall.

It even had a 4 knee wall and second floor loft.

Only 2 posts inside were actually structural the rest were just for looks.

Very nice project when it was done.

Not a bad way to go if $ is tight and the customer wants high R Value.

And yes the Romans used concrete, don't know if they invented it though. Some of the largest Roman structures still standing had concrete in them.