Thanks for the responses.
The external walls are being built up with ICF. The way the standard interface works, is that the hollower bears 80mm onto the concrete core of the ICF wall below. We then build up ICF walls from this level for the next wall pour.
The main objective is to stop the concrete from the ICF flowing freely and uncontrolled into the cores. If there is no 'stop' there, there is nothing to prevent it.
Airtightness in my understanding shouldn't be effected in reality, as either way the cores will be sealed by the external wall pour. It's more to do with managing the flow of concrete into the core.
