I'm not sure where all this is going. But ..
If you have to remove the insulation then that would seem like you have to take down the inner skin
In which case you can rebuild it and reuse the same insulation
I can't see why it would be needed to demolish the wall and then rebuild with other insulation, but if so you could then potentially need to line the inner face with insulation too
Or you can do a sap calc.
But for anyone else reading, rigid board cavity insulation is one of the crapest things to use for DIY extension building. Stick with quilt
If you have to remove the insulation then that would seem like you have to take down the inner skin
In which case you can rebuild it and reuse the same insulation
I can't see why it would be needed to demolish the wall and then rebuild with other insulation, but if so you could then potentially need to line the inner face with insulation too
Or you can do a sap calc.
But for anyone else reading, rigid board cavity insulation is one of the crapest things to use for DIY extension building. Stick with quilt