10mm will probably be OK on studwork but, personally, I would stick with 12.5mm due to your stud spacing.
You should bear in mind though that 10mm or even 12.5mm isn’t going to give you much of an increase in thermal efficiency if you use it on an external wall. If you really want to make a difference, I would consider using 20mm board on there; more expensive sure but, after all, you only going to be doing it once.
You should bear in mind though that 10mm or even 12.5mm isn’t going to give you much of an increase in thermal efficiency if you use it on an external wall. If you really want to make a difference, I would consider using 20mm board on there; more expensive sure but, after all, you only going to be doing it once.