Hmmmmm....I am not quite sure what you are asking? Are you asking if you can put a 1' thick light clay wall on the inside of the polyiso wall? Or are you asking if you can plaster onto the wall?
I would not recommend putting a light clay wall on the inside against the poliso. With this I don't think the wall would dry properly, or would take a long time. A normal light clay wall can take 6 weeks or more to dry fully.
If you are asking if the iso board can be plastered onto, then yes this is an option. I just helped do this on a friends house about a month ago. It worked really well!
I hope that answers your question?
Cheers,
Tim
