Internal Wall Insulation - retaining existing plaster?

[jayc89]

Looking into IWI for some of our existing rooms. Thinking we'll likely use the "warm batten" method, but regardless of the method, almost all recommend retaining the existing plaster.

 

In places, our existing plaster is over 25mm thick. Knocking this off reduces the floor space last by ~ 22% so quite considerable.

 

What's the purpose of retaining the plaster? Would a sand + cement parge coat serve the same purpose?

Edited April 4, 2022 by jayc89