Old Plaster as insulation?

A silly perhaps, but a serious question.

I don't have any insulation at the moment. None. The dwelling is 50's brick veneer single story (South East Melbourne) .

I am completely renovating my house - changing plaster, floors. What someone has told me - keep the old plaster on the ceilings. Plasterers will lay new plaster on battens. My reason for removing the old plaster was the mass of the plaster itself - but what's another ton for the joists, and stumps, right?
The air space between the old and new plaster will work like air in the double glazing windows.
There is cost involved in adding battons, but will it be offset by reducing rubbish removal cost of old plaster?

So, my questions are:
1. The mass of keeping old plaster hanged - is it a problem?
2. Will the air gap work as double 'glazing'?

And hey - if any good plasterers interested - please pm me, 20sq house in SE Melbbourne.

Thanks


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