rendering the back of an open fireplace

[penguinman]

I need to re-render the rear of a large fireplace. The previous rendering, not done by me, cracked and blew with the heat. Is there anything I can put into the new rendering mix to minimise this problem. I have looked at vermiculite boards but this would be over £100 for this size of fireplace.

Any suggestions gratefully received

Thanks

 

[maaaaaaaaat]

100 pound,cheap i think.