Backfilling a cast iron fireplace advice

Quite often here at Castfireplaces we get asked questions regarding installation and how to do things. We are fireplace retailers and know about the products we sell so we can often give good advice. Today by email we were contacted about back filling a fireplace:
“We bought a couple of gallery cast iron gas fires from you a couple of weeks back…just now in the process of having them installed, so far looks lovely – thanks for the great service. However, I have a general installation question….
I intend to backfill the void behind the cast iron inset with a 1-5 mixture of cement and vermiculite. I have read somewhere that I need an air gap between the vermiculite and the cast iron. Apparently, one trick is to place some cardboard between cast iron and the mix – the cardboard is supposed to burn off, leaving a gap. Do you know if it’s recommended to leave a gap? I would have thought having a gap would reduce the ability of the vermiculite to remove heat via conduction / prevent cracking of the cast iron. But then with no gap would the iron be able to expand okay.”

So what is backfilling? I hear you all ask. It’s actually the process of filling the chamber behind the fireplace, making sure it’s all filled with vermiculite and cement, so the fire doesn’t heat the chamber behind and pushes all the heat forward.
It is an essential part of fireplace fitting and, if not done correctly, damage will occur to the cast iron back. The customer is right cement and vermiculite is tightly packed into the void with a thin piece of cardboard behind the cast back. The reason for this is that you do not want the vermiculite mix sticking to the firebrick that can be replaced.
Once done, it’s topped off so that any soot will fall into the fireplace and not sit behind it as it’s flammable.
So some good advice and our customer is correct in his knowledge and should have a fireplace fitted that can be signed off by building controls.
We are here to help at Castfireplaces, that’s why ordering a fireplace from us is another great choice.