Is it ok to use Celotex behind my cast iron fireplace to help fit it?

[jgkgolf]

I'm fitting a cast iron fireplace which needs to be kept about 7 cm off the wall to fit into the rebate in the surround. I was going to use slabform rockwool but celotex is better as it keeps it nicely in place while I fit mortar around it. It may be used as a solid fuel fireplace in the future. I presume this is perfectly safe to use as it is just another form of insulation?

[Andrew Gabriel]

I can't really picture what you are describing, but the basic polyisocyanurate foam burns, giving off cyanide. I believe the products such as Celotex and Kingspan have fire retardents built in, but I still wouldn't put them near anything hot. Foam certainly melts, and although fire retardents may stop the foam sustaining a flame itself, it will still burn when subject to a good external heat source.

[Andy Wade]

Agreed. The usual insulating material to use behind firebacks etc. is vermiculite granules - still available as 'Micafil':

make the rigid material they call Micafil concrete you mix the granules with OPC (ordinary cement) - 1 vol of cement to 6-8 vols of vermiculite (no sand) - and add an absolute minimum of water to make a workable mix. Allow to cure and dry for a couple of weeks before lighting a fire, and build up slowly from small fires at first.

[The Natural Philosopher]

it doesn't.

It chars and doesnt give off much at all.

But you have to get it preety hot.