can any one tell me the best way to backfill a cast iron insert for my fire place. I intend to place a7.5kw livingflame gas fire inside as a room focal point.
The mix should be not too strong. 1:2:4 of Lime:Sand:broken brick is ok. Or, 1:1:12 Lime:Cement:Vermiculite chips is commonly used for an insulating mix. Mix it up fairly dry - not like wet concrete - and don't compact it.
It is also a good idea to leave some kind of expansion joint between the wall and the back edge of the fill. A sheet of 10mm polystyrene or some similar compressible material woud do the job.