Cavity opening due to installing a fire

[ct75]

I firstly installed a gas fire with an outside flue to my home, this meat knocking a hole in the wall and having a box on the outside to take away the fumes, as I live high up when it rained (most of the time) there was a tremendous noise coming from the metal box outside as rain was hitting it, I got rid of the fire and bricked up the outside wall, i have now placed an electric fire in its place which is also sunk into the internal wall. I am now suffering from a draught from the cavity at the rear of the fire, what is the best material to fill this with please as I am losing a lot of heat in the living room with a constant cold trickle of air coming through.

Thanks in advance

[gucciman]

i would have thought you could have used loft insulation to fill the cavity but you need to check with some of the builders on here as im not 100% certain that its flame retardant.