Hi,

(Sorry, I posted this to the wrong category earlier)

I've seen the previous messages on this topic but none seem to apply to my case.

I have an unused chimney stack in an uninsulated external wall, feeding two decorative fireplaces, with pots intact (no caps at present). I'd like to insulate the breasts and stop draughts. My loft is converted and I have a pir-insulated roof.

I've seen suggestions from Tony covering cases where the loft isn't converted, suggesting airbrick in loft and blocking chimney at ceiling level. This doesn't apply as I have a warm loft.

Should I stuff insulation into a bag and push this through pots to at insulated roof level, plus install vented caps? And then leave flues open to the fireplaces? Or fill flue with eps beads - and if so, down to a vented register plate just above the fireplace?

Thanks!

If you are proposing to fill the chimney then you will have to consider the weight of a vertical column and the compressibility of EPS beads at the base of the column. Have you considered the use of expanded glass beads instead as they have a much higher compressive strength, are indestructibe and therefore have a constant insulation value in the long term and above all are completely fireproof. They have minimal water absorption and vapour permeability is still available through the voids between the beads which will remain open over time.

I presume you are going to insulate the internal of the chimney breast. I also presume that the back of the fireplace internally will not be insulated.
Just a thought before you go and block every thing up have you considered fitting a sealed wood burner in the fire place i.e with an air input into the back which you could duct from outside. You mention a register plate so is this in place already?
You will then have a ventilated system through the stack to the outside.
I have done this with 2 stacks but in my situation had to bring the duct under the floor. I am dead pleased I did this as I have a draught free house a source of heat and a ventilated chimney that takes the air from outside.

Take the chimney down to below you PIR level in roof.
Continue PIR detail over old stack, reroof over.
Punch into both flues in the loft, clean them over their entire length down to the fireplaces below.
Either punch into the chimney breasts in lower rooms or fit closure plates in the fireplaces.
Install ducting, or flue liners over entire length of both.
backfill with..
a) vermiculite (if the external finish is properly weatherproof) or
b) leca (if your less confident) or
c) leave unfilled and ventilated top and bottom (if you know its going to ship water)
If you ever want/need to move air or heat between floor you now have a sensible route.
Do consider fire regs first though as you will have breached fire compartments.