We decided we wouldn’t need our open fire once we did all our people powered retrofit measures (3years still ongoing!). The chimney stack needed some work & also cast a pretty big shadow for proposed pv panels.
And it was the source of a howling draught!
Our trusted roofers removed the outside stack to around 1m above loft floor (~600mm above insulation layer), and made good the hole in roof.
We narrowed the fireplace ‘throat’ with engineering bricks & cement board leaving a breathable gap filled with a modified ‘chimney sheep’ reinforced with 1.5 mm s/steel mesh.
Then filled chimney breast from top with Geocell foam glass bubbles (ex Mike Wye).
It took 5x100 l bags of the 2-4 mm size bubbles level to just above loft insulation level, capped it off with layer of mesh to keep out spiders etc!
Apart from the stack removal it’s a reasonably easy diy job and seems to be an effective insulating breathable solution if you want to keep chimney breast.
Made a huge difference to whole house comfort in removing a major draught!