We have been told of leca used to fill unused chimneybreasts for insulation / moisture control properties.
Is this something anyone can shed any light on?

We have a shared chimney stack with our neighbour, and originally there would have been backing chimney breasts as well. Ours were removed by a previous occupant, while the neighbour's are still present but have been covered up (and aren't in use!). Is there any benefit to us (or her!) of filling her chimney stacks with leca? (and/or the remaining stack we have - it is cut off in our loft).

We have a moisture problem in our downstairs room where the chimney breast has been removed and are wondering if it could be coming through her unused chimney breasts (at least in part). We have had a roofer to look at the stack and it does need repointing which may not be helping the moisture, but if we are going to re-cap it we might as well fill it at the same time.

I've filled my disused flue with leca. It's not very good insulation, but has to be a lot better than an empty tube open to the outside air. What I'm not sure about is how this affects moisture levels. I planned to put a humidity sensor in for a winter before filling it up and monitor that after filling too, but entirely failed to get round to it, so just filled it in in the end. It's dead easy to do. It will get a sensor eventually.

My chimney has two flues and the other is still in use (now lined). That put me off using better insulation with low melting point like polybeads as I'm not sure how hot it gets adjacent to the other flue. Rockwool would be fine, but would it stay dry enough to be effective? leca is pleasingly robust against both heat and damp so seems well-suited for this application. Putting in a foamglass thermal break at the loft level (my chimney is internal) would probbaly be a good idea too, but is a lot of work for limited benefit. Maybe one day...

hapless - exactly what I'm presuming has happened here (we're semi detached though, but the stack is shared).