During our back to brick renovation in our terrace it was discovered that the chimney of our party wall neighbours is not lined/is just the original bare brick. They have a gas fire place connected to this, in the ground floor living room. (We raised concerns re carbon monoxide poisoning - they refused to have it lined/decommissioned, even though we offered to pay.)
We’ve recently realised that we can hear conversations in their living room from the bedroom on our side, which is directly above the living room. As we’ve gutted and replastered we can only imagine this is travelling up the chimney.
Any ideas how we can sound proof this? Would a chimney balloon help? It’s bad enough that we can follow along with their conversation.