Soundproofing chimney breast for home cinema

[Thregwort]

In my mid-terrace house built 1910 there are solid (single brick and cement render) party walls which unfortunately allow the neighbours' talking at normal volume to be heard quite clearly by me. As I have just bought a big plasma tv, I do not want the noise from a cinema sound system to get through to the neighbours, and so I am sound-insulating the walls throughout with Wickes heavy density rockwool boards.

My question concerns the (unused) fireplaces and chimney breasts, at present just hardboard paneled and papered over. It occurs to me that these empty chambers will simulate a proper party (cavity) wall, being - with the neighbours' mirror image, THREE brickwork walls in total, but surely acoustically, be a bit echo-ey, and so I propose filling them up with loft-type glass-fibre insulation, the type that comes in big rolls.
I am aware of the need for ventilation in unused chimneys, but as I am mid-terraced, I wouldn't have thought condensation would be an issue. There are no damp patches anywhere in the building.
Before I go ahead, if anyone can see a downside to my plan, I would be grateful to hear from them.

Thanks in advance

 

[footprints]

If you still have chimneys open at the top you will get damp problems if you close off ventilation at the bottom.
Perhaps if you put soundproofing in the void, run a section of flexible flue liner or something similar through the middle to maintain some air flow.
By the way it's nice to hear about someone taking trouble to consider the neighbours when installing equipment.

Regards,
footprints

 

[Thregwort]

Thanks, Footprints, that's a good plan - I would keep all the benefits of a void filled with fibreglass, and yet have the centre section kept open with flue liner tubing.
I'd be happy with that - I can't imagine it would negate the sound deadening qualities very much, and it would solve another thought I had about how to shove the floppy fibreglass up the chimney without it becoming all bunched up.
I could prefabricate a big 'sausage' by wrapping the fibreglass spirally around the flue liner and just hold it lightly together with e.g. masking tape, and then shove it up the chimney, knowing the masking tape would naturally disintegrate after a few weeks, thereby allowing the whole body of fibreglass to expand to fill the space.