Decommisioning a chimney

Hi All,
I'm looking for advice re closing off and decommissioning a chimney. I've already searched the forum on this but nothing addresses the airtightness setup (that I could find).
My situation is that I've moved a stove and am now left with a 150mm hole in the wall. I realise that it's best to put a vent in here but I'm also wondering if there are any other options. I'm aiming to make the house as airtight as possible (HRV installed) and feel that a continual open vent at the back of a wardrobe might not be a good idea. On the other hand I don't want this area to be prone to dampness and a source of mould.
Could one fill the flue with some kind of insulation or what do you think in general?
Or how small can the vent be?

As usual any advice/comments are much appreciated.


Regards & Thanks
Franm

Fran,
There are a number of issues here. Perhaps the best way to address all the issues would be to seek professional advice.

If you propose to block up a chimney flue you must ventilate the flue in the room which it once served, usually through provision of a hit and miss vent. Failure to do so will allow condensation to form within the flue with the possiblity of a dirty condensate running down your flue, over time this may leach into the fabric of your habitable rooms. There really is no way around this requirement for ventilation, unless the actual stack were to be partially demolished and reduced to below the roof level (preventing the ingress of moisture).

Your best bet would be a clay cap for the top, the cap will have 3/4" holes around the side to allow ventilation, it also stops rain getting down.
If you have a head for heights you should be able to pick one up at any builders providers at a reasonable price.

Thanks folks for all the replies and advice.
Regards
Franm