Chimney breast advice

[Bungalowben]

I’m after some advice on insulating a bedroom with a chimney breast on an external wall which has been bricked up.

 

My thinking is to build a stud wall which follows the back wall, alcoves and the chimney breast, 50mm of celotex and then plasterboard. Keep it warm and allow me to drop cables for switches easily.

 

Couple of questions which I’d appreciate advice on please:

1) Will building the stud wall and stuffing with celotex cause any issues if I leave a gap behind? Say 100mm gap behind 50mm celotex?

 

2) The chimney needs to be vented. If I put a 9” x 6” vent in, will there be sufficient airflow behind the stud wall?

 

Or, does this all scream problem waiting to happen?