Ventilation behind stud wall

[baljci]

Hi there,

Looking for some advice. I am building a bathroom in a stone wall room that is part subterranean. I am building a stud walls 300mm away from the stone wall and adding foil backed insulation board between the studs and covering with foil tape so no water vapour can reach the cavity and condense on the wall. 

My question is, would it be prudent to add ventilation holes (maybe even a fan) inside the cavity to suck air outside to keep it fresh so mould cant ever grow. 

Is this a good idea? Or should I just completely seal up the cavity between the studs and the stone wall? 

Many thanks!

Essentially, would it be a good idea to add the vents as per the image:

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[Mr Punter]

It is good to vent the cavity.  It will allow the stone to dry and will help prevent mould.