Wall insulation for sandstone flat

[whalley1]

For the past couple of winters I've noticed that the wall under my lounge bay-window has been really cold. I then discovered that the "wall" is actually just 3mm ply nailed onto a basic frame in front of the rough sandstone external wall. As a tenement flat (third floor), the outstide red sandstone is smooth but the internal face is naturally rough - so the space between this ply and the stone varies between about 3cm and 8 cm.

So, the question is, what kind of insulation can I put in this cavity to provide a little more protection than 3mm plywood gives? It had been suggested to use a "space blanket" type of covered glass fibre insulation, but then someone else said that glass fibre causes damp in sandstone. Is that true even of the stuff covered by foil/plastic? And if so, what could be used instead?

Any suggestions would be gratefully received!

 

[AMc]

My (inexpert) opinion is that you won't get much damp on a 3rd floor bay unless something is leaking in which case you have more to worry about. Rising is out of the question and penetrating would probably be apparent.
As I understand it mineral wool doesn't cause damp but won't prevent it either as it is permeable to vapour if there's moisture then it will move through it.
If you want to be really sure you can stuff the cavity with sheepswool insulation (or an old jumper?) which is breathable and will absorb and release moisture.

 

[whalley1]

Thanks for that. I'm reassured that although I know NOTHING about building materials, my logic circuits are still working! I like the idea of the woolly jumpers but think I'd need an awful lot of them for the whole bay so it will be something on a roll I would guess.
Thanks again!