Insulate existing cavity walls or leave for ventilation

[pstunt]

I am looking to get the forum's views on whether or not to insulate an existing cavity wall or leave it as an empty void.

 

We are at the foundation stage of converting an 1940's ( ish ) brick stable which was built probably by the National Coal Board, as it's related to an old coal mine.  We are not allowed to knock down the main original building for historical interest reasons, but we are allowed to knock down and rebuild some extensions and put 2 new extensions on the rear of the building. The agreed plans on the inside of the existing walls are to install 70mm Xtratherm SR/TB (foil faced) insulated dry lining finish with a lightweight plaster skim, to achieve a U-Value= 0.22 W/m2K. We would like to achieve better than this but the main building is very thin at 4.6m and therefore we dont want to use up internal space on insulation.

 

We are considering the idea of filling the current cavity wall with insulation ( Possibly blown in ? ) but my concern is that it may be left as a void for ventilation  moisture reasons. My builder is going to discuss with our architect but hopefully someone here may have some informed opinion. 

 

The walls have airbricks on the outside and inside but certainly the inside ones will be sealed up. Not sure what the plan is for the outside ones. I will assume they will be blocked off.

 

 

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