Technical Booklet G Resistance to the Passage of Sound·Page 33·2.36
Junction requirements for wall type 1 with external cavity walls
When an external wall is a cavity wall with masonry inner leaf, the outer leaf may be any construction type, and the cavity should be closed with a cavity stop unless fully filled with mineral wool or expanded polystyrene beads.
Where the external wall is a cavity wall – (a) the outer leaf of the wall may be of any construction; and (b) the cavity should be closed with a cavity stop (see Diagram 2.5) unless the cavity is fully filled with mineral wool or expanded polystyrene beads (seek manufacturer's advice for other suitable materials).
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/regional/Technical Booklet G - Resistance to the Passage of Sound.pdf