Technical Booklet G Resistance to the Passage of Sound·Page 80·3.74
Cavity wall with timber frame inner leaf
For cavity walls with timber frame inner leaves, the cavity must be closed with a cavity stop, and the inner leaf finish must be two layers of plasterboard (minimum 10 kg/m² each) with all joints sealed with tape or caulked with sealant.
Where the external wall is a cavity wall – (a) the outer leaf of the wall may be of any construction; (b) the cavity should be closed with a cavity stop; and (c) the wall finish of the inner leaf of the external wall should be two layers of plasterboard, each sheet of plasterboard to be of minimum mass per unit area 10 kg/m2, and all joints should be sealed with tape or caulked with sealant.
Source — /Users/richardhill/Documents/planning-arch-project/data/documents/regional/Technical Booklet G - Resistance to the Passage of Sound.pdf