In a terraced or semi-detached house, the separating wall needs to be able to resist the passage of sound, irrespective of whether or not the neighbouring house has a habitable room in the roof (sound can travel through the wall into your neighbour's roof void and through the ceiling, and vice-versa).
If the existing wall is
- 225mm (9") coursed brick or stone,
- 2 leaves of 100mm thick brick or dense blockwork with 50mm cavity, or
- 2 leaves of 100mm thick aerated concrete blockwork with 75mm cavity,
then this will be adequate if lined with 12.5mm plaster or plasterboard. All gaps and holes in the wall need to be filled prior to plastering or plasterboarding.
If the existing separating wall is not adequate, then the best solution is a separate timber stud wall.
- The existing wall should be rendered with sand & cement to seal any gaps,
- 50mm x 50mm timber stud wall, fixed only to the floor and roof structure (not to the wall) at least 13mm from the face of the wall,
- The perimeter of the wall should be sealed with tape or mastic,
- Mineral fibre at least 50mm thick between studs,
- Lining of 2 layers of 12.5mm plasterboard on the room side only, with joints staggered.
If there is more than one room in the loft, then any of the following walls will need to be sound insulated:
- any wall between a bedroom and another room or bedroom; and
- any wall between a WC and another room (but not if the WC is an en-suite to a bedroom).
This can be achieved by installing mineral fibre (at least 25mm thick) between the studs.
The new floor needs at least the following sound insulation measures:
- Floor boarding of at least 22mm thick chipboard, or at least 28mm thick softwood floorboards, and
- 100mm thick mineral fibre between the joists.
If there is a new ceiling under the floor, then it needs to be at least 12.5mm thick plasterboard.
As with thermal insulation, the mineral fibre should not be compressed to fit into a gap, as this reduces its effectiveness