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).
Walls
A lining of 12.5mm plaster or plasterboard will be adequate for soundproofing if the existing wall is one of the following:
- 225mm (9") coursed brick or stone
- 2 leaves of 100mm thick brick or dense blockwork with 50mm cavity
- 2 leaves of 100mm thick aerated concrete blockwork with 75mm cavity
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 which meets the following conditions.
- The existing wall should be rendered with sand and cement to seal any gaps.
- The timber stud wall should be at least 50mm x 50mm, 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 must be at least 50mm thick between studs.
- There must be a lining of 2 layers of 12.5mm plasterboard on the room side only, with the 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
- 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.
Floors
The new floor needs to meet the following sound insulation measures:
- floor boarding of at least 22mm thick chipboard, or at least 28mm thick softwood floorboards
- 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