Our Brisbane townhouse block shares a party wall between each unit, and the certifier says the wall system we picked does not meet the sound rating. What rating does a wall between two Class 1 dwellings need, and does it matter that a bathroom sits against a living room on the other side?

The certifier is very likely right, and the bathroom is probably why. A wall between two Class 1 dwellings has two separate tests, and your layout triggers both. [1]

• (a) Rating: the wall must have an Rw + Ctr (airborne) of not less than 50.
• (b) Construction type: the wall must be of discontinuous construction if it separates a bathroom, sanitary compartment, laundry or kitchen in one dwelling from a habitable room (other than a kitchen) in the adjoining dwelling.

A living room is a habitable room, so a bathroom against it switches on (b). [1] Meeting Rw + Ctr 50 is not enough on its own. A wall system that was chosen because its test report shows 50 can pass (a) and still fail (b).

The two provisions people conflate. The rating and the discontinuous construction rule are separate requirements joined by "and". [1] Figure 10.7.1 shows it plainly: where bedroom meets bedroom, or bedroom meets garage, the wall needs Rw + Ctr 50 only; where a bath or laundry meets a living room or bedroom across the line, the label is Rw + Ctr 50 and discontinuous construction. [1] So the requirement can change along the same party wall, room by room. Check the plans against the rooms on both sides, not just the wall type.

How the 50 is proven. Two ways: [2]

• Laboratory test results, rated in accordance with AS/NZS ISO 717.1; or
• Deemed-to-satisfy wall forms in 10.7.5 to 10.7.8, built in line with the relevant parts of 10.7.3.

The explanatory information says the forms in 10.7.5 to 10.7.8 are typical examples, and proprietary systems can qualify by testing instead. [2] Ask the supplier which route their documentation follows, and whether it covers discontinuous construction or only the rating.

Built details the certifier will also look at. 10.7.3 applies to any sound insulated wall: [3]

• junctions with perimeter walls and roof cladding sealed;
• masonry laid with all joints filled solid (except complying articulation joints);
• two layers of plasterboard with joints staggered, and all joints taped and filled solid;
• steel framing not less than 0.6 mm thick and studs not less than 63 mm deep unless 10.7.5 to 10.7.8 say otherwise;
• in timber framing, noggings must not bridge between studs supporting different wall leaves, and perimeter members fixed and caulked with no voids.

That nogging rule is a common way a discontinuous wall is undone on site.

Height and extent. The wall must run to the underside of the roof above, or to a ceiling that itself provides the required sound insulation. [1] The explanatory information notes the insulation is needed for wall common to the two dwellings, but not the part in the subfloor. [1] The same rules apply to a wall between a Class 1 dwelling and a Class 10a building not associated with it. [1]

Why this is the governing rule. Under NCC 2022 Volume Two, complying with Part 10.7 of the Housing Provisions satisfies the sound insulation Performance Requirement H4P6. [4] No Queensland variation to Part 10.7 was found. Queensland's transport noise corridor rules deal with the external envelope, which is a different question from the party wall. [5]

Practical next step. Mark every room on both sides of each party wall. Wherever a bathroom, toilet, laundry or kitchen backs onto a living room, bedroom or other habitable room, ask the supplier for evidence of Rw + Ctr 50 and discontinuous construction for that section. Then check the wall goes to the roof underside and no noggings cross between the leaves.

Queensland edition note. Queensland is on NCC 2022 Amendment 2 until 30 April 2027, and NCC 2025 takes effect from 1 May 2027 with no election window. The Queensland Development Code applies alongside the NCC. [7] NCC 2025 keeps the same Rw + Ctr 50 and discontinuous construction requirements; the only change is that the test standard is cited as AS ISO 717.1. [6]