Condensation management in a new SA house

Where Part 10.8 comes from, and South Australia’s position

Each of the three clauses in Part 10.8 carries a reference to a corresponding NCC 2019 clause: 10.8.1 to 3.8.7.2, 10.8.2 to 3.8.7.3 and 10.8.3 to 3.8.7.4. Whether a particular requirement below, such as a flow rate or a run-on timer, was already in NCC 2019 is not set out in this note.

The index of South Australia’s variations to the Housing Provisions, in Schedule 8, lists Parts 4.2, 9.2, 10.2 and 13.1 and the referenced documents. Part 10.8 is not among them. South Australia also varies NCC Volume Two separately; that schedule is not covered here.

Two of the three clauses turn on climate zone. The ABCB publishes the climate zone map the NCC uses. This note does not state a zone for any South Australian location; for a particular site it is read from the map and confirmed with the certifier.

External walls: 10.8.1

Where a pliable building membrane — the wrap fixed over a wall frame — is installed in an external wall, clause 10.8.1(1) requires it to "comply with AS 4200.1", "be installed in accordance with AS 4200.2", and "be located on the exterior side of the primary insulation layer of wall assemblies that form the external envelope of a building."

Clause 10.8.1(2) then sets how readily material outside the insulation must let vapour through. A membrane, sarking-type material or insulation layer on the exterior side of the primary insulation layer must have a vapour permeance of not less than "in climate zones 4 and 5, 0.143 µg/N.s" and "in climate zones 6, 7 and 8, 1.14 µg/N.s." The explanatory information says Class 3 and Class 4 vapour control membranes, as defined in clause 5.3.4 of AS 4200.1, meet the first figure, and Class 4 membranes meet the second. It also notes that "many foil-faced insulation products have a low vapour permeance."

Clause 10.8.1(3) covers walls without a membrane: "Except for single skin masonry or single skin concrete, where a pliable building membrane is not installed in an external wall, the primary water control layer must be separated from water sensitive materials by a drained cavity."

Exhaust systems: 10.8.2

Clause 10.8.2(1) sets minimum flow rates for an exhaust system installed in these rooms: "25 L/s for a bathroom or sanitary compartment" and "40 L/s for a kitchen or laundry." Subclause (2) says where the air goes: exhaust from "a kitchen, kitchen range hood, bathroom, sanitary compartment or laundry must discharge directly or via a shaft or duct to outdoor air." Subclause (3) applies the same to a venting clothes dryer. The explanatory information notes that this does not apply to other types of dryer, "such as condensing clothes dryers".

Subclause (4) applies to a bathroom or sanitary compartment that is not ventilated under 10.6.2(a) — that is, by openings, windows, doors or other devices that can be opened, with a ventilating area of not less than 5% of the room’s floor area, opening to one of the spaces that clause lists. There, an exhaust system that is not run continuously must "be interlocked with the room’s light switch" and "include a run-on timer so that the exhaust system continues to operate for 10 minutes after the light switch is turned off."

Subclause (5) requires make-up air for a room with an exhaust system under (1) that is not ventilated under 10.6.2(a): "via openings to an adjacent room with a free area of 14,000 mm²" or in accordance with AS 1668.2. The explanatory information gives a worked example: an opening with that free area can be achieved by a 20 mm undercut to a 700 mm wide door. Subclause (6) requires make-up air in accordance with AS 1668.2 for a room with a venting clothes dryer under (3), again except for rooms ventilated under 10.6.2(a).

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Roof spaces: 10.8.3

Clause 10.8.3 applies "in climate zones 6, 7 and 8". In those zones a roof must have a roof space located immediately above the primary insulation layer (or immediately above sarking of a stated vapour permeance, or immediately above ceiling insulation meeting 13.2.3(3) and (4)), with "a height of not less than 20 mm", and either ventilated to outdoor air "through evenly distributed openings in accordance with Table 10.8.3" or located immediately under the tiles of an unsarked tiled roof.

Table 10.8.3 sets the openings by roof pitch, per metre of the roof’s longest horizontal dimension. For a pitch of 15° or more and less than 75°, it is "7,000 mm²/m provided at the eaves and 5,000 mm²/m at high level", with a further 18,000 mm²/m at the eaves for a cathedral ceiling. The clause does not apply to a concrete roof, a roof made of structural insulated panels, or a roof subject to Bushfire Attack Level FZ requirements under AS 3959.

Whether 10.8.3 applies to a given house in South Australia depends on the climate zone of the site, which this note does not state.

Where these decisions are made

Most of Part 10.8 is out of sight in a finished house: the wall wrap is behind the cladding and the ducts are above the ceiling. Each is a product choice or a routing decision that belongs in the drawings and specification, where the membrane’s class, the fan’s flow rate and the duct’s end point can be stated and checked against the clause before the frame is closed in.

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