NCC 2022 Volume One - Building Code of Australia Class 2 to 9 buildings

F8

Part F8 Condensation management

Part F8 Condensation management

Introduction to this Part

This Part is intended to reduce the risk of illness or loss of amenity due to the occurrence of condensation inside a building. It does this by requiring features that enable moisture-laden air to be removed from inside the building and the building structure.

Notes

From 1 May 2023 to 30 September 2023 Part F6 of NCC 2019 Volume One Amendment 1 may apply instead of Part F8 of NCC 2022 Volume One. From 1 October 2023 Part F8 of NCC 2022 Volume One applies.

Objectives

The Objective of this Part is to safeguard occupants from illness or loss of amenity as a result of excessive internal moisture.

Applications

F8O1 only applies to a sole-occupancy unit of a Class 2 building or Class 4 part of a building.

F8O1 Objective

Part F8 aims to limit the amount of condensation that can accumulate within a building by managing vapour flow through the building fabric. It only applies to residential building classifications which are considered to be more susceptible to the accumulation of moisture due to the building’s intended function and use. The majority of moisture within a building is produced from washing (bathrooms and laundries) and cooking.

Condensation is a physical phenomenon that occurs naturally wherever and whenever the physical conditions are conducive. Mould often grows where condensation forms and cannot dry out within the built environment. The principle physical drivers are air pressure, temperature and humidity. These same physical conditions can occur within all built structures, in all climate types within the building (its internal environment), within its intermediate zones (subfloor and roof space zones) and within the building structure (floor, walls, ceilings and roofing materials). These natural processes cannot be stopped from occurring where these conditions exist. However, buildings can be designed, constructed and used in a way that manages vapour pressure, condensation risk and subsequent mould growth.

Human occupation of a residential building creates approximately 10 litres of water vapour per person per day. In an average family home with two adults and one child this equates to 30 litres of water vapour within the built fabric per day. This comes from people breathing, cooking, boiling water, washing and bathing. 

Interstitial condensation can affect the structural integrity of a building, but its presence often goes undetected until such time as the cost to remedy becomes significant. The most effective means to reduce the problem of interstitial condensation is to provide a pathway for water vapour that avoids the accumulation of condensation. To remove the risks associated with condensation and to maintain indoor air quality, moisture laden air needs to be removed from the building and expelled out to the external environment.

The design, construction and use of a building can create conditions that lead to a building experiencing prolonged periods of damp, which leads to poor indoor environmental qualities, (potentially affecting occupant health) mould, and building degradation. Occupant behaviour, for example opening windows and doors to ventilate the building, can significantly impact a building’s moisture generation rate but cannot be regulated by the NCC. The NCC’s Part F8 Condensation Management requirements were included in the NCC to assist in addressing the risks associated with condensation in residential buildings.

Functional Statements

A building is to be constructed to avoid the likelihood of excessive internal moisture accumulating within the building structure.

Applications

F8F1 only applies to a sole-occupancy unit of a Class 2 building or Class 4 part of a building.

Performance Requirements

NCC Blurbs

In a sole-occupancy unit of a Class 2 building or a Class 4 part of a building, risks associated with water vapour and condensation must be managed to minimise their impact on the health of occupants.

Notes

  1. Refer to the guidance in the “Condensation in Buildings Tasmanian Designers’ Guide” – current version available at www.cbos.tas.gov.au. This Guide must be read in conjunction with the NCC.
  2. The strategies listed in the Guide exceed the NCC requirements for condensation management, however are strongly recommended to assist in minimising condensation in cool climates like Tasmania.
NCC Title
Condensation and water vapour management
NCC State
TAS
NCC Variation Type
Replacement
NCC SPTC Current
Condensation and water vapour management

Risks associated with water vapour and condensation must be managed to minimise their impact on the health of occupants.

Applications

F8P1 only applies to a sole-occupancy unit of a Class 2 building or Class 4 part of a building.

F8P1 Condensation and water vapour management

Class 2 buildings and Class 4 parts of a building must be designed to mitigate the risks associated with the amount of moisture that accumulates internally.

Verification Methods

(1) Compliance with Performance Requirement F8P1 is verified for a roof or external wall assembly when it is determined that a mould index of greater than 3, as defined by Section 6 of AIRAH DA07, does not occur on—

  1. the interior surface of the water control layer; or
  2. the surfaces of building fabric components interior to the water control layer.

(2) The calculation method for (1) must use—

  1. input assumptions in accordance with AIRAH DA07; and
  2. the intermediate method for calculating indoor design humidity in Section 4.3.2 of AIRAH DA07.

F8V1 Condensation management

F8V1 describes a means of verifying that the effects of internal moisture will not accumulate to a degree which would detrimentally impact the health, safety or amenity of a building’s occupants as described in F8O1.

F8V1 addresses the management of condensation in roof and external wall assemblies, aiming to minimise the risk of mould growth. Compliance with F8V1 is verified through ensuring a mould index greater than 3 does not occur on specific surfaces. This mould index is defined by Section 6 of AIRAH DA07. To achieve this, the provision specifies two key points for calculations: first, input assumptions must conform to AIRAH DA07 guidelines; second, the intermediate method described in Section 4.3.2 of AIRAH DA07 should be used for calculating indoor design humidity. Verification with F8V1 is a is a means of demonstrating compliance with Performance Requirement F8P1.

Deemed-to-Satisfy Provisions

(1) Compliance with Performance Requirement F8P1 is satisfied by complying with Deemed-to-Satisfy Provisions F8D2 to F8D5.

(2) Where a Performance Solution is proposed, the relevant Performance Requirements must be determined in accordance with A2G2(3) and A2G4(3) as applicable.

Explanatory information

The intent of these requirements is to assist in the mitigation of condensation within a building. The implementation of a condensation management strategy may not prevent condensation from occurring.

To specify the minimum construction requirements to assist in the mitigation of condensation in buildings.

F8D1 Deemed-to-Satisfy Provisions

Where a solution is proposed to comply with the Deemed-to-Satisfy Provisions, F8D1 clarifies that compliance with F8D2 to F8D5 will achieve compliance with F8P1.

Where a Performance Solution is proposed, the relevant Performance Requirements must be determined in accordance with A2G2(3) and A2G4(3) as applicable. (See commentary on Part A2).

The Deemed-to-Satisfy Provisions of this Part only apply to a sole-occupancy unit of a Class 2 building and a Class 4 part of a building.

To clarify that Part F8 only applies to Class 2 buildings and a Class 4 part of a building.

(1) Where a pliable building membrane is installed in an external wall, it must—

  1. comply with AS 4200.1; and
  2. be installed in accordance with AS 4200.2; and
  3. be located on the exterior side of the primary insulation layer of wall assemblies that form the external envelope of a building.

(2) Where a pliable building membrane, sarking-type material or insulation layer is installed on the exterior side of the primary insulation layer of an external wall it must have a vapour permeance of not less than—

  1. in climate zones 4 and 5, 0.143 µg/N.s; and
  2. in climate zones 6, 7 and 8, 1.14 µg/N.s.

(3) Except for single skin masonry and 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.

Explanatory information

F8D3(2) requires some wall materials on the external side of the primary insulation layer to have a minimum level of vapour permeance. Vapour permeance is measured in µg/N.s (micrograms per newton-second).

Class 3 and 4 vapour control membranes (as defined by clause 5.3.4 of AS 4200.1) meet the vapour permeance requirements of F8D3(2)(a), while Class 4 vapour control membranes meet the vapour permeance requirements of F8D3(2)(b).

Open-cell insulation, such as mineral wool or fibreglass, typically has a high vapour permeance, while closed-cell insulation such as polystyrene typically has a low vapour permeance. Many foil-faced insulation products have a low vapour permeance.

To set out the requirements relating to pliable building membranes, sarking type material and secondary insulation.

F8D3 External wall construction

All pliable building membranes installed in an external wall must comply with AS/NZS 4200.1 and be installed in accordance with AS 4200.2. In climate zones 4, 5, 6, 7 and 8 pliable building membranes must be vapour permeable to assist in the transfer of moisture from the internal to the external environment. This recognises the higher level of risk associated with the build-up of internal moisture in colder climates particularly due to the inadequate ventilation of internal spaces. Any pliable building membrane ought be installed with suitable provision to allow for the drainage of accumulated condensate to a drainage point external to the wall.

Where a pliable building membrane is not installed, a drained cavity must be provided (except for single skinned concrete or single skinned masonry walls). Alternatively, a design can be assessed using Verification Method F1V1.

(1) An exhaust system installed in a kitchen, bathroom, sanitary compartment or laundry must have a minimum flow rate of—

  1. 25 L/s for a bathroom or sanitary compartment; and
  2. 40 L/s for a kitchen or laundry.

(2) Exhaust from a kitchen, kitchen range hood, bathroom, sanitary compartment or laundry must discharge directly or via a shaft or duct to outdoor air.

(3) Where space for a clothes drying appliance is provided in accordance with F4D2(1)(b), space must also be provided for ducting from the clothes drying appliance to outdoor air.

(4) (3) does not apply if a condensing-type clothes drying appliance is installed.

(5) An exhaust system that is not run continuously and is serving a bathroom or sanitary compartment that is not ventilated in accordance with F6D7 must—

  1. be interlocked with the room’s light switch; and
  2. include a run-on timer so that the exhaust system continues to operate for 10 minutes after the light switch is turned off.

(6) Except for rooms that are ventilated in accordance with F6D7, a room with space for ducting a clothes drying appliance to outdoor air in accordance with (3) must be provided with make-up air in accordance with AS 1668.2

Explanatory information

A range hood installed in a kitchen must comply with F8D4(2).

Part F6 includes other ventilation requirements which must be met, including a requirement for make-up air to be provided to mechanically ventilated rooms in accordance with AS 1668.2.

To reduce risk of condensation associated with mechanical ventilation.

F8D4 Exhaust systems

F8D4 requires that kitchen, bathroom, toilet and laundry exhaust fans achieve specified minimum airflow rates. Requiring higher minimum airflow rates for laundries recognises the potential for much greater air moisture in the room and therefore the increased risk of condensation. This provision also requires that exhaust be discharged directly to outdoor air. If a space for a clothes drying appliance is provided, ducting to the outdoor air is also required unless a condensing-type appliance is installed. Non-continuous exhaust systems in bathrooms or sanitary compartments are to be interlocked with the light switch and feature a 10-minute run-on timer if it is not naturally ventilated. Lastly, make-up air must be provided as per AS 1668.2 where a room has space for a ducted clothes drying appliance and is not ventilated in accordance with F6D7.

(1) In climate zones 6, 7 and 8, a roof must have a roof space that—

  1. is located—
    1. immediately above the primary insulation layer; or
    2. immediately above sarking with a vapour permeance of not less than 1.14 μg/N.s, which is immediately above the primary insulation layer; or
    3. immediately above ceiling insulation which meets the requirements of J3D7(3) and J3D7(4); and
  2. has a height of not less than 20 mm; and
  3. is either—
    1. ventilated to outdoor air through evenly distributed openings in accordance with Table F8D5; or
    2. located immediately underneath roof tiles of an unsarked tiled roof.

(2) The requirements of (1) do not apply to a—

  1. concrete roof; or
  2. roof that is made of structural insulated panels; or
  3. roof that is subject to Bushfire Attack Level FZ requirements in accordance with AS 3959.
Table F8D5 Roof space ventilation requirements
Roof pitch Ventilation openings
<10° 25,000 mm2/m provided at each of two opposing ends
≥10° and <15° 25,000 mm2/m provided at the eaves and 5,000 mm2/m at high level
≥15° and <75° 7,000 mm2/m provided at the eaves and 5,000 mm2/m at high level, plus an additional 18,000 mm2/m at the eaves if the roof has a cathedral ceiling
Table Notes
  1. Ventilation openings are specified as a minimum free open area per metre length of the longest horizontal dimension of the roof.
  2. For the purposes of this table, high level openings are openings provided at the ridge or not more than 900 mm below the ridge or highest point of the roof space, measured vertically.

To set out the ventilation requirements relating to roof spaces.

F8D5 Ventilation of roof spaces

F8D5 specifically targets climate zones 6, 7, and 8, and prescribes requirements for roof space ventilation. The roof space should be located immediately above the primary insulation layer, or sarking with specific vapour permeance or above ceiling insulation meeting certain criteria. The space must have a minimum height of 20 mm and should either be ventilated to the outdoor air as per Table F8D5, or be under un-sarked roof tiles. Exceptions include concrete roofs, structural insulated panel roofs, and roofs subject to Bushfire Attack Level FZ as per AS 3959.

Ventilation openings are to be evenly distributed to avoid creating pockets of stagnant air. 

Openings created by roof cladding profile can help meet ventilation opening requirements. Examples include openings underneath a metal roof cladding profile can or inherent openings in a profiled tile roof, provided these openings are directly connected to the roof void to be ventilated.

For roof pitches greater than 10 degrees, total low level ventilation openings must be greater than total high level ventilation openings to minimise drawing air from occupied spaces and ensure intake of outdoor air.