Part F8 Condensation management

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.

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.

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.

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).

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.

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.

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.