By Paddy Healy – Director, Credwell Energy
Introduced in 2019 as “Part F6 of the NCC 2019 Amendment 1, the BCA 2022 has reorganised its content, and the relevant information is now located in “Part F8”. This updated version of the Building Code of Australia Code will be applicable from 1st October 2023. One notable addition to this version is the inclusion of the “Mould Growth Index (MGI)” in the verification method, which serves as a benchmark and offers guidance for compliance when aiming for this pathway.
The BCA 2022 also brings more clarity to the vapour control classifications, addressing a growing concern related to water vapour trapped within the building envelope. Sealing in buildings has improved significantly and with the use of vapour-impermeable building materials, uncontrolled air infiltration and insufficient makeup air for exhaust systems this has led to visible issues like mould growth, as well as unseen risks, such as damp timber studs.
In cases where there is a deviation from the Deemed-to-Satisfy (DtS) requirements, the verification method performance solution F8V1 can be employed. This solution offers an alternative approach to ensure compliance with the code’s provisions. By incorporating these changes and improvements, the BCA 2022 aims to enhance building standards, address emerging challenges, and promote safer and more sustainable construction practices.
Any perceived risk of condensation that is expected within the building envelope can be investigated and reduced using this performance solution.
Deemed-to-Satisfy Provisions (DtS)
Building Membrane Location
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- Must be located on the exterior side of the primary insulation layer on the external walls. A primary insulation layer is the most interior insulation location (eg: batts in external stud wall).
Vapour Permeance
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- Any building membrane/sarking-type material or insulation layer installed on the exterior side of the primary insulation layer of an external wall must have a vapor permeance of –
- 143 ug/N.s in climate zones 4 & 5 – Class 3 – Vapour Control Membranes (VCM)
- 14 ug/N.s in climate zones 6, 7 & 8 – Class 4 (VCM)
- Mineral wool/fiberglass are high vapour permeance. Rigid board insulation and foil-faced insulation products have low vapour permeance.
- Any external wall assemblies with rigid board insulation and/OR foil should be investigated with a Performance Solution to determine if they will result in undue internal moisture accumulation.
Minimum Mechanical Exhaust Rates
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- Min 25 L/s for a bathroom.
- Min 40 L/s for a kitchen or laundry.
Discharge
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- These exhausts must be discharged directly to the outside and not allowed to recirculate internally.
Roof Ventilation
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- Sufficient ventilation of these roof spaces with a defined opening area and location of these openings if the mechanical exhaust is discharged into them.
Performance Solution – F8V1 Verification Method
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- This verification modelling is achieved using DA07 approved software in a holistic modelling approach. This holistic modelling considers weather files that include driving rain, relative humidity, and solar radiation data.
- The software uses this climate data along with, orientation, wall/roof assemblies, surface transfer co-coefficients, initial moisture content, liquid transfer coefficients, water vapour diffusion factors & material thermal conductivity (moisture & temperature dependent) to determine the water vapour flux through the assemblies modelled thus identifying any potential water build-up.
- For BCA 2022, compliance is achieved once it is shown that the Mould Growth Index (MGI) is 3 or less for any wall/roof assemblies investigated.
- This MGI must not occur on the inner surface of the water control layer or any fabric interior to this.
- The time span is modelled for 5-10 years to show the cumulative effect of any moisture build-up.
- The sensitivity of materials in the assemblies are also used as an input to determine the likelihood of this mould growth. Steel for example would not be susceptible to mould however it can make humid moisture-laden air reach dew point and thus condensation within walls in some circumstances which can affect the materials around it.
Please contact Credwell at info@credwell.com.au or call 02 9281 8555 for any further advice on condensation analysis/water vapour control within buildings.