Mould Growth Provisions for NCC
Mould Growth Provisions for NCC 2022
First published in LinkedIn on January 8, 2023
Article by Clarence Macalister and Dr Mark Dewsbury. Thanks to Jessica Allen for assistance.
What is a mould index, and why is it in the National Construction Code in 2022?
As Australia moves towards more energy efficient buildings, mould and condensation issues have increased. Without specific changes in construction practice, this trend could continue, creating widespread structural, health and liability issues.
Since mid 2019, the NCC has included a performance requirement to minimise the effects of condensation related moisture in buildings (NCC 2019 P2.4.7; updated to NCC 2022 H4P7). In NCC 2022 the preformance requirement states:
H4P7 Condensation and water vapour management: Risks associated with water vapour and condensation must be managed to minimise their impact on the health of occupants.
In NCC 2022, the “deemed to satisfy” provisions around condensation management have increased (NCC 2022, part 10.8). These specifically refer to vapour permeable wall membranes, roof ventilation and mechanical ventilation of bathrooms and kitchens.
For systems which do not use the DtS provisions, there is a new verification method for condensation management which uses hygrothermal modelling software to predict mould growth (NCC 2022 H4P5). The software modelling determines how a particular element (like a roof or wall assembly) will perform based on the construction lay ups and temperature and humidity data for the local climate- with the specific benchmark of mould coverage (the “Mould Index” or “Mould Growth Index”).
The parameters used in the modelling are stipulated in AIRAHs DA07 “Criteria for Moisture Control Design Analysis in Buildings” document. This is derived from the US equivalent document with the same working title (ASHRAE standard 160).
Software modelling of building performance is nothing new- it has been part of the NCC since 2004 through the NaTHERS energy efficiency provisions. Hygrothermal modelling software has been used internationally for predicting building performance for over 30 years. The reason mould index is used (as opposed to the occurrence of condensation) is that mould can occur in conditions when condensation will not. Condensation occurs when relative humidity reaches 100%, and mainly on non-porous surfaces (e.g. glass, foils and aluminium). Mould growth on the other hand can occur in humidities as low as 70%, and on both porous AND non-porous surfaces. Mould is also the major health risk “trigger” for occupants of buildings.
As Australia moves towards more energy efficient buildings, mould and condensation issues have increased. Without specific changes in construction practice, this trend could continue, creating widespread structural, health and liability issues.
Since mid 2019, the NCC has included a performance requirement to minimise the effects of condensation related moisture in buildings (NCC 2019 P2.4.7; updated to NCC 2022 H4P7). In NCC 2022 the preformance requirement states:
H4P7 Condensation and water vapour management: Risks associated with water vapour and condensation must be managed to minimise their impact on the health of occupants.
In NCC 2022, the “deemed to satisfy” provisions around condensation management have increased (NCC 2022, part 10.8). These specifically refer to vapour permeable wall membranes, roof ventilation and mechanical ventilation of bathrooms and kitchens.
For systems which do not use the DtS provisions, there is a new verification method for condensation management which uses hygrothermal modelling software to predict mould growth (NCC 2022 H4P5). The software modelling determines how a particular element (like a roof or wall assembly) will perform based on the construction lay ups and temperature and humidity data for the local climate- with the specific benchmark of mould coverage (the “Mould Index” or “Mould Growth Index”).
The parameters used in the modelling are stipulated in AIRAHs DA07 “Criteria for Moisture Control Design Analysis in Buildings” document. This is derived from the US equivalent document with the same working title (ASHRAE standard 160).
Software modelling of building performance is nothing new- it has been part of the NCC since 2004 through the NaTHERS energy efficiency provisions. Hygrothermal modelling software has been used internationally for predicting building performance for over 30 years. The reason mould index is used (as opposed to the occurrence of condensation) is that mould can occur in conditions when condensation will not. Condensation occurs when relative humidity reaches 100%, and mainly on non-porous surfaces (e.g. glass, foils and aluminium). Mould growth on the other hand can occur in humidities as low as 70%, and on both porous AND non-porous surfaces. Mould is also the major health risk “trigger” for occupants of buildings.
Mould spores are everywhere. They will germinate and grow (even if only at a microscopic level) if temperature and humidity conditions are suitable. These are influenced by factors such as climate, usage of the building, ventilation and the construction of the walls, rooves etc. Temperature and humidity are the core metrics for hygrothermal modelling software, and these are accurate predictors for mould growth on a given substrate type.
Mould growth can be dynamic and will often “cycle”, peaking in homes in winter and decreasing or even reversing in summer (assuming a cool-temperate Australian climate like Canberra, Melbourne, Adelaide Hills, Albany or Hobart). However, these cycles may build up the level of growth over several years, with progressive annual increases. It may take ten years to reach a “plateau” level of growth.
Mould growth can be dynamic and will often “cycle”, peaking in homes in winter and decreasing or even reversing in summer (assuming a cool-temperate Australian climate like Canberra, Melbourne, Adelaide Hills, Albany or Hobart). However, these cycles may build up the level of growth over several years, with progressive annual increases. It may take ten years to reach a “plateau” level of growth.
What happens in the first few years of occupancy may not accurately determine the long term infestation of mould. This is why a 10 year assessment period is used.
The assessment is based on an internationally recognised “mould index”, which ranks the coverage of mould. Using temperature, humidity and material data, the rate of mould growth and cover on a given surface and be predicted. The mould index system uses a 0 – 6 rating- with zero being no cover, and 6 being complete dense cover of mould. If built or designed incorrectly, it is possible to reach a mould index of 4 (i.e. 50% cover) within a few years of occupancy.
The assessment is based on an internationally recognised “mould index”, which ranks the coverage of mould. Using temperature, humidity and material data, the rate of mould growth and cover on a given surface and be predicted. The mould index system uses a 0 – 6 rating- with zero being no cover, and 6 being complete dense cover of mould. If built or designed incorrectly, it is possible to reach a mould index of 4 (i.e. 50% cover) within a few years of occupancy.
Mould growth approximating MI 3 -4 in the roof space of a two year old house in Launceston, Tasmania, This would be a “fail” under NCC 2022 verification method (H4P5). Photo C Macalister.
To meet the NCC 2022 verification method, the mould index for a given assembly and climate must be less than 3, when simulated over a 10 year period. 3 indicates the visible presence of mould on a surface up to 10% of its area.
So, if a construction system other than the NCC deemed to satisfy provisions are used, the modelling of the assembly must predict less than 10% visible mould on any given surface after a ten year period.
So, the big question – will meeting the deemed to satisfy provisions prevent this level of mould growth in a typical Australian home? Several industry experts suggest “probably not”. Modelling like that in the graph above (a “code compliant” building until mid 2023) validates this expectation.
To make that crystal clear- leaders in the field in Australia suggest meeting NCC 2022 DtS provisions for condensation management may not guarantee compliance with the performance requirement.
What does this mean for the average building designer or builder?
1) The NCC is the MINIMUM level of performance, and is not necessarily a guarantee of good performance- and mould and condensation is no exception.
2) Based on international experience with increasing energy efficiency and computer modelling, the issue of mould and condensation is only likely to worsen if there is not a major change in the design and construction of buildings.
So, the big question – will meeting the deemed to satisfy provisions prevent this level of mould growth in a typical Australian home? Several industry experts suggest “probably not”. Modelling like that in the graph above (a “code compliant” building until mid 2023) validates this expectation.
To make that crystal clear- leaders in the field in Australia suggest meeting NCC 2022 DtS provisions for condensation management may not guarantee compliance with the performance requirement.
What does this mean for the average building designer or builder?
1) The NCC is the MINIMUM level of performance, and is not necessarily a guarantee of good performance- and mould and condensation is no exception.
2) Based on international experience with increasing energy efficiency and computer modelling, the issue of mould and condensation is only likely to worsen if there is not a major change in the design and construction of buildings.
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