Moisture accumulation and mould growth for 6-Star and 7-Star housing (Research Report 4: Final Report)
This report is the final report for the research activity that explored the simulation-based analysis of moisture and mould that may be occurring within the external wall systems of 6 Star and 7 Star houses in Victoria. A simulation approach is taken due to the urgency identified by design and construction
professionals regarding the presence of mould and moisture in new homes. To date, most of the research in this area has been forensic analyses of homes that have concerning amounts of moisture and mould. Forensic analysis can be used to calibrate simulation tools, but not to inform change. Once a new home is constructed, owners often have a financial commitment with a financial institution for one of more decades. Similarly, the financial institution owns the building, until the loan has been paid. Whether it be the building occupants, the financial institution or the community at large, there is an expectation that a new home should have a life in excess of 50 years. However, since 2014 new homes in all Australian states and territories have had concerning amounts of moisture and mould on interior surfaces and within the interstitial spaces (external wall, subfloor and roof space systems). International research since the 1930s has identified that when an interior air temperature and relative humidity are different to an exterior air temperature and relative humidity, water vapour diffuses through the external wall, floor and ceiling systems. If this water vapour is not managed, mould can grow, and moisture can form. Moisture and mould within building interiors and interstitial spaces has been known for decades to affect human health and lead to the decay of the building fabric.
In 2009, the World Health Organisation stated that there should be no visible presence of mould within building interiors or interstitial spaces. Nath noted in 2018 that Australia had twice the Asthma rate of other OECD nations and Rickaby noted in 2021 that £1B of the UK National Health System expenditure
could be attributed to treatment of occupants in mouldy housing. As we heat and cool our homes, we create these differences in external and internal air temperature
and relative humidity. But for reasons of thermal comfort and human health, this should occur. Buildings that have interior temperatures below 15°C or above 28°C have been found to impact human health. From a thermal comfort perspective, when people are reasonably sedentary, (i.e., sitting on a
couch), we feel cool when the air temperature is less than 20°C and we will turn a heater on. Subject to our location in Australia, we may feel hot when the interior temperature is above 25°C and we will turn on the air conditioner. These expectations are not wrong, and in many developed nations, the interior thermostats for heating and cooling homes is set to 21°C for heating and 24°C for cooling. Many Australian workplaces use these same thermostat set points for employee and visitor/customer comfort.
In Australia, the focus of the National Construction Code on reducing heating and cooling energy in the recent years has not been for human health reasons, but as an action to reduce Australia’s greenhouse gas emissions. This has led to a singular focus rather than a multi-faceted focus. The multifaceted
focus would respect that as we create well insulated and better sealed buildings we must consider not only the heating and cooling energy use, but we must also consider how we manage
water vapour diffusion through the built fabric, how mould may grow when the relative humidity is above 60% in interstitial spaces, how moisture will form and the envelope systems must be designed to allow drainage and drying, and that the interior air quality (temperature, relative humidity, pollutants) needs to be maintained, such that the interior relative humidity does not exceed 70%. These differences rely on the minimum requirements within the weather proofing, energy efficiency,
ventilation, and water vapour diffusion sections of the national building regulations.
Recently, Australia has adopted a similar regime with the first ‘condensation’ regulations included within the Health and Amenity clauses of NCC 2019, followed by some enhancements in NCC 2022. Whereas NCC 2019 focussed on condensation, NCC 2022 has moved the focus to a simulated MI value. Internationally a building envelope system that has a simulated MI of 3, or more, should not be constructed. In some nations, an envelope system with a simulated interstitial MI of 2, or more, should not be constructed. Envelope systems with a simulated MI between >1 to <3 generally require further analysis and investigation. Generally, a MI of 3 or more, indicates the visible presence of mould to the unassisted human eye.
In this research, hygrothermal (heat and moisture) and bio-hygrothermal (mould growth) simulations have been completed for nine typical low rise residential external wall systems commonly constructed in Victoria. The use of hygrothermal and bio-hygrothermal simulation to inform building envelope design is quite common in North America, Japan, Korea, the United Kingdom, and many nations within Europe. Within this field, Australia is up to 70 years behind some other developed nations.
Research Report 3 identified that only 43% of non clay masonry NCC climate zone 4 and 69% of non clay masonry NCC climate zone 6-8 external wall systems, with a southern (non-equatorial) orientation, and an air change rate of ACR10@n50, met the minimum MI value of <3, as specified in NCC 2022. The clay masonry veneer wall systems, which already require a vented and drained cavity, showed 0% of NCC climate zone 4 and 25% of NCC climate zone 6-8 external wall systems, with a southern (nonequatorial) orientation, and an air change rate of ARC10@n50, met the minimum MI value of <3, as specified in NCC 2022. The results from the hygrothermal and bio-hygrothermal simulations highlight the need for:
- Greater water vapour permeance properties of weather resistive pliable membranes, as climates become cooler and/or building air tightness is increased.
- The need for a vented and drained cavity, to ensure the moist cladding system does not contact the weather resistive pliable membrane, to allow outward-bound water vapour to leave the insulated envelope, and to promote ventilation to remove water vapour and assist in the drying potential of the external wall system.
- As buildings become more airtight there is the need to include an interior water vapour control membrane and automated mechanical ventilation.
Recommendations
The research has highlighted many aspects of hygrothermal and bio-hygrothermal considerations for existing and new buildings that need further development and application in Australia. These include, but are not limited to:
- The Tasmanian experience in 2014, identified the urgent need for training and awareness across the design and construction sectors. The state government instigated training activities
via the HIA, MBA, AIBS, AIA, EA and other non-aligned groups. The first published design guide was published in 2014, with an update for 6 Star and 2018 and a further update for 7 Star
planned for 2023. These measures significantly shifted the design and construction professions and avoided what appeared to be growing concerns regarding the significant presence of condensation and mould in new homes between 2010 and 2016. A similar approach is needed in Victoria.
- Detailed measured values of water vapour diffusion properties need to be printed clearly on packaging and material data sheets for all weather resistive pliable membranes. This will
enable design and construction professionals to make better informed choices regarding the climatic suitability of weather resistive barriers.
- All external wall systems should demonstrate that a southern (non-equatorial) oriented wall system has a simulated MI of <3.0, and preferably less <2, as required in some other
developed nations.
- This research has explored principles of increased vapour permeance, the addition of a vented and drained cavity and interior vapour control membranes. However, several wall systems did not achieve and MI of <3. Other aspects including increasing wall insulation, adding an overcladding insulation product, increasing the vapour permeance of weather resistive pliable membranes and increasing ventilation capacity of the vented and drained cavity needs to be explored.
- A preliminary analysis of the relative humidity values within the 2022 NatHERS RMY climate files identified that some climates had extended periods of external relative humidity above 80%. This condition makes it difficult for the wall system to diffuse moisture in an outward manner. This leaves the need for either greater hygrothermal buffering inside the building
and/or greater amounts of ventilation in the vented and drained cavity. As an example, in some countries a clay masonry veneer external wall is required to have five times (5 x) the ventilation capacity of a timber clad external wall system.
- There is a need for government to provide regulatory guidance in relation to the climate data required for this type of verification method. This is because different sources of RMY and EPW files provide very different values for all climatic inputs. At this stage, there is no perfect climate data file, but to ensure all certified users of transient hygrothermal simulation tools are using the same climatic inputs will provide design and construction professionals with repeatable simulation results.
- As noted in the conclusion, this research has used construction material physical properties from international databases. There is an urgent need for manufacturers to measure and provide physical propertie
Funding
Transient hygrothermal and bio-hygrothermal risk analysis for housing in temperate and cool temperate Australia : Victorian Building Authority
History
Publication title
Assessment of mould growth risk in new Victorian homesConfidential
- No
Department/School
Architecture and DesignPublication status
- Published