Indoor mould has been increasingly observed in Australian residential buildings, both in houses and apartments.

Its presence indicates an underlying moisture problem. This can either come from water ingress through defective cladding, plumbing failures or condensation due to a lack of management of water vapour.

The occurrence of mould in homes and apartments underscores several areas of concern.

First, defects arising from new buildings may potentially signal either a lack of awareness of the health implications of mould among building practitioners (architect/designer, building surveyor and builder) and/or inadequate practices with regard to documentation, certification and construction.

Second, problems may also reflect a lack of adequate guidance and regulation through the National Construction Code (NCC). It was only in 2019 that condensation management provisions were introduced into the Code for the first time. Prior to that, buildings that were constructed under earlier versions of the Code were not subject to any performance requirements in relation to condensation management.

Finally, the importance of providing healthy, mould-free environments for living, work and study has increased as more people are now working from home for at least part of their working week.


More and More Mould

In Australia, there have not been any large-scale longitudinal studies to quantify what appears to be an increasing trend of mould occurrence.

However, the overall picture from researchers such as Dr Nicole Johnston, the UNSW City Futures Research Centre, and the annual state-based surveys by the Australian Apartment Advocacy, all point to defects being common in apartment buildings. When these defects are broken down by type, water-related defects regularly top the list.

Timeline of major events in the development of building standards around condensation and mould.

(Timeline of major events in the development of building standards around condensation and mould.)

In 2016, the Australian Building Codes Board (ABCB) conducted a survey involving more than 2,000 building practitioners. From that survey, it was estimated that problems with condensation management are present in about a third of new houses and apartments.

In the 2016 ABCB Condensation Scoping Study, it was identified that increasing code requirements for energy efficiency had caused condensation in other countries such as the US, UK and Canada. Buildings that were more airtight delivered better thermal performance but retained more water vapour. In Australia, higher instances of condensation were found to have occurred following the introduction of mandatory minimum energy efficiency performance in new Australian buildings. This was particularly the case following the introduction of 6-star mandatory energy efficiency requirements under the Nationwide Housing Energy Rating Scheme (NatHERS).

Over the next few years, Australia is set to move to 7-star mandatory requirements as a result of updates to the NCC in 2022 (NCC 2022). The new requirements will come into force across varies states and territories across 2023, 2024 and 2025.

This raises a further question. Even with the aforementioned condensation management provisions in 2019, will condensation management efforts be sufficient to offset the greater accumulation of vapour from yet even more airtight homes?

A further concern relates to the high rate of builder insolvencies which has occurred across Australia over the past two years. This has led to incomplete buildings being left exposed to the elements and has resulted in microbial contamination prior to occupancy as well as moisture entrapment in building materials. Many of these materials (such as particle board, plywood bracing, timber truss and frames) were not intended for long term exposure.

Water ponding on slab due to driving rain. Some of the bottom plates are already in decay.

(Water ponding on slab due to driving rain. Some of the bottom plates are already in decay.)

Mould Causes

Mould can arise from several causes.

These can include:

  • water ingress from failed weatherproofing and waterproofing
  • water loss from plumbing failures, particularly from burst braided hoses
  • rising damp from poor site drainage or failed basement tanking, and
  • high humidity (in warm and humid climates) as well as condensation in cool climates.

I should highlight that water – that is liquid water – is not the only possible cause of mould growth. High levels of humidity in the environment also enable mould to germinate.

Three consecutive years of La Niñas, from late 2020 to mid-2022, brought record rain and widespread major flooding to Australia. We also experienced longer periods of continuous elevated humidity. These are exactly the types of conditions which enable mould to germinate.

In 2022, there were a number of very high rainfall events. These flooded thousands of houses in Australia. However, many houses in regions that either were not flooded or only had minor water ingress experienced mould growth on numerous contents and surfaces. Having investigated the weather records from the Bureau of Meteorology (BoM), I found long periods of elevated humidity for a period of two to four months after heavy rainfall especially in Queensland. It is likely that some of these houses were subject to long periods of high outdoor humidity that enabled mould to germinate on any surfaces where the substrate supported it.

Thus when all factors are considered, it is likely that the majority of residential buildings in Australia have mould problems to at least some degree.


Modelling mould germination risk — when climate zones are too coarse

Sedlbauer of the Fraunhofer-Institute for Building Physics developed an isopleth (or contour) method to model the combination of temperature (x-value), relative humidity (y-value) and germination period (z-value) on cellulose based material. When Bureau of Meteorology  observations are overlaid on these isopleths, it is possible to evaluate the risk of mould growth. This is based on how frequently the weather observations have continuous periods that would support mould germination.

This is illustrated in the above plots of BoM weather observations for a 12-month period ending July 2023. Red bubbles indicate the number of days of continuous high humidity with the size of bubble scaled to length of period, its position is the average T and RH of that consecutive period.

Brisbane and Byron Bay are both in the same climate zones to achieve condensation management compliance to the NCC: NCC Climate Zone 2 for the Deemed to Satisfy vapour permeability specifications of pliable building membranes; and NatHERS Climate Zone 10 which may be used for the condenstion Verification Method. Even so, the 2 locations show vastly different mould germination risks using the mould isopleth method.

Due to the sensitivity of building materials to mould germination in a matter of days, the broad categorisation of climate zones (8 in NCC, or 69 under NatHERS) is inadequate to anticipate the risk which can be derived from the use of a much more granular method through BoM recorded observations (over 600 weather stations in Australia).

Mould cost and insurance

Establishing causality is key when assessing whether mould damage is covered by insurance.

Where mould growth is caused by high humidity in the property rather than water damage, many insurance policies are unlikely to cover the cost of damage which arises out of this.

On the flip side, most insurers will cover the replacement of mould affected building materials and contents which are deemed unsalvageable if the contamination is directly related to, or resulting from, an insured event such as flood (unless specifically excluded), storm or escape of liquid.

Where things become more complicated are circumstances where water ingress under two concurrent conditions. These are: (1) non-compliant design or non-compliant construction; and (2) an insured event. In these situations, the storm event to which the damage relates may not have resulted in water ingress had the design or construction been carried out in a compliant manner.

In the Victorian Building Authority (VBA) funded Scoping study on the nature and extent of moisture damage in houses & apartments in Victoria, it was found that common areas of non-compliant design or work occurred in roof plumbing and balcony water-proofing.

If claims for mould continue to escalate (frequency and cost), it could potentially lead to higher premiums as well as limits or exclusions on mould coverage.

At a recent building surveyor conference I attended, an insurance broker mentioned that some professional indemnity (PI) insurers are considering excluding mould complaints from PI cover of building practitioners. Such a move would leave practitioners personally liable to defend such complaints from homeowners.

We should also remember that whilst building laws have been slow to change, rental laws have progressed ahead. In Victoria, the Residential Tenancy Regulations 2021 has updated minimum standards that require each room in the rental property to be free from mould and damp caused by or related to the building structure.

As a result, the industry is now caught in this no man’s land of meeting the minimum standards of the NCC whilst not meeting the minimum standards for rental.

What’s our response?

For all these reasons, I have been appealing to building surveyors to exceed the minimum standards for the public good. In turn these surveyors need to be supported by building regulators, building appeals boards and tribunals so that surveyors are suitably empowered to discharge the full remit of their statutory powers. This, for instance, can be the prohibition of any material that is found to be unfit for purpose (r.120) or to cause inspections at any stage (s.35), or to issue building notices if of the opinion that the building is a danger to the health of any member of the public (s.106).

Building designers (including architects) and builders may also consider increasing their knowledge in psychrometry and water activity. This will enable them to learn how to design or “build out” risks associated with condensation and microbial contamination.

Educators, the TAFEs and universities, should also keep up with the changing demands of the industry in order to provide job relevant graduates.

Mould is a big issue, and it is to be found in most buildings.

Mould is always somebody’s problem. Let’s make the fix everybody’s business.


By Tim Law, PhD, Technical Lead — Building Sciences,  Restoration Industry Consultants


In 2021, Dr Tim Law led a research project to understand the nature and prevalence of indoor mould damage in homes and apartments across Victoria for the Victorian Building Authority. The final report from that study can be read here.