Many will be familiar with the concepts of evaporation and condensation in everyday life.

When applied to buildings, however, these principles need greater analysis.. 

This article explores some common misconceptions around condensation and mould.

At the outset, it should be acknowledged that all myths have an element of truth. Accordingly, the following does not adopt a true-false statement but instead involves an ‘it depends’ discussion where nuances and qualifications are important. 


Myth 1 New houses will not have condensation

Some believe that condensation will not occur in new homes. 

Unfortunately, this is misguided. 

In 2014, researchers at the University of Tasmania used detailed monitoring and microbiological sampling to investigate condensation in three newly constructed homes – each of which was undergoing its first winter.

In all three homes, significant condensation was discovered. In one, dripping originating from the underside of the low-pitched roof felt like a light drizzle.

The study revealed a lack of guidance in the NCC (National Construction Code) around condensation management.

In a further study funded by the Australian Building Codes Board (ABCB) in 2016, a survey was sent to building practitioners and received more than 2,000 respondents. This found that condensation was widespread across Australia and that in this aspect our code was decades behind international best practice.

Whilst some recommendations from that report were incorporated into NCC 2019, these were initial steps only which are part of an ongoing work program, They do not, in themselves, prevent condensation and mould entirely. 

Indeed, at an introductory NCC Seminar, Graham Moss, Director for NCC Provisions at the ABCB, acknowledged that these measures – even if followed correctly – were not a ‘magic bullet’ and would not, in themselves ‘necessarily kill condensation in your buildings’ or ‘prevent condensation in all cases’.

Furthermore, simply having measures specified in drawings does not mean that this will equate to what is constructed. Whilst the NCC requires use of vapour permeable membranes in residential buildings in Climate Zones 6, 7 and 8, for example, there is no mandatory inspection stage which will verify that these have in fact been installed. 

Hopefully, Australia will catch up to international best practice in coming years.. 

For now, however, some condensation is likely to occur in new homes.

The constantly wet, and often dripping underside of the roof.

Inside the roof and walls where there is extensive condensation and mould growth on the timber structure in the house that has been occupied for about 2 months.


Myth 2 A home that is energy-efficient will not be mouldy

In 2010, the airtightness of buildings was upgraded amid changes to the NCC which both increased the stringency of energy efficiency provisions and introduced a new standard for construction in bushfire-prone areas. 

This was considered beneficial as greater airtightness not only reduced heat loss and delivered better thermal comfort but also helped to prevent cinders from blowing into the building.

As this happened, however, copious amounts of condensation emerged in a growing number of homes. The persistent damp which resulted has led to other problems with mould and its deleterious effects on human health.

Largely speaking, this happened because the move toward greater airtightness was not accompanied by an adequate understanding of vapour management.

Indeed, the ABCB-funded condensation scoping study found that design decisions around energy efficiency were the primary factors which affected most aspects of new home design. This included risks associated with vapour management, condensation, moisture and mould. That study concluded that there was a need for an integrated approach whereby each step of improvement in energy efficiency was accompanied by suitable measures surrounding vapour control and moisture management.

Until this happens, and until designers better understand building physics, there is a risk that increased energy efficiency will be accompanied with condensation and mould.

Ceiling insulation showing change of plane in ceiling insulation from raked ceiling to flat ceiling. The plasterboard (marked in red outline in right figure) should have been insulated with wall insulation to the same insulation as the ceiling insulation, R-4.0 in this case.

Myth 3 There is no harmful mould if you cannot see it

In cool climates, condensation is most significant where surfaces are coldest. Often occupants associate condensation with single-glazed windows or thermally unbroken aluminum window frames. However, water vapour is insidious: it can move with air, it can diffuse through air, and it can permeate through porous material. 

Given that most buildings have materials that contain high-carbon cellulose-based mould food, the addition of water pretty much guarantees mould growth. 

We often don’t see it only because the action is happening in the interstitial spaces between internal finishes and external cladding.If you do see mould on the walls or ceiling, that’s likely to be the tip of the iceberg. The site of colonisation is in the dark damp interstitial spaces on the reverse side. Many of these spaces are either difficult or unpleasant to access. You are more likely to smell mould before you see it. 

As an aside, the colour of mould is immaterial. The term “toxic black mould” is never used by mycologists (fungal scientists). This fascinating macro photography time lapse shows mould waxing and waning as if the conidia were trees, and changing colours as if the colonies were seasonally changing deciduous forests. 

Regardless of colour, mold is an indication of a moisture problem. And any moisture problem will eventually have fungal colonists that are the stuff of nightmares

Health Canada advises to control humidity, repair any water damage in residences to prevent mould growth and to thoroughly clean any visible or concealed mould growing in residential buildings. These recommendations apply ‘regardless of the mould species found to be growing in the building.’

Mould settle plates from houses in Tasmania. After 1 week these moulds grew from invisible spores that settled on plates exposed for 1 hour. Identified genera: alternaria, aspergillus, cladosporium, penicillium, zygomycete.


Myth 4 My new house is built with new materials so it will not be water-damaged

Construction material exposure on site is of particular concern during COVID. Due to strong demand and COVID related supply disruptions, there is a shortage of building materials and longer wait times for materials to arrive on site. 

This is problematic as many materials such as engineered timber, truss nail plates and particleboard flooring are not suitable for prolonged exposure to the elements.

Engineered timber

Laminated veneer lumber (LVL) is a common engineered timber product used in structure. It is generally unsuitable for external use, and would in those situations require special preservative treatment. 


Nailplates in trusses

Timber trusses are fastened by nailplates. When wet timber expands, then contracts after drying, these nailplates are progressively pushed out in a ratcheting fashion. This mechano-sorptive nailplate backout phenomenon is well documented. It was found that the strength of a nailplate decreased by some 25% for every additional millimetre of gap between nailplate and timber. When these trusses were loaded to nearly full design capacity, the trusses failed (ruptured) after about 20 wetting-drying cycles. This presents a problem not only for prolonged exposure of timber trusses before the roofing is laid, it is also a problem when condensation occurs inside the roof space and thus subjecting the trusses to wet-dry cycles.


Particleboard flooring

Particleboard flooring is an economical and popular flooring substrate. 

It is manufactured to specification in AS1860.1 (2017) “Particleboard Flooring Part 1: Specifications”. In this standard, surface water absorption is to be tested to AS4266.1 s.13 which determines the increase in mass to the sample in contact with water on the surface of 115 minutes. 

The cut edges, however, are not tested. These are a known area of weakness to water entry, so manufacturers factory-seal the edges with wax and include the following instructions, “Where panels are not factory sealed, and where panels are cut to size on site, the edges should be sealed with adhesive used to bond the panels to the joists”. 

Where weather exposure may occur during construction, the standard requires a Class 1 Particleboard flooring. Even so, it is not suitable for conditions of moisture content permanently in excess of 16% (AS1860.1, 5.a.iii) or exposure beyond 3 months of normal weather (AS1860.2, 11). The acceptable exposure time could be even shorter in extreme weather. 

Should builders encounter protracted delays, become insolvent, or neglect sites due to an overcommitted schedule, the particleboard flooring could be sitting in puddles of rain for days to weeks – much more than the tested 115 minutes – and could be exposed for more than three months between flooring installation and lockdown stage.


5. An owner is without recourse if mould were to be found to be caused by condensation

In most circumstances, this statement is mostly correct.

Mould which arises from condensation is different from mould arising from other building defects. In itself, mould is explicitly excluded from home and contents insurance. The limitation is that the insurer is only responsible for restoring a building to a prior state prior for a ‘listed event’ covered by the policy. With the right coverage, one could be covered for water ingress from damage to the roof or arising from plumbing leaks. In such cases there will be compensation by the insurer.

However, with condensation, it could well be a problem with building design and construction — for which there is no listed event and no prior state to ‘restore’ something to. Mould remediators who are not familiar with identifying condensation causes may temporarily remove the mould, only to have it reappear next winter. The amount of condensation that occurs in buildings can be profuse, amounting to an indoor drizzle. One just has to look in the roof space on a cold morning. In the case of many houses built during the period after 6-star energy efficiency provisions were mandated in 2010, and before condensation provisions were introduced in 2019, raindrops, or more accurately dewdrops, might be falling on your head. 

Published in 2015, the VBA Guide to Standards and Tolerances, which has been republished in Tasmania, New South Wales and Queensland, places responsibility on occupants with this guidance:

“Where the requirements of the Building Code of Australia (BCA) have been complied with, the responsibility for controlling condensation by maintaining adequate natural or mechanical ventilation through the use of openable windows, exhaust fans, or other means, is the responsibility of the owner.Condensation is defective if the builder has not complied with the relevant clauses of the BCA.” (18.03)

Condensation under the roof sarking as a result of design decisions. In this instance there was no amount of ventilation that could have prevented this.

In states that have adopted the National Model Building Act in entirety, such as Victoria, the Building Act has provisions for a building surveyor to serve a building notice if a building poses danger to the “health of any member of the public or any person using the building”. 

Tasmania’s modified acceptance of the Model Building Act in 2000, on the other hand, omits the clause on health. Instead, in Tasmania, the Public Health Act empowers the environmental health officer (EHO) to make  the determination whether “premises are so unhealthy that no person can safely occupy them”. Following this determination, an interim closure order is issued until a building surveyor provides a rectification report, for which the building will have to be either rectified or closed.

The difference between the Victoria Building Act and Tasmania Public Health Act boils down to this: is the building unhealthy to anyone, or to everyone? Is there a problem if 1 in 10 occupants suffer, or only when all 10 in 10 do? The burden of proof in Tasmania is to show that every user of a building becomes sick from an unhealthy premise. In Victoria, a building notice can be served so long as any person is sick from the building. 

It can be seen that in terms of ensuring that buildings are fit for occupation health wise, the precautionary principle is more prominent in states like Victoria.


Architectural scientists have an adage: we can only manage what we can monitor, and monitor what we can measure. Without research funding we don’t have enough answers as to how to solve the building-mould-health problem.

For this reason, the public should pressure state health and growth departments to make this a research priority. Public funds need to be directed to research into making buildings, new and old, safe and healthy. 

At the end of the day, the cost of unhealthy buildings falls on the public.