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NCC 2019: The Condensation Management Loophole

Updated: Jun 12, 2019

Sitting in VCAT the other day, a tribunal system in Victoria that decides the fate of civil pseudo-legal cases, I was quickly reminded of the phrase “Loopholes – the breakfast of lawyers" as I watched a certain legal practitioner navigate the vagueness of law to find purpose for their client and justify those massive hourly rates.

As the proceeding wrapped up without conclusion, walking back up King Street in the west of the Melbourne CBD, I received a call from Samantha Anderson (awesome condensation nerd) on NCC 2019, and the natural ambiguities of loopholes as we head into the great Australian condensation season (generally May – October).

Today, if condensation was a rugby or soccer season, Victoria, the ACT and Tasmania would be fighting it out for top position, with New South Wales and Queensland not too far behind. Western Australia and South Australia? Let’s call them the wild cards as i haven't a clue!

If we are honest, such ambiguities are ever-present, embedded and part of the fabric so to speak, within not only the Australian design and construction industry but those overseas too. Loopholes are simply part of the game and almost the default ‘value management’ mechanisms of construction, particularly in design and construct contracts.

However, loopholes are also natural outcomes from any complex, legislative system, particularly when developing a document to be more readable yet succinct. So before we point fingers as to where they come from, find a system that doesn't have them first!

The subject matter of the call was Condensation Management - Volume 1, but the conversation was equally applicable to Volume 2. To cut through the word count, it was applicable to Class 1 (dwellings), Class 2 (multi-resi) and Class 4 (a dwelling part of another building) buildings.

The positive, was the inclusion of much needed Condensation Management provisions, the inclusion of wall constructions as a contributor to the challenges and the need to control water vapour diffusion and leakage into and through building components.

We have lots of residential buildings across Australia with major condensation problems, now entering remediation strategies as a result of too many factors to mention here. From mould feasting on internal surfaces within 6 months of occupation to icicles forming inside roof structures, any added provision for condensation management in colder climates (Climate Zones 6, 7 and 8) to assist mitigation should be welcomed by all.

For those new to the subject, the drive of vapour and leakage through wall systems in colder climates is generally from the inside to the outside during the winter season. Therefore, a more permeable wall structure allows moisture to be lost to the outside of the wall structure typically into a cavity, where it drains or evaporates to ultimately dry out during summer. Here is a good, clear write up by Jason Quinn on condensation control layers and the basics… read more.

The not so positive loophole and point of pain for this conversation, however, is in F6.2 (Vol 1) Pliable Building Membranes. Here is what NCC 2019 states:

“Except for single skin masonry and single skin concrete, where a pliable building membrane is not installed in an external wall, the primary water control layer must be separated from water sensitive materials by a drained cavity”

What does this state? Well, of course, open to interpretation and provide your comments to this post, but I would dissect it as follows:

1.     First, if you install a vapour permeable pliable membrane (as classified by AS/NZS 4200.1) in Climate Zones 6, 7 and 8, you have met the Performance Requirement and are awesome. Your wall will be more durable and has a lower risk of corrosion and mould. Put your feet up, the jobs done!

However, if you are tempted by the ‘other products’ outside of the AS/NZS 4200.1 definition for membranes or similar products used in place of a dedicated membrane, you are looking straight into the loophole of sin (in this case) and taking the following risky route:

2.     Except for external single skin wall types (made of up the water control layer alone and no other materials), and if a pliable building membrane (as classified by AS/NZS 4200.1) is NOT installed, you must;

3.     provide an unspecified ventilated cavity (assume 25mm minimum) to drain and dry any potential condensation, to ensure separation from the water sensitive materials (materials with an inherent capacity to absorb water vapour, with timber products used in the definition only although insulation is also sensitive in some conditions).

So, if choosing the risky untested route and you have said your Hail Mary's, a project would be assuming the exterior skin is the water control layer. Indeed, I would go onto assume that this exterior skin, as a primary water control layer, would also be unventilated given its primary purpose is to control moisture and further exacerbate mould risk. So what does this mean? Of course, here comes the charts generated via hygrothermal modelling (WUFI)!

Let’s have a look at Class 1 and Class 2 homes, with 3 bedrooms and typical market floor areas in Melbourne, comparing a market leading vapour permeable membrane (blue) that allows vapour diffusion (good) and untested vapour barrier type product with a reflective foil (orange). As the reflective foil is impermeable, we can assume it does not allow for vapour diffusion (bad).

As you may have guessed, the conclusion in both Class 1 and Class 2 buildings is simple. Even though a well-ventilated cavity exists that separates the potentially water sensitive materials, the non-tested vapour barrier effectively blocks vapour from diffusing through the insulation layer and into the cavity to drain. It's poor design and not worth the risk!

With the dots representing the external face of the insulation (exterior 10mm) over June only (extracted from a 5 year study), we can clearly see that the untested vapour barrier interface with the insulation is always above 95% RH and thus a mould and vapour saturation frat party! This is not only bad practise, it is a clear detriment to the health and amenity of the occupants and fails to meet the Performance Requirements of F6 Condensation Management.

On the other hand, the market leading performance vapour permeable membrane (blue) shows daily variation and a drying cycle that keeps the mould party in check and meets the Performance Requirements of F6 Condensation Management.

Now, this is not to say that vapor barriers have no role in Australia, as this would be incorrect. When vapour diffusion is primarily coming from the outside to the inside, a case can be had but this only becomes likely at an invisible line between Brisbane and Townsville. But in most cases, raise an eye brow at least as the gods won't save you if you install this stuff!

In summary, if you are going to 'value manage' a project, replacing vapour permeable membranes with other untested products is not the way to do it. I love to bend the rules. Sure, that is what they are for. But physics is not in the habit of lying. Indeed, this loophole can strongly be argued to not meet the Performance Requirements of NCC 2019 - Condensation Management. I will leave that to the Building Surveyors to debate.

If you agree, lets commit to the formal resolution of loopholes, a Proposal for Change, and seek to get this updated in 2022. Rome wasn’t built in a day folks...... but they were laying bricks every hour!