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  • Writer's pictureS.Samson

Mould Growth in Buildings

Updated: Sep 12, 2023

As Australians start to demand better performing homes and the NCC (National Construction Code) requires certain insulation values and membranes we must watch out that we do not create a sick building, which in turn can also make us sick.

I have been building residential building in all shapes and forms since 2014. Since that time, I have come across many building assemblies that could quite easily cause mould growth. I have also seen where mould has formed due to water ingress or poor vapour management.

Note: In this article we are talking about building assemblies in colder climate zones where it is predominantly cool on the outside and warm on the inside. Also I will be keeping things simple and only talk about vapour pressure and not relative humidity. For more about relative humidity and vapour pressure see further reading at the end of the article.

Why do we want to avoid mould growth?

Some moulds can produce toxins, specifically mycotoxins, which have been generally accepted by the WHO and NHS to cause respiratory symptoms and allergic reactions.(1)

Structurally mould growth is more superficial, indicating a moisture problem which could cause rot or rust.

So where is the issue?

The issue starts with insulation. This is not to say we shouldn't add insulation, on the contrary we definitely should. But when we add insulation to buildings it changes how the building assembly performs hygrothermally (the term 'hygrothermal' refers to the movement of heat and moisture through buildings). In Australia we have developed, over time, a system that works, although not very well thermally. This is the common ventilated facade.

We have an internal layer, often plasterboard or brick, a ventilated cavity, and an external layer, often weatherboard, brick or asbestos. As humidity moves from the warm side to the cold side it will ventilate out before it condenses, and in extreme circumstances if it does condense it could then easily dry out.

Now lets fill the stud cavity with insulation, put on a sarking and clad with lightweight cladding and see how that changes things.

The building becomes generally warmer on the inside (in a cold climate) due to the insulation. This can increase the vapour pressure (vapour pressure corresponds to the amount of water the air is holding). A warmer internal temperature results in more vapour in the air, due to human activity in the home, its ability to store more vapour, and comfort, as we don't like to breathe dry air.

As the air, warmth and vapour pass through the building assembly towards the cold side and reach the end of the insulation they are met by a cold point, usually the cladding. Now the vapour pressure has dropped so much due to the temperature drop it cannot hold the same amount of water that it did when it was warmer. the vapour must now leave the air and return to its natural state, water. This is how condensation forms.

The fact that most cladding, whether it be metal, painted fibre cement, masonry or timber are the least vapour permeable part of the building assembly does not help. This is because it is often also our rain screen and so it has to be waterproof.

These two factors work together to create the foundation of the problem. The external facade is the coldest point and also the least vapour permeable. The vapour cannot pass through the material before it condenses.

The more insulation we add the more we exacerbate this issue as the warmth from the inside of the building finds it harder to get to the outside, making the building components on the outside effectively colder.

If our cladding is directly fixed to the studs there is very little air gap between the cladding and the sarking. additionally the cladding and the sarking are usually touching almost everywhere because the insulation is pushing out against the cladding. So now the first cold point that the vapour encounters is the (often) foil faced sarking.

The sarking will still act as a backup rain screen (or weather barrier) if there are any leaks in the cladding but any vapour can and will condense on the inside of the sarking. Now we know how mould growth can form on the inside of our weather proof membrane.

How bad is this problem?

If we use WUFI 1D (2) software to model the hygrothermal performance of this building assembly we would see that we would have significant mould growth in the first year on the pine timber studwork and mould growing on the insulation within ten years. This is using Adelaide climate data, 7mm FC cladding, a vapour permeable weather proof membrane, 90mm stud cavity filled with glasswool insulation batts and 10mm gyprock.

Mould growth takes longer on the insulation as the insulation does not provide good food for the mould, but if it will grow on the insulation we know we have serious issues.

Now lets say we built this house new and we made it reasonably airtight. The building would have far less air changes and so less of the vapour that is generated in the building by the occupants would be removed as a natural function of the building.

The moisture and problems now become so significant that WUFI cannot calculate beyond a year into the future as it has run out of places to put the moisture.

We have to take all this with a grain of salt as this is only a computer simulation and may not represent individual cases but it can give us a good indication of what a problematic building assembly looks like.

How do we fix the problem?

We have to ventilate the facade. All our facades should be ventilated, and just adding battens and attaching the cladding is not good enough, we have to create a path from a low point to a high point for the air to pass through and remove any vapour or moisture. The problem is there is nothing in the NCC that says we must do this, so builders will keep making mistakes. Even the installation instructions on a most cladding products do not refer to or require any sort of ventilated cavity. For some reason we can install foam (which has no vapour permeability) directly to sarking and studs. If we install CSR Heble it requires us to have metal top hats that usually run horizontally but no gaps in the top hats to allow air to pass by. Many of the James Hardie details allow us to install their claddings directly to sarking and studs. Designers and architects usually direct materials to be installed as per the manufacturers specification to reduce their liability. And this is why cladding often gets installed the wrong way.

Additionally if we want to build an airtight building we should introduce a vapour control layer on the inside of the building assembly to slow down the transfer of vapour into the building assembly so that we can reduce the amount of moisture that is transported to those cold surfaces. This is important because now there is less drying opportunity from the inside of the building. Commonly a good layer of render can perform this function on masonry construction.

We should also have appropriate air extraction in the problematic moisture generating areas of the home. On a very airtight building we may require some sort of makeup air to replace the air extracted in case you don't have any windows open but I think I will elaborate on this topic another time, stay tuned.

How can I easily identify a 'bad' system?

Firstly ask your builder to explain to you how they plan to ventilate between the weather proof membrane (sarking) and the external cladding. This is different to providing an air gap on battens. The air must be able to pass through unrestricted. For example weep holes on a brick facade will not suffice. The openings must be big enough and often enough to allow adequate ventilation. If there is no air gap or no ventilation you will likely encounter problems over the life of the building. If you would like to see the detailing of a well ventilated fibre cement cladding system check out CSR Cemintel (3).

Secondly if you see a builder using a foil faced sarking on an insulated wall assembly you know they are doing it wrong. Its funny because you see it everywhere. I would never use a foil faced sarking and if I did it would only be adjacent to an air cavity such as a roof space. The manufacturers themselves point out that you should not use foil faced sarking in walls (4).

The NCC, since 2019, requires vapour-permeable barriers in climate zones 6, 7 and 8 (NCC F6.2). In other words, it’s a different story in cooling-dominated climates.

What do I do if I find out my builder is building a 'bad' system?

The best thing to do is point them towards the your home website, specifically the cladding page (5) and the condensation management section. This is an easy reference point and the best government literature that explains how to build a correctly functioning wall assembly. Demand that they use this detail.

Wrap Up.

We want to build buildings that will withstand the test of time, keep us comfortable and not be a health hazard. We cant always rely on the minimum standard set out in the NCC. Installation instructions can be poorly researched and are often optimised to comply with the minimum standard, be cost effective and easy to construct. Arming yourself with this knowledge will help you to ask the correct questions or renovate your house correctly and produce a healthier environment for yourself and your home!



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