Most of us could share a scary story or two about mould and mildew in buildings.

It’s unfortunately a common problem, and it sometimes seems to show up for no reason. Mildew in the shower is probably the most common example, but many old and new buildings have mould in very visible places like window sills or in the corner of a room.

Try to get rid of it and it just comes back. Clean the shower and it will be back two weeks later. But growth of mould is a biological process, enabled only by physical conditions. Keeping the biology in check means controlling the physics of moisture and heat.

It’s interesting to hear some of the conversations here in Australia relating to condensation and mold, including documents and surveys put out the Australian Building Codes Board to gauge the severity of the problem by asking people to give feedback. But it’s interesting to me mostly because I grew up in America, where the same conversations have been going on for decades. It’s true, the physics causing condensation in buildings are the same in the southern hemisphere. In many cases, the building practices that created the problems in the US are similar here.

The first place to concentrate on the problem is the building envelope. Where does the blame lie for a failed enclosure? If the designer creates a good design but the construction quality is bad, is it the designer’s fault? Is the inverse also true? Realistically, all share responsibility in delivering a working final product and the parties can help each other by following commissioning principles from start to finish.

It’s time for a scary story about mould and mildew in buildings. The picture below is from a roof parapet on an aquatic centre. The building had major problems with HVAC control and condensation. Though it was only a few years old, the enclosure showed signs of rusting structural members and copious mold and mildew, and the building HVAC was struggling to keep the indoor climate bearable. How did this happen?

There are three main reasons condensation happens in envelope assemblies – vapour diffusion, cold surfaces, and air leakage. Let’s tackle them one by one and see how building codes in the US could inform those in Australia to improve things.


First, we can discuss vapour diffusion, which is the drive of moisture from areas of high vapour pressure to areas of low vapour pressure, including through materials. For example, when it’s cold outside in the winter, there is a tendency for water vapour from the warm humid indoors to permeate out through the walls toward the relatively cooler and dryer outdoors. In cold climates, some serious effort must be taken to prevent vapour diffusion because it can result in significant condensation in building assemblies.

In US building codes, a vapour control layer is specified to prevent this phenomenon, and the practice is rooted in historical experience. Builders have long insulated homes to improve comfort, but when serious levels of insulation were used for energy conservation, this changed things. Insulation works by preventing heat transfer. The function that keeps heat indoors also keeps it from warming outer surfaces, letting those cool. Sometimes, those surfaces become cool enough to act as condensing surfaces. The result is interstitial condensation – the same phenomenon feared in Australian buildings.

In cold climates in the US, building codes in many climates for decades have required vapour retarders and vapour barriers. The difference in the name relates to the extremity of the function – vapour barriers let almost no water vapour through, while vapour retarders are semi-permeable.

In the aquatic centre, the builder had been advised to deal with some tricky gaps in the envelope with an expensive flexible membrane. The membrane is not a vapour barrier – it is vapour permeable. Applied correctly, these are great materials for some assemblies because they let trapped moisture escape. Here though, there is constant vapour pressure from the warm humid aquatic interior pushing the moisture out. The vapour-permeable membrane just lets the water vapour through, where it condenses on colder surfaces. If vapour control layers are used correctly, they keep water vapour where it’s supposed to be.


Unfortunately, if vapour control layers aren’t put in the right place right place, you’re going to get condensation anyway. The picture below shows a roof parapet under construction. See the foil-faced  wrap on the outside of the assembly? That would be classified as a vapour retarder – it lets some moisture through but has some resistance. Still, because investigation revealed that a great deal of condensation is happening on the inside of the wrap at this building, it is clear that there is another source of condensation besides vapour diffusion.


Further inspection showed an almost total lack of insulation in the parapet other than the wrap, even though it had been specified in construction drawings. The lack of insulation leaves many of the surfaces cold during cooler weather, which makes them vulnerable to condensation. The image below shows where this condensation can show up in this assembly. The dark red line shows that condensation can form on the inside of the wall wrap if it is not kept warm by insulation. To summarize decades of American experience for this Australian building: a vapour control layer must go on the warm side of the insulation. You need to keep the water vapour away from the cold side where it will condense.


The last major problem that leads to condensation in building enclosures is air leakage. Even if the correct materials to inhibit vapour transmission are specified and installed in the right places, unless they are sealed together to form a continuous barrier, they will allow moisture-laden air to slip around and reach condensing surfaces.

Fortunately, we have a tool to help verify that air leakage is minimized, called a blower door. It is a tool that can measure and help find air leaks in a building. It’s so useful that much of the developed world has blower door testing as part of the building code. It’s that important.

At this building, the blower door test was used to measure how leaky the building was. An obvious culprit for leakage was the roof parapet, which lights up like a neon sign under an infrared camera. So much air leaks out from the aquatic center that, standing on the roof, you can smell the pool chlorine and hear kids splashing around inside. If you open up the cap and look inside, you would see the moisture, mould, and rust in the first picture. This building is only a few years old.


The lessons here are obvious. Design and install adequate insulation to keep surfaces in the assembly above the dewpoint. Use vapour barriers where needed to prevent water vapour diffusion through the insulation to cold surfaces. Use a blower door to check that your air barriers are as tight as possible to prevent moisture-laden air from circumventing your other moisture control efforts.