Is mould in our homes causing asthma? There is no doubt in my mind that household mould is a considerable contributor to the respiratory problems we have in New Zealand.

So why is this being tolerated? Mould will most often form on the cold surfaces in a building such as aluminium window joinery, single glazing or the corners of a room facing south. This is because these are areas with high thermal bridging where the indoor warmth is being conducted out from the building with little resistance.

Warm, moist internal air will condense on these cold surfaces; the colder the outdoor temperatures, the colder the thermal bridge becomes and the more marked the condensation will be. Condensation forming on thermal bridges is a simple matter of physics. Air holds moisture called humidity, and the hotter it gets the more moisture the air can hold. If you place a cold item such as a chilled glass in a hot humid space it will immediately mist up with condensation. The same applies within the spaces of a building.

There can be a hot humid environment on the indoor side of the wall and a cold damp environment on the other side. As the heat energy flows out of the building taking the moisture vapour with it, the moisture will condense on cold surfaces within the wall construction. A surface mould problem on the plasterboard is usually the tip of the iceberg.

Leaky weathertightness envelopes have led to the premature decay of many houses in New Zealand, however the issues from internally driven moisture (or interstitial condensation) have yet to be tackled as they are not being recognised as a risk by the industry at large.

This goes against good common practice in many other developed countries in the world where similar building practices and climates do exist. A recognised method of predicting mould growth in a particular building element is through hygrothermal analysis using a computer simulation program called WUFI, (Wärme und Feuchte Instationär), developed by the Fraunhofer Institute Germany.

The WUFI programme takes into account the specific New Zealand climatic conditions and allows us to simulate any number of building wall or roof elements. WUFI is a common procedure used to ensure the durability and indoor environmental quality of certified Passive Houses. However, when commonly accepted construction details used in New Zealand are modelled with WUFI, some very concerning results come to light.

We first looked at the use of internally strapped and lined concrete masonry walls with the help of WUFI modelling. No matter what part of New Zealand you are in, this approach has been shown to be problematic, and it gets much worse the further south you go. Adding more insulation or even a moisture control layer does not help. This problem can easily be remedied by moving the insulation layer to the outside face of the masonry.

Brick veneer or weatherboard construction with a drained cavity is also commonly used with insulation and plasterboard linings. Once again the results from WUFI modelling are very concerning, and show how readily mould can develop in this common construction detail in many circumstances.

To fix the problem, our industry needs be looking at best international practice, undertaking more detailed hygrothermal analysis, such as WUFI modelling, of our common construction details in order to ensure they do not pose issues with condensation and mould during the early design phase of our projects.

When retrofitting buildings, it is imperative that the upgrades are checked and do not unwittingly lead to the premature decay of the building framing. Only with more care and attention to detail will we rid ourselves of building mould and the associated health issues.