Passive House Course Day 4 - Mositure

Indoor air temperature and humidity influences mould and water penetration into the building fabric. Our lack of understanding in Australia is going to lead to some major building issues as we develop more insulated and air tight standards. In a way we’ve been lucky so far that our buildings perform so poorly from an air tightness point of view that most moisture within the walls will eventually escape. 

There are a few things that we need to understand about the properties of water to understand how our buildings should be designed;

• Condensation will form on the surface of a material that is 12.6 degrees or lower.

• Typically an internal wall or window surface can be cooler than the indoor air. For example your bedroom may be 16 degrees overnight, but the glass could drop below 10 - even if double glazed. As we all know this glass will gather condensation and ‘fog up’.

• Cold air holds less moisture, so the hot air passing over a cold surface will reduce the air temperature close to the surface, and the moisture will be lost from the air, clinging to the surface. In this case glass, or even the aluminium window frame - which is also a very good conductor of the outdoor air temperature!

So the key is to provide a continuous thermal envelope, a consistent barrier protecting the indoor temperature, minimising weak points where condensation can gather. 

One thing I'll be sure to focus on is bathrooms. Typically colder rooms, often not heated, and definitely high in humidity. Like a middle child - trouble waiting to happen ;-)