Once a house becomes  energy efficient and airtight, you are likely to trap humidity inside and this can result in a big issue.

This is a topic that is just starting to surface here in Australia, but it’s a common issue, and other countries have gone through the same before us.

If you want to know more about it, just google “sick building syndrome”.

Investigate what happened in Europe about 20 – 30 years ago. Or what’s currently happening in New Zealand homes now. People are getting very sick with asthma, allergies, autoimmune diseases and so on.

Once you start building a semi-airtight, good performing house, you create lots of issues if you don’t look at the ventilation during the design phase.

This is not a new or unknown issue. It’s happened everywhere in the world, from California to New Zealand and in many parts of Europe. So, Australia is likely to follow suit as we move towards more energy efficient building methods. 

Just by living in our homes, by breathing, showering and cooking, we produce a huge amount of humidity. And if this humidity is not expelled out of your house, it can and will create water and mould issues over time.

It won’t happen overnight. Often it can take years for water to build up inside your structure. For mould to grow, before it even breaks through the plasterboard. 


I know, people are thinking “I just need to open my windows a bit more often”. But sadly, that won’t be enough. In order to get all the humidity out of your home you would need to do a proper cross ventilation at least twice a day. For 10 – 15 minutes. And no, keeping your awning window open won’t cut it. You would need to open doors or casement windows or opposite sides of your home. To really get it all out.

Sadly, in most parts of Australia, you would almost never do that.

Sure, that is not a problem on a mild day. But what do you do when it’s 30 degrees out side? Or 10 degrees? Or on a stormy day? No, you would not open the windows, if the weather is inclement. The humidity is still there and will be trapped inside your home.

In summary, I would always recommend to add some sort of mechanical ventilation into your home. Even if you can’t afford to go all the way to PassivHaus. I would rather save a bit on insulation or the windows, than not getting a ventilation system.

Even if you can’t afford a certified Passive House HRV (heat recovery ventilation unit) that is no problem. Or you can get a decentralized system, they are a bit cheaper.

My biggest tip would be don’t skimp on the ventilation.


If you are still not convinced and think you can manage the humidity via opening windows, then at least do yourself and your family a favour and install a good sensor in your home. That will track humidity levels as well as CO2 content. Therefore, you will at least know if there is a potential problem and if you have to do something about it. It might mean opening the windows more often. Or reverting to some sort of mechanical ventilation system.

The decentralised HRV units can even be retrofitted to help conquer excessive humidity.