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  • Writer's pictureRydePassiveHouse

Do You Have to Keep Your Windows Closed?

Updated: May 18

From my conversations with visitors to the house, I get the impression that there is a question about opening windows in an airtight home. Some people seem to believe that living in a passive house means you lose the freedom to open the windows when you want. This may not be a question in environments in Australia where either temperatures, urban noise, or air quality does not allow for a comfortable lifestyle. However, in the mild climate of Sydney and away from major transport infrastructure, in a part of Ryde where urban consolidation has so far been constrained, the answer may not be so clear. I certainly thought about when and why we were opening the windows, when I was contemplating my options for a new house.


Like most people, we mostly used the windows to ventilate the house and manage the internal temperate. In winter, despite being an old leaky house, the build up of stale air in the bedrooms overnight was noticeable by the smell and feel of the air in the morning. This was probably made worse by heating the room. To compensate, we would open the windows widely to let fresh air in. In too would come the cold, leading to a cold house during the day. The outdoor air temperature would slowly rise throughout the day, and warmer air would enter the house, but there was an uncomfortable lag while we waited for the internal temperature to rise.


In summer, if we were at home, we would again open the windows widely to prevent the build up of heat in the house during the day.  Otherwise, and for security reasons, the windows would be closed completely or left slightly open. On particularly hot days, we would need to leave windows open, and I would always worry about security overnight. An alternative would be to close up the house again and run the air-conditioner overnight.


Opening windows widely to ventilate a house has now been made more difficult by the NSW Government’s window safety laws, which restrict window openings for windows 2m or more above ground to 12.5cms. Incidentally, this is something to think about if you are planning to “night purge” heat from your passive house. To avoid this, we used large balcony doors onto a balcony. Clerestory windows could also be a good option. 


Bathroom and laundry windows were always open, even if just slightly, to manage moisture and prevent mold forming. Again, leading to a cold bathroom in winter and a possible security risk.


Also, it’s a bit hard to yell at your kids in the back yard when the windows are closed. There is that, too.


So, I guess the question to ask is, do I need to open my windows in a passive house? For me at least, the answer is no. But we still do, if we want.


Ventilation is taken care of by the heat recovery ventilation system. It is very impressive. The core temperature of the house is quite stable and recovers well, even when the windows have been open for a while. We usually open a couple of bedroom windows slightly in winter to lower the temperature to about 18°c. In summer, it is quite possible to air condition the house all day and then open up after the outdoor air temperature has dropped, or not run the air-conditioning during the day and open up to night purge any built up heat.


The issue of moisture build up in the house, including in the wet areas, is less straightforward.  This is usually not an issue in the hotter part of the year when the windows are closed and the air-conditioning is in use, as it will remove moisture from the air. From what I have been told, the magic number for relative humidity in the house is 50%. This is the number required to ensure that moisture moves through the wall and vapour-permeable membrane to the outside world. I have found this a bit hard to achieve without running a dehumidifier for long periods. My compromise has been to run the dehumidifier while people are showering and for a period after. I also check intermittently and run it if the relative humidity is over 60%(ish).  


Although it is tempting to think that you should throw open the windows to let in the fresh air. How fresh, and clean, is the air in Sydney? I am certainly no expert and not able to comment with any authority, but I was very surprised by the state of the HRV filter after 10 months and the amount of dust that had been picked up by the extract air vents over 12 months.


I’ve changed my mind. I now say, “kids, go inside and get some fresh air!"

Fine particles trapped in the Zehnder HRVS F 7 filter and dust in the conical extract air filters.

External view of the conical filter in the extract air filter.


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