When must windows be open in your Passive House? Answer: whenever you like and in these two circumstances

17 June 2024 by Rachel Rose

Aranet 4 CO2 monitor in a Passive House

As any Passive House homeowner knows, you can open your windows and doors whenever you want. The mechanical ventilation is delivering fresh, filtered, pre-warmed (or cooled) air 24/7 which means opening windows is not required for indoor air quality (IAQ). Not that New Zealanders are very good at that anyway.

But there are two specific circumstances in which it’s important you do have the windows open.

#1 Until the MVHR is commissioned

First is before you move in, when the building is at lock up stage but the MVHR has not been installed or commissioned. It’s vital the builders are leaving windows open (tilting is sufficient) to create airflow to remove any moisture in the house. That moisture will come primarily from materials being brought into the house, but all the tradies breathing will also contribute.

Depending on your eaves and the direction of driving rain, it is possible for rain to get in through an inward tilted window during particularly heavy weather (I learnt this the hard way). If your particular windows don’t lock in the tilt position, the builder will need to chock them open to prevent them slamming if the wind gets up, especially if you have cross ventilation. 

#2 Whenever the power goes out

Second, in the case of a power cut you’ll need to crack the windows to get some fresh air. How quickly you’ll need to do that and for how long depends on the size of your house and the number of people in it. For a brief outage, say up to 45 minutes, you’re unlikely to have a problem. Ventilation is needed to prevent CO2 building up to undesirable levels. Ideally, CO2 would stay below 1000ppm—this article goes into more detail. Humidity may be a secondary issue.

This is another case where monitoring is really valuable. I’ve written before about the Aranet 4 monitor I bought when we were camping out during the house build. It’s been just as useful now that we’re living in our certified Passive House home. We recently had an overnight power cut lasting 12 hours and it was concerning how quickly indoor CO2 levels climbed. This was easily abated by some manual ventilation (but we didn’t love the cold draughts).

There are other monitoring options, including permanently fixed monitors that record and store a variety of data points. Check whether they will work if the power is out and if data is displayed on the unit or only via your phone. Your MVHR control panel may report indoor temperature and relative humidity but obviously that data isn’t available when the power is out.

I’m really happy with our Aranet 4 choice. It’s not cheap but it’s a proper scientific instrument. (Cheap CO2 monitors use a proxy measurement and my research suggests they are not accurate enough to be relied on.) We can use Bluetooth to retrieve and save some weeks of data at a time and can get near instant readings for CO2, temperature and relative humidity anywhere in the house by simply moving the little device around.

Lately we’ve been experimenting with switching our MVHR into away mode (0 on our Steibel Eltron) for a few hours at a time when the house is lightly or not occupied. We’re experimenting to see how this might impact heating demand. Humidity is buffered very well here because of the materials we used, so the sole data point we need to respond to is CO2. If our CO2 levels are under 1000ppm (preferably 800ppm), we’re getting all the ventilation we need.

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