Table of Contents
| Opening windows in the Passive House? – It’s possible and allowed!
Can you open the windows in a Passive House? A frequently asked question – here is the brief answer:
Yes, you can open the windows.
Yes, you may open the windows.
This article provides evidence: research findings from occupied Passive Houses.
Want to see a short video first?
Windows in the Passive House: of course they can be opened
All Passive Houses which have been certified by the Passive House Institute have openable windows. Each room has at least one such window where possible; a turn-and-tilt fitting is optimal. This is, what we recommend - and it does not counteract the ventilations system, which is designed to guarantee good indoor air quality even if the windows are closed.
Why openable windows?
There are many reasons why occupants like to open the windows: to call out to someone, to hear the birds singing, to air the rooms… The wishes of the occupants have priority even if they lead to increased energy use (as a rule this doesn't happen in a Passive House, see below).
In German-speaking countries, Passive Houses do not require any active cooling in the summer (no air conditioning). In spite of that, they remain pleasantly cool even during very hot periods – provided that the windows can be appropriately shaded during the day and a high level of air exchange is possible at night. This is most easily done by opening the windows. These measures are not only necessary in Passive Houses – this is normal behaviour in traditional buildings too.
Like early June – year round
In Central Europe at the beginning of June, with typical June weather, heating is no longer required in a conventional house. Although the average outside temperature is only around 17°C, more comfortable temperatures are prevalent inside the house due to internal heat sources and solar radiation through windows. Sometimes it can even become too warm. The occupants then have to open the windows. When it becomes too cold, the windows are closed. At the beginning of June in Central Europe almost all houses are like Passive Houses. This time might be called the Goldilock-period (which is short for a conventional building; we use the term “Goldilock period” in analogy to the so called “Goldilock zone”, the zone in a planetary system, where the temperatures are not too hot and not too cold - just right to offer life a suitable habitat),
In a real Passive House, the period during which no heating is required at all stretches across a much longer period of time. Depending on the location, occupation and solar radiation, this period begins in March or April and ends in October or November. The “Goldilock period” in a Passive House is an extended period, lasting several months. During this period, in a Passive House the windows are used just like they are in conventional houses in June:
“ I open the windows when I feel like it. If it gets too cold, I shut the windows. Without any heating, it quickly becomes warm again inside (within 2 minutes)”,
as a long-term Passive House residents states.
And what is it like in winter?
In Central Europe, the heating period in a Passive House corresponds almost exactly with the calendar-defined winter: heating is required from mid-December to mid-March (depending on the regional climate, usage and building details, this can vary by ± one month). During the rest of the year, the heating system is turned off and the windows are used to ventilate as described above.
Many studies which have been published in previous years show that occupants open windows much less in the middle of winter than they do at other times (see illustration). This is understandable because opening a window on icy days or during storms or rain does not improve comfort.
|Measured data regarding the opening of windows in the Passive House
development in Wiesbaden: this varies from user to user, but in winter it is much less
than in summer – also in low-energy houses. The functioning of the houses
has not been affected the various uses observed here.
(from [Ebel 2001] ; extract here (in German))
The “normal” window opening behaviour in winter does not even provide for a sufficient exchange of air to guarantee an acceptable quality of indoor air. This is indicated by the frequently occuring moisture damage in traditional houses and by direct measurement of the indoor pollution, as well as direct measurement of the air changes. That is why the housing sector provides brochures to inform people about the necessity of carrying out intensive window ventilation on a regular basis. Without a ventilation system, at least every 6 hours purge ventilation has to be carried out , during the day and at night too - see Types of ventilation. For ordinary users this is bothersome and sometimes even impossible, which is why traditional buildings are much less ventilated in winter.
In the Passive House, this problem is easily solved: a ventilation system with heat recovery ensures an adequate supply of fresh air. The moisture generated in the house and indoor pollution are completely removed – whether the residents open the windows or not. The windows are only opened if there is a real reason for the users to do so.
And there is no excessive heat loss in spite of that?
Of course, opening the windows in winter increases the heat losses and also the heating demand of a house if the heating is in operation, thats true in Passives Houses, too – just as it is true in a conventional house.
What scientists find surpising is the fact that this additional loss is insignificant in 95% of all cases . The evaluation of the measured data showed that the window opening times in Passive Houses do not allow conclusions about the heating consumption levels..
Window opening times do not have a significant influence on the heating
energy use. So what alse causes the differences in energy use?
Thorough research has given an explaination - you find it in the following article: Energy efficiency of the Passive House Standard: Expectations confirmed by measurements in practice
Source: [Ebel, Kah 2003] .
In all studied cases the additional heat losses through the open windows during the heating period are not siginificant enough to impair the function of the Passive House. The energy consumption increases somewhat – usually by 1 to 2 kWh/(m²yr) and in extreme cases by up to 17 kWh/(m²yr) – but it remains extremely low in comparison with the consumption values usually found in conventional houses. The differences in consumption caused by the various temperature requirements in the home are much greater.
|This illustration shows the measured values for the heating consumption in the Passive House development in Wiesbaden in the same year in which the window opening times were recorded.
- The individual consumption values are certainly different, but
opening of windows hardly plays a role for these differences.
- The largest single consumption is “actually” quite small:
24 kWh/(m²a) equivalent to less than 2.5 litres heating oil per square metre and year.
The average is 1 litre (also from [Ebel 2001] ; Extract here (in German)).
On the far right (separate) the average heating consumption of a comparable housing development built according to current energy saving regulations (EnEV in Germany or OIB in Austria) are shown for comparison. More details about the comparison of measured values with those in “ordinary new buildings” can be found on the following page Energy efficiency of the Passive House Standard: Expectations confirmed by measurements in practice.
It is therefore wise to build a Passive House - and one doesn't need to change one's habits for that. Ventilation by opening windows doesn't have to be carried out – but even without it, fresh air and a high level of comfort always exists in the home.
It sounds so simple, but doesn't conform to the picture which most people have of an energy saving house, or even an “extremely” energy saving house.
That is understandable and rightly so, because sceptism about the new and the unknown saves one from disappointments.
In the case of window ventilation in the Passive House,long-term experiences of many users are available. Here are some statements by a Passive House resident in his own words:
“'Whether we want to open the windows in winter or not, and for what length of time, remains up to us . This has an insignificant influence on the Passive House concept, especially as no-one wants to keep the windows or doors open for more than a few minutes at minus temperatures <below frost>, even in ordinary houses. As a rule, opening the windows is not required, as the ventilation system (not to be confused with an air conditioning system) continuously provides for an adequate quantity of fresh air . The indoor climate has a better quality of air than an average new construction, even though airing several times a day in winter is not necessary here. Mould due to humidity cannot occur in the Passive House. In the summer, the Passive House is just like a normal house where windows are opened at night for cooling. The house where I live was planned with 13.6 kWh per square metre per year, and according to measurements it achieves 12.9 kWh with actual user behaviour and without any restriction of the user behaviour: Passivhaus 'Wohnen & Arbeiten' (in English).“
Now that the Passive House is no longer in the research and development phase – and in fact has been actual practice for decades – it is a easy to find out what it is really like: Passive Houses can be viewed and residents can be questioned about their experiences. On International Passive House Days property developers and owners of Passive Houses make it possible for those who are interested in experiencing a Passive House for themselves to visit these. The Passive House is a concept for everyone. Every property developer, architect, building constructor or investor can and may build a Passive House.
[Ebel 2001] W. Ebel, M. Großklos, T. Loga, K. Müller, B. Steinmüller: Wohnen in Passiv- und Niedrigenergiehäusern, IWU, 1. Auflage 2001 (An extract is available for free (in German): Wiesbaden Passive Houses settlement.
(In this development a lot of different measurements have been made in order to check indoor air quality, opening periods of windows and the actual energy consumption. A big part of the conclusions drawn in our article comes from these measurments).
[Ebel, Kah 2003] W. Ebel, O. Kah: Tracergasmessungen: Auswirkungen von Fensteröffnung bei kontrollierter Lüftung; im Tagungsband der 7. Passivhaustagung Hamburg, Passivhaus Institut, 1. Auflage 2003
(tracer gas measurements allow for a precise determination of actual air flows; the method has been used to determine the real amount of fresh air provided through an opened (or titled) window under otherwise normal operation of the dwellings. In this research we have learned, that the actual energy loss through a single opened window in a passive house is by far not as high as it has been suspected. That is one reason, why users may operate their windows just like they want to - as long as they close the windows again, when they start to feel uncomfortable.).