Natural ventilation – does it work?

While mechanical ventilation is sometimes perceived as problematic, expensive and possibly even energy-guzzling, natural ventilation often seems to be seen as – well – “natural” – a safe, old-fashioned,  reliable default solution. In this article for Passive House Plus I had a look at this assumption.

Theoretical modelling suggests that natural ventilation is likely to be rather unreliable, with the same building at risk of both under- and over-ventilation under different weather conditions. But what happens in practice?

The first problem I had was finding some data: there is very little of it.

In the studies I was able to find, it turned out that indoor air quality in naturally ventilated homes (including levels of relative humidity, oxides of nitrogen, and volatile organic compounds, for example) is not what it should be. (I also found some studies from schools raising similar concerns, but there wasn’t room to write about these as well).

For example, a study of 22 homes built to the 2006 Part F regulations for ventilation found that about half of them failed to achieve their recommended background ventilation rate even with all vents open/fans running as intended; pollutants exceeded the guideline levels in a number of them.

But what was really worrying was that when the researchers first arrived, they found that many of the vents were closed, and many of the extract fans (both in bathrooms and kitchens) had been disabled at the isolator. Similar findings appeared in all of the studies I was able to track down.

Unfortunately we do not seem to be very sensitive to the high relative humidity and other pollutants than are, nonetheless, dangerous to our health – but we are sensitive to draughts, and noise. This means that vents get closed and fans shut off, and our living conditions are unhealthier than they should be as a consequence.

What was interesting was to find that poor indoor air quality is not a new problem; studies dating back to long before airtightness was much of an issue, showed similarly poor indoor air quality and low ventilation rates. So the comfortable belief that natural ventilation is somehow “tried and tested” probably needs revisiting.

We seem to be facing a ventilation performance gap as worrying as the energy performance gap. As with energy performance, designers seem to have checked compliance with the Part F recommendations, looked at the results from their modelling exercises, and assumed this means a building is going to be properly ventilated in practice. As with energy performance, monitoring data shows this just isn’t the case.

At least the energy performance gap is now on the agenda – and we are even getting some practical solutions discussed. But despite the huge costs to the nation of respiratory diseases like asthma, and numerous other ailments worsened by poor air quality, ventilation is too often taken for granted. It’s probably time this changed.

 Read the article in pdf here Natural ventilation – does it work?

My thanks to Passive House Plus for the use of this document.

I have now added a post containing links to a number of the references used in this article, here.


9 thoughts on “Natural ventilation – does it work?

  1. Kate,

    Thank you for the helpful & important discussion. I, too, have searched for data on the problems of IAQ in “naturally” ventilated buildings. (I prefer to call them “randomly” ventilated, as I feel it conveys a more realistic image of what happens.) While this study was focused more broadly on crawl space performance, it did reach some alarming conclusions about what was happening in these test houses (up to 40% of the interior air originating in the crawl space, for example):

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