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.

 

Can Passivhaus teach the policymakers to love the occupant?

Ventilation was much in my mind as I dipped in and out of conversations and seminars at Ecobuild this week. And I began to notice a disturbing tendency for airtightness measures and ventilation to be discussed quite separately, with the costs and benefits of each addressed almost as though the two were unrelated.

Again and again, ventilation appeared to be an afterthought, or a problem, instead of an integral part of the assessment of building performance. Several people also lamented that  “well, you can get draft proofing done on a Green Deal, but ventilation doesn’t meet the golden rule, so who will pay for it? – it can’t be funded”. And alarmingly, this dangerous split seems to be reflected in the very structure of the Green Deal.[1]

This strikes me as rather like saying “you can get this operation that you need, but there is no money to sew you up afterwards”.

Ventilation seemed almost to be resented, an ‘obstacle’ in the way of deeper carbon cuts, limiting what could be achieved. All that fresh air spoils the building performance, it seems.

This put me in mind of a similar flavour you find in discussions about ‘comfort taking’. Once again, the pesky needs of the occupants are getting in the way of the true goal of carbon cutting. Thus in its Green Deal Impact Assessment  DECC complained that “comfort taking” leads to “underachievement in real-world energy savings”.[2] Is it really an “underachievement” to make someone’s house more comfortable?

In both these instances, there is a not-so-hidden subtext that the base animal needs of building occupants are a real hindrance to the low-energy, zero carbon perfection we are seeking. And in a way, you can see how if your job is to achieve these goals, a building with no occupiers would solve a lot of your problems.

So it might be unexpected in a way, that it takes Passivhaus, possibly the most shamelessly geeky and science-based standard of them all, to be the one that not only embraces the occupant, but actually starts from the occupant perspective. My understanding of Passivhaus design is it is built from the premise that occupants want comfortable, healthy buildings that are cheap to run. And yet, somehow, it looks as though Passivhaus manages to deliver the lowest energy and (I think you could easily argue) lowest emissions buildings of the lot of them.

Which is why (or at least one of the reasons why), as I remarked on Twitter this morning, I found that Passivhaus crept into my mind quite often, when I was pondering my uneasiness with what was being said outside the Passivhaus sessions. I think the Passivhaus lot are on to something.



[1]  I was told that for post 1919 buildings there is not anything in the Green Deal that requires the advisors to assess ventilation, even when they recommend draftproofing. There are especially serious concerns about the mass-scale installation of solid wall insulation, where the installer alone is responsible for the ventilation strategy, with no back-stop liability resting with a Green Deal provider. This is an issue that needs more than a blog to tackle – hopefully I will get the chance to come back to this soon.

[2] DECC June 2012 Green Deal Impact Assessment