Passivhaus buildings are sometimes criticised by environmentally minded designers as being “hermetically sealed” boxes that can’t “breathe”. These designers have the best intentions of creating healthy and energy efficient buildings where people have direct control over their indoor environment and a close relationship with the outdoor environment. And they fear the passivhaus standard won’t allow a building to provide this. Luckily, their fear is unfounded. Let’s take a closer look at the two aspects of this criticism:
What does it mean for a building to be “hermetically sealed” and does the Passivhaus Standard require this?
What does it mean for a building to “breath” and does the Passivhaus Standard allow this?
Hermetically Sealed
What does it mean for a building to be “hermetically sealed”?
This is the definition of “hermetically sealed” from the Cambridge Dictionaries Online:
hermetically sealed
adjective /hɜːˌmet.ɪ.kliˈsiːld/SPECIALIZED (engineering) describes a container or space that is so tightly closed that no air can leave or enter it
DISAPPROVING separated and protected from very different conditions outside, in an unnatural way:
We drove past a row of squalid shacks on the way to our hotel, where we slept in air-conditioned, hermetically sealed rooms
From the dictionary definition we can see where the concern might arise. And it’s also quite clear that this is usually applied to buildings that don’t have opening windows and rely solely on air-conditioning to provide the air to breathe inside. It’s worth noting though, that the typical hotel room from the example that has no opening windows and relies on air-conditioning, probably isn’t actually very airtight!
Does the Passivhaus Standard require a building to be “hermetically sealed”?
On first glance it might appear that a passivhaus building does need to be hermetically sealed, as the Passivhaus Standard requires buildings to be completely airtight. However, a closer look reveals that a passivhaus building is not actually airtight; it is the building fabric that is airtight. This might seem like a subtle difference but it is important – you can open windows (and doors!) in a passivhaus building.
However, the only way to test if the building fabric is airtight is to close all the windows and doors and ensure that the building is hermetically sealed while being tested. This is to make sure that the walls, floors, roofs, window seals, door seals etc., do not leak air, and thereby waste energy and cause discomfort once the building is in use. Assuming the building passes the air test and the fabric is measured to be airtight, windows and doors can be opening as and when necessary or desirable, and of course the ventilation system operates moving fresh air into the building and taking stale air out.
And the Passivhaus Standard does not require air-conditioning for the ventilation system. For comfort and energy efficiency reasons mechanical ventilation with heat recovery is almost always required to meet the passivhaus standard. However, opening windows can form part of the ventilation strategy, as I’ve written about before. The ventilation system provides a consistent supply of fresh outside air to the inside, it doesn’t re-circulate stale indoor air.
A passivhaus building isn’t hermetically sealed.
What is a Building that “Breathes”?
It isn’t always clear what someone means when they say that a building needs to “breathe”. You could take it quite literally and debunk it as myth like Allison Bailes has done on the Energy Vanguard Blog. And as Martin Holladay also has on Green Building Advisor.
Or you could understand it to mean that a building needs a fully designed ventilation system, which is true for any kind of building. I wrote about that in a previous post about airtight buildings and ventilation.
Or you could understand it to mean that buildings need to have “breathable” construction.
“Breathable” construction
What is “breathable” construction? It is actually a misnomer for what should correctly be referred to as vapour permeable construction. This means using the right vapour permeable materials (predominantly, but not only, natural materials) in the right combinations to ensure that the walls, floors and roof can dry out. The drying out takes places as water vapour gradually moves from within the construction element to the outside, or to the inside of the building. This ensures moisture doesn’t get trapped inside any of the construction elements of the building. The moisture could come from the building materials used (for example timber), or from the weather during the construction process, or from humidity inside or outside the building once it is in use. It is important that moisture doesn’t get trapped in the construction elements as trapped moisture can cause damage to the building materials or can lead to mould and associated health risks for the people in the building.
Sometimes there is confusion about “breathable” construction and airtight construction, thinking that these are mutually exclusive. However, once the correct term, vapour permeable construction, is used it becomes clearer that this isn’t the case. It is absolutely possible for construction to be both vapour permeable and airtight (that is, draught-free).
Can a passivhaus building have “breathable” construction?
Or rather, can a passivhaus building have vapour permeable construction? Quite simply, the answer is yes. Many passivhaus buildings do have vapour permeable construction. It is undoubtedly true that there are passivhaus buildings that don’t have vapour permeable construction, but the Passivhaus Standard doesn’t require one or the other.
A Passivhaus Building is not a Hermetically Sealed Box.
Most, if not all, designers of passivhaus buildings want the same outcomes as environmental designers: healthy and energy efficient buildings where people have direct control over their indoor environment and a close relationship with the outdoor environment. Many passivhaus buildings have “breathable”, that is, vapour permeable construction.
Passivhaus architecture is environmental architecture.
What differentiates passivhaus architecture from other environmental architecture is science, building physics to be more specific, and the rigorous certification process. The environmental aspirations are designed, modelled, checked and built as intended.
Passivhaus buildings deliver what they promise.
Do you care about environmental architecture? Do you sometimes wonder if passivhaus might be at odds with environmental design? I write twice a month to dispel myths such as this – we both want the same thing when it comes to changing the world for the better through architecture.
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The concern may be based on the term “airtight” which is used as the technical description of the fabric of low energy buildings: In building construction “airtight” means a tightness, which is in the range of less than 1 m³/h at a pressure of 50 Pa for a m² of the building envelope. Now: If you look at systems like aircraft or spacecraft, if these would be so “untight” like a building envelope which is considered to be quite tight, these craft would never fly (… the air would be gone within a minute). Therefore: What we call “airtight” even in the case of a passive house is not very “tight” at all; yes, its tighter than most old buildings are; “airtight” in this sense of building construction is just tight enough in order to avoid any danger from drafts and to avoid structural damage by air in- and exfiltration.
To have an impression of the quality of tightness needed: be aware, that a commonly used inside plaster ranges well below 0.2 m³/h/m² @50Pa – and therefore is tight enough even for a passive house. And, of course, the plaster is a vapour permeable construction, giving an example of the combination “air tight” – “but water vapour permeable”, the breathable construction Elrond Burrell has been talking about.
Hi Elrond,
Great article yet again, but just one thought:
You note “Passivhaus architecture is environmental architecture” and I wonder what you mean by ‘environmental’? If you’re equating Passivhaus with energy efficient, comfortable buildings that have much lower CO2 emissions than ‘normal’ buildings than that makes sense.
But if you’re equating Passivhaus with low-impact, healthy, non-toxic construction then there’s a bit of a problem.
You see (which I know you do), a Passivhaus standard building can be built out of anything. You could, for instance, build a Passivhaus building from spent uranium rods covered in urea formaldehyde as long as it achieved the PHPP criteria. Daftness aside, you could also build it from treated timber, blown synthetic insulations and lots of off gassing plastics – which many do. While it would most certainly be energy efficient and have low CO2 emissions it wouldn’t necessarily be very environmentally friendly.
So here’s the thing: Passivhaus is probably the single best energy efficiency standard that exists in the world. But for a building to really be green – for it to be ‘environmental’ – it has to do a heck of a lot more than just be energy efficient and comfortable.
And so it seems I beg to differ: Passivhaus architecture is not necessarily environmental architecture.
This isn’t semantics; just a clear mission to make sure that we all keep our eyes open and our heids screwed on when we start talking about this stuff.
Hi Sam,
Thanks for the great comment.
Firstly, it highlights a particular problem: there is no agreed definition for “environmental architecture”. Hence my provocative statement is subjective. And I personally agree with you on all the aspects that need to be addressed for “environmental architecture”.
Secondly, climate change is the overriding environmental concern that we must address above all others. If we create architecture that is “low-impact, healthy, non-toxic” but isn’t radically energy efficient, and isn’t comfortable to occupy (bearing in mind comfort and long term health are related), then all the other ‘environmental considerations’ run the risk of appearing frivolous as our planet rapidly becomes uninhabitable and our species heads towards extinction. Our continued profligate use of energy in buildings is a massive cause of CO2 emissions and hence climate change. And leaving aside direct effects on our own species momentarily, the impact of climate change on the wider environment in all its forms is already massively destructive.
So the single best thing we can do for the environment is to emit radically less CO2. And for architecture, energy in use (in the majority of cases) is by far the largest cause of CO2 emissions. Any architecture that purports to be “environmental” must start with radically reducing energy use and CO2 emissions as the top priority, and once that is tackled, other aspects should follow.
So I stand by my statement: Passivhaus architecture is environmental architecture.
Best wishes, Elrond
PS my manifesto covers similar ground, as does the story behind my blog.
Hi Elrond,
Good debate! I suspect that we’ve different approaches and ideals when it comes to green building.
You’re absolutely spot on when you note that “If we create architecture that is “low-impact, healthy, non-toxic” but isn’t radically energy efficient, and isn’t comfortable to occupy…then all the other ‘environmental considerations’ run the risk of appearing frivolous…” The ‘best’ possible building would naturally be one that was energy efficient, constructed from non-toxic materials and that was beautiful to use.
But I fundamentally disagree with the notions that a) “climate change is the overriding environmental concern that we must address above all others” and b) the single best thing we can do for the environment is to emit radically less CO2″.
Firstly, bee population decline (for example) is just as important an environmental concern as climate change; secondly, there really isn’t a single best thing for the environment – there are lots and lots of wee things that all make a tiny difference individually but that add up to create much greater benefits.
As humans we really can’t afford to lavish ALL of our attention on single-mindedly trying to reduce CO2 emissions without making sure we get all of the other, equally important, bits right at the some time. (Such a reductionist approach will only bite us on the bum, as history has shown us before.) As building designers these other bits include the low impact, non-toxic, healthy stuff, vapour-permeable construction, biodiversity, social inclusion, etc etc etc.
But back to the original thread…your article focuses on how Passivhaus buildings CAN (and should) have opening windows and how they CAN be built from natural vapour-permeable materials. As you also note, there are a few PH buildings in the UK that do exactly this (e.g. http://passivhausbuildings.org.uk/viewproject.php?id=312 and http://passivhausbuildings.org.uk/viewproject.php?id=323).
What I’d like to see is the Passivhaus Institut acknowledging the health and comfort benefits to occupants of using non-toxic materials and vapour-permeable construction – as well as energy efficiency – to create healthy low energy buildings. (After all, we don’t exactly want toxic, unhealthy buildings!) Your Manifesto says exactly this too.
It’s a shame I’m up here and you’re down there as we really need to get a pint!
Sam
Hi Sam,
Sorry I’ve only just got around to replying to you again.
I think actually we have very similar approaches and ideals. We may appear to diverge slightly here because of my writing subject but probably not much else.
I agree we can’t be single minded to a point of being reductionist, however, we do need to focus on key issues and respond to them well. Too often we attempt to do a little of everything with a result that nothing really gets tackled properly. And part of my point in writing here is to help people see why passivhaus is so important and makes a tangible difference. Environmental issues can seem too broad and too overwhelming and are too often expressed in depressing disempowering ways, we need more practical tangible approaches everyone can grasp.
Nice choices of projects to share by the way. 🙂
Next time you’re down this way or I’m up that way we’ll have to find a watering hole.
Cheers, Elrond
[…] spaces. In contrast, in the passivhaus buildings it has been noted that this doesn’t happen. The balanced ventilation and airtight fabric means the cold outside air isn’t drawn in, there are no cold draughts created and there is no […]