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?
What does it mean for a building to be “hermetically sealed”?
This is the definition of “hermetically sealed” from the Cambridge Dictionaries Online:
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.
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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