This is a Passivhaus Basics blog post that gives an overview of a specific aspect of the Passivhaus Standard.
The Passivhaus Standard requires airtight construction. What does this mean?
Essentially it means a draught-free building envelope.
A clear airtightness strategy is required to achieve this. The airtight line needs to be continuous even when formed of different materials. And it needs to be joined up, even where there are penetrations.
Sometimes airtight construction gets confused with how a building is ventilated or with ‘breathing construction.’ This post clears up these particular confusions.
And why does the Passivhaus Standard require airtight construction?
Airtight construction is draught-free construction. It is an essential part of the Passivhaus Standard to protect the building envelope, to ensure radical energy efficiency and to provide exceptional comfort.
Integrated design is a key element of successful passivhaus design, as I wrote about in a previous post. Certified passivhaus designers and consultants are often architects, building services engineers or sustainability specialists. So how important to the integrated design process is the structural engineer? The answer is: vitally important!
There are significant benefits in having a structural engineer who is also a certified passivhaus designer on the team as this post explores. Where this isn’t possible, at the very least the structural engineer needs a good understanding of passivhaus and the importance of their role in the design process.
Structural engineering has a significant impact on design simplicity, thermal continuity, airtightness and more.
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?
I often read and hear people, including passivhaus experts, state that once a building is airtight it requires mechanical ventilation. This is not strictly true. And there are many examples of passivhaus buildings that operate using ‘natural ventilation’ during the summer months. An airtight building significantly reduces air movement through the building fabric: walls, floor, roof, window frames and junctions etc.. And we should never rely on this kind of air movement to provide the air that we breath inside a building, for ventilation, that is. (Fancy drinking the water that seeps through a leaking roof? It’s kind of similar.) A ventilation strategy and system that genuinely does work, and is used properly all year round, is needed in any building, airtight or not. Once you add comfort and energy efficiency into the equation, this almost always means mechanical ventilation with heat recovery will be required for some of the year. The comfort and energy efficiency benchmarks of the Passivhaus Standard certainly mean this is the case.
And this is where airtightness and mechanical ventilation converge – both are needed for comfort and energy efficiency.