Posted On18 Mar 2019

Airtightness, Airtightness, Airtightness

What makes a building low energy?

As most readers know, Passivhaus is one of the leading ways to realistically reduce the need for heating or cooling energy in buildings, both new build and retrofit. I say “one of the” but I don’t personally know of any other that works. Minergie in Switzerland is OK but it’s really a Passivhaus copy.

So what are the fundamental propositions of Passivhaus? I would say there are three:

  1. The building must be airtight. Wow – living in a plastic bag! How horrible! Well actually its quite comfortable. No draughts, constant temperature, maybe a bit weirdly uniform. In fact, after the initial exposure, most people don’t even notice the environment internally, which is a definite success! After all, thermal comfort = lack of thermal stress. But how can we ensure that the air is fresh and smells and excess humidity are removed?
  2. This is the second point: The building must have a controlled ventilation system to ensure indoor air quality is maintained at all times. So far, the only systems we have used are mechanical, since all the natural ventilation systems we have found are unable to provide a consistent ventilation rate, instead vary dramatically with the weather. People generally will open windows only when the internal air quality conditions are quite bad.
  3. And the third is insulation. Lost of insulation, all joined up to itself like a good polar suit all round, no leaks anywhere. And that’s where airtightness comes in again. If air can leak through to the other side of our insulation, then its not doing anything. Bob Lowe reported this as “convective bypass”, but it’s basically crummy workmanship (or to be kind, installation without any understanding.)

So why have I titled this Airtightness, Airtightness, Airtightness? Well if we look at the three options above, they all end up requiring airtightness to make sense. The heat recovery aspect of ventilation is a waste of time if you home has a leakage of much above 3 achr at 50Pa. And if the insulation has air washing around both sides due to a lack of airtightness, its not going to do anything.


low energy building = airtightness