Breathable and airtight construction explained…..

If you are interested in building to low energy standards, particularly the exemplary Passivhaus standard, you may well have come across terms such as ‘breathable’ and ‘airtight’ construction. Our clients often come to us confused about the meaning of these terms and wonder how they can be applied to the same building!

And indeed they are confusing terms…. and definitely sound like they should be mutually exclusive! It would be good therefore, to start with a simple explanation of what these terms mean when used in reference to construction, before we can understand how they work together to create high performance, healthy buildings.

Let’s start with airtightness. An ‘airtight construction’ sounds like it might create a horrible, stuffy environment, and some of the myths around not being able to open windows in a Passivhaus, stem from this thinking. In fact, an airtight building fabric is about minimising warm air leakage and incoming cold draughts through material junctions and service penetrations in the building fabric and controlling the ventilation of spaces through both natural and mechanical means.

As insulation levels in our buildings improve, the importance of air leakages through key junctions and service penetrations becomes increasingly important. If warm air is able to leak out of our buildings, it undermines the value of the insulation and occupants will be left with higher heating bills than expected and a cold draught on the back of their necks!

If however, through careful detailing and high quality workmanship, we can create an airtight and thermally efficient building fabric, we can control the temperature of the fresh air entering our buildings and reduce our cooling or heating demand depending on the season. Natural and mechanical means of ventilation both have their place and can be used together to create healthy and highly efficient buildings. While using cross ventilation from opening windows might work well in the summer to provide fresh, cool air, mechanical ventilation with heat recovery is a good way to provide fresh air in winter months that has been pre-warmed by the heat in the outgoing stale air, thus retaining the heat within the internal spaces efficiently.

So if an airtight fabric is so important to energy efficiency, why then do we talk about ‘breathable construction’?

The term breathable in this context refers to moisture rather than air. If moisture gets into a building fabric and is unable to escape or dry out, it can cause structural decay and can create unhealthy internal environments through mould growth, mites and viruses; this is exaggerated in poorly ventilated buildings.

 To avoid this, we use materials that are ‘breathable’. These are materials that have the following characteristics:

-       Vapour permeability – they allow water vapour in the air to pass through the construction

-       Hygroscopicity – the ability of a material to absorb and release moisture

-       Capillarity Action – the ability to release absorbed water through its capillaries

All materials have these abilities to some degree, but they are negligible in many of the most commonly used building materials, and unless a material displays all these characteristics, it can’t be described as ‘breathable’.

 The natural fibres in bio based materials are particularly good at regulating moisture, absorbing and releasing it according to the humidity levels and helping to maintain a healthy internal environment that is neither too dry nor too damp. If the vapour resistance of the materials within a construction build-up decrease in the same direction as the vapour drive (usually the warm side to the cold side), then water vapour in the internal environment will be able to follow the path of least resistance through the structure to evaporate safely in the open air. This ability to ‘breathe’ helps reduce the risk of interstitial condensation, and together with a good ventilation strategy, will help maintain a healthy indoor air quality and a robust building structure.

 Maybe from now on we could start using the terms draught-free and moisture regulating… not quite as catchy, but definitely more accurate!!