'Breathability in buildings is not really about air. It is about water: water as a gas and water as a liquid; water inside the building, water outside the building, and water in the walls, floors and roofs themselves (though not about water in pipes!). It is not only about how water moves through structures (water vapour permeability), but also about the ability of materials to absorb and release water as vapour (hygroscopicity) and about the ability of materials to absorb and release water as liquid (capillarity). Water affects everything in building from the health or decay of building fabric, through to the thermal performance of the building and to the health of occupants. Particularly as we try to increase the airtightness, thermal performance and indoor air quality of our buildings, breathability has become a critical issue, affecting all areas both of new build and of renovation.' [Neil May, 'Breathability: The Key to Building Performance']
Why build a breathable building?
The main areas where water needs to be considered in building design are: external protection from weather; moisture within the building fabric; condensation on the inside of external walls; and moisture generated within the building.
The main strategy used in modern buildings is to try and exclude water completely: both weather from getting into the building from the outside and moisture generated within the building from getting into the fabric from the inside. In theory this seems fine. However, it relies on this being absolutely achieved, as it makes no provision for any moisture which does find its way into the building shell to get out; and the problem is that in reality it is incredibly hard to achieve. It is usually designed using impermeable layers and it relies on good detailing at all junctions (particularly between different materials) and a very high level of workmanship.
This is a common example of what can go wrong: a timber framed building is rendered with a cement render. Cement render is not very flexible and as the timber frame moves hairline cracks tend to form in the render. Water finds its way into these cracks and once it gets behind the render, it runs down the inside of the wall. If there is no provision for this water to get out, then it can cause the timber sole plate to decay.
Buildings were not always designed in this way. Historically, the timber framed building above would have been rendered with a soft lime render. This is much more flexible than cement, so tends not to get the hairline cracks like cement render. In addition, the lime is capillary open and absorbs water (rather than trying to exclude it like cement render), which later evaporates out. This helps to protect the timber frame, and keep it dry.
The idea of a breathable building is that it does not trap moisture, it accepts that the building is exposed to moisture and allows for it to dry out. It does this without compromising the water or air tightness of the building. We try to take advantage of the breathable properties of different materials, and juxtapose them to help protect the building fabric and thus create a durable and long lasting building.
Indoor Air Quality
Breathability is also important for another reason - it prevents mould from forming, which is harmful to human health and can affect the quality of the internal air.
Indoor air quality is also affected by the materials used to construct the building. A lot of common building materials are pollutants - the most well known example is standard paint. Previously, and still to a large extend, the use of these materials was generally not a problem because buildings were well ventilated. However, in our quest to make buildings more energy efficient we are making our buildings ever more airtight. This means the pollutants escaping into the air become trapped inside the building. We think the solution is to use natural materials which give off no, or very little, pollutants.
Other benefits of a natural & sustainable building
There are many other benefits of using natural materials, these are just a few:
Clay has the ability to regulate the internal relative humidity level to the most comfortable level for humans.
Having a well insulated and airtight house means you need less heating, which is cheaper.
We try to use local material, which means there are fewer carbon emissions required to transport the materials to site.
A lot of the materials we like to use require less energy to produce that standard building products - for example lime vs cement, sheeps wool, hemp, straw.
Where possible we also try to use recycled materials.
Natural Building Technology (http://www.natural-building.co.uk ) have a good article with more information on breathability at the following link:
Breathability: The Key to Building Performance by Neil May