Talk to anyone about old buildings and they tell you how important it is to let them breathe. Having only recently understood what this means, I find myself using the word breathability in the same way as I use the word sustainability – with the assumption that everyone knows what I’m talking about. So, for those who are interested, here’s a quick post to explain why buildings need to breathe.
There is an old Sanskrit proverb that says, “breath is life, if you breathe well you will live long on earth.” This applies to old buildings. Before 1919, when technologies such as damp-proof courses, cavity walls and cement made its way into construction, buildings were built using materials that absorbed and released moisture. Water vapour that crept up through walls from the ground would evaporate because of sunlight and heat flow from open fires. In very simple terms, breathability is about preserving this equilibrium between absorption and evaporation. Nowadays, why it’s hard to maintain breathability in old buildings is because modern construction materials such as cement, gypsum plaster and celotex are not microporous and are designed to repel moisture rather than absorb and re-release it. If you use cement render on an old solid wall then as moisture creeps into it from the ground it can’t be evaporated and will sit trapped within the wall and cause damp. This is why it is important to use materials that are suited to the construction type of your house.
Breathability is referred to a lot in terms of paint. Using a non-breathable paint on a lime-plastered wall will disturb its moisture equilibrium but how do you know if a paint is breathable? There’s no universal standard to measure breathability in paint. The most accepted one is the SD value which measures the rate of transfer of moisture and air. The lower the SD value the more moisture is able to pass through it which is a good thing. A truly breathable paint will have an SD value ranging from between 0.01 to 0.5. The most breathable paints tend to contain fewer synthetic materials and be better for the environment and your health. The most breatheable wall paints I’ve found are made by Edward Bulmer, Earthborn, Keim and Farrow & Ball.
You might be thinking though that breathability only matters in old houses and to a certain extent you’d be right. However, the latest thinking is that having interior walls that absorb moisture when humidity is high and release it when humidity has dropped results in a healthier, more comfortable living environment. Many architects are now designing buildings using materials that have high levels of breathability.