When we talk about “breathability” in buildings, we aren’t referring to the movement of air within a building, rather the movement of water.
Water moves in and out of buildings constantly as both a liquid and as a gas, and is practically everywhere; inside the building, outside, in the walls, floors and roofs. Breathability is one of the most important relationships between the building and the world and it affects almost everything to do with the building health and performance.
Why is breathability so important?
Water moves through structures as part of water vapour permeability, but the types of materials used in the building can vary how water is released. The hygroscopicity of a material describes its ability to absorb and release water as vapour while capillarity refers to movement of water as a liquid.
Water affects everything in a building from the health or decay of building fabric, through to the thermal performance of the building and to the health of occupants. As we try to increase the airtightness, thermal performance and indoor air quality of our buildings, breathability has become a critical issue. All areas of new build and retrofit are affected by it, so it is crucial that we get it right. Our strategy for dealing with water in the air and in the fabric is central to the success or failure of the building as a structure that endures, performs, nurtures and protects (the main functions of buildings).
Why Moisture is Harmful
Damp external walls can have considerably lowered thermal resistance, while surface condensation on the inside of houses causes moulds which are harmful to our health. We therefore have 4 basic areas where the effect of water on building performance is considerable:
- On the outer surface: Rain penetration and other external conditions
- In the middle: building thermal performance and interstitial conditions
- On the inner surface: surface condensation
- Inside the building: indoor air quality
Why Airtightness Matters
For many of us, as users of buildings, the area where breathability matters most is Indoor Air Quality (IAQ) and the effect on human health. This issue becomes absolutely critical as we strive to make our buildings more energy efficient by airtight design. Airtight design is not about unventilated design. It is about ensuring no unplanned air leakage occurs through the fabric. Without a degree of airtightness, the insulation of most buildings is pointless.
As we try to reduce heat loss through buildings to a greater extent, the issue of airtightness becomes more and more important, however it is not only about heat loss. It is also about the migration of moisture into the fabric of buildings, and potentially about loss of thermal performance and interstitial condensation.
Building for The Future
Until we integrate this biological and physical understanding of water in buildings and its effects on performance and health, we are in danger of designing, constructing and repairing buildings which are going to fail in some, if not many, ways. New buildings and renovated buildings need to be built from low energy, minimally processed bulk natural materials. Materials such as timber, earth, stone, straw and other natural fibres are not only the best materials from an environmental point of view. They are also the best materials from a performance point of view. They provide us with houses which are simple to design, build, maintain, and which give health and satisfaction to those who live in them.