This can be a confusing term for two reasons. First, because it refers to moisture not air and secondly because the exact physical processes which go on in a building are complicated (and still being researched). However the metaphor of breathing captures the important idea of continual movement – moisture in itself might not matter, just as long as it doesn’t take up permanent residence in your walls.
Broadly, breathability is the ability to buffer moisture – to soak it up and then release it - thus avoiding a situation where water vapour is retained by the fabric of a building. The technical term for this property is hygroscopic. Natural materials tend to be more hygroscopic – breathable - than synthetic ones. The term “vapour permeable” also gets used in this context: it means the rate of passage of water vapour through solid materials.
What is a “breathing (or breathable) wall”?
This term refers to a specific type of construction, not to an ordinary wall with retrofitted breathable insulation. A “breathing wall” will be made up of a gradation of vapour permeable materials – the materials towards the inside will be less vapour permeable than the materials towards the outside (5:1 is the minimum ratio often quoted). What a true breathing wall does not have is an impermeable vapour barrier. The standard wall construction includes a “vapour check” layer close to the inside of the wall.
This might sound a bit counter-intuitive: surely the less vapour permeable layers should be on the outside, to keep out the rain? Think about what happens. The interior of a house (in a climate such as the UK’s) usually has a higher relative humidity (RH) than the outside because it contains (exhaling, cooking, bathing), people. It’s a very general rule that, left to themselves, things will always migrate from an area of higher concentration to one of lower concentration, so there will be movement of water vapour towards the outside (or, to be more technical, the interior has a higher vapour pressure). At times of high interior RH, water vapour will enter the walls, lowering the RH (both high and low RH cause problems and the ideal range is 40-60%). At times of lower interior RH, almost all of the water vapour will migrate back into the interior to be taken away by air currents. Any water vapour that might have managed to penetrate the wall deeply will be drawn out towards the exterior because of the lower permeability of the outer layers, so no harm done. Any water coming in from the outside will be unable to get very far as it is pushing through layers of decreasing permeability.
Breathability in practice
Implicit in the previous paragraph is the important point that the breathability of materials interacts with both temperature (which affects RH) and ventilation (one of whose functions is to carry away water vapour). Mark Brinkley (author of the sustainable-construction manual The HouseBuilders Bible) made the following comment on the online forums of the Sustainable Building Association:
"What we are talking about here is essentially the interplay of water, air movement and heat within the building fabric. We know they are related, but we don't yet fully understand how.[…] Think about it. Breathing walls, humidity buffering, vapour barriers, breather membranes, lime v cement, natural materials v synthetics, even Actis and the multifoils. They all sit in the gap that exists between theoretical physics and our limited understanding of what actually happens on site."
That sounds a bit worrying – we can’t completely predict what will happen! – but it isn’t really. For standard situations – insulating a normal loft, or cavity wall for example - we do understand what is going to happen and for non-standard situations a reasonable prediction can be made. The important point is to always bear in mind that a building is a dynamic system, not an inert box.