I have come across a few people who when talking about breathable house structures state that they keep their windows open a bit and the chimney helps. The term breathability / breathable does conjour up images of lungs and hence the passage of air through a wall. However, what we are talking about is moisture transfer.
The mainstream industry narrows this down to the transfer of water vapour. However, when looking at older solid walled buildings we actually need to think of breathability as the transfer of moisture. This means water as a liquid as well as a vapour. Anyone with an old house will recognise this as being a major difference and one that can cause a host of problems: After all rising damp, driven rainwater ingress, etc are liquid water issues.
So in order to make good decisions we must accept that for older houses we must think of breathable walls as one that allow the movement of water through their structure.
The general rule of thumb (even for modern timber framed buildings) is that the outer elements of a wall should be between 4 and 5 times more breathable than the inner elements. This means that a gradient of moisture transfer is maintained in the structure where water is encouraged to move from the inner elements to the outer ones. By allowing water to pass through to the outside keeps the inner elements free of damp. The outer elements are allowed to get wet, but the thickness of the structure means that any external water should remain in the outer elements (issues like poor maintenance can still affect the workings of breathable walls).
Materials have different porosity’s and hence understanding how ‘breathable’ they are is really important. Other factors like surface area, orientation, shading, climate are also important, but the breathability of building materials is key. I shall not go into detail here, but understanding the principles of breathability is vital to making informed decisions over renovation / refurbishment work.
If we close off the breathability of a wall by using materials like: insulated plaster board, cement renders (especially those with water-proofing in), conventional paints, we are starting to interfere with how it works. These modern materials are designed to be waterproof (even if they have a degree of water vapour breathability). So if we apply them to the outer elements of buildings we are asking for trouble. The way out for water from the inner elements is blocked and hence the concentration of water will start to increase in the wall. This is made even worse and potentially dangerous when liquid water is not stopped from entering the wall (e.g. rising damp, wind driven rain, leaks from guttering, poor seals around windows / doors etc). This water just sits in the wall and can start to rot joist ends, transmit heat more effectively, cause mould, …
Even making the inner leaf less breathable by using conventional paints, modern insulations etc can cause problems. Breathable walls mainly dry to the outside, but water is also given off to the internal environment. By stopping this you will find that paint can start to peel off, that condensation issues are made worse and ultimately that the moisture gradient changes from internal to external to being external to internal!
Messing with a solid walls’ natural breathable state is fraught with hazards. Maintaining the wall’s breathability is therefore really important. So don’t get fooled by conventional materials that state that they are breathable and suitable for solid walls, they are only using measurements for water vapour. You need porous materials for porous walls.