‘Breathability’ is not a strictly defined scientific word. Rather, it is a blanket term used to describe materials which exhibit a range of characteristics. The most relevant of these when it comes to building materials are, porosity, moisture diffusion, vapour permeability and hygroscopy.
In this context, these characteristics relate to the ability of a given material to deal with moisture – basically its ability to evaporate moisture away from itself.
Porosity in Lime Mortars
Lime scores much more highly than cement when it comes to these characteristics and this is one of the reasons why it is an appropriate material to use on pre 1900 buildings.
The pore structure of cement is relatively closed; whereas that of lime is relatively open and interconnected. This is porosity, and affects how well the material can wick liquid moisture away from itself.
Moisture Diffusion in Lime Mortars
Water will penetrate the pores of cement, but will not evaporate readily. If a cube of lime mortar and a cube of cement mortar are both saturated and then left in the same environmental conditions, the cube of lime will dry a lot faster than the cube of cement. This is moisture diffusion, and describes how well the material can lose moisture through means of evaporation.
Vapour Permeability in Lime Mortars
Vapour permeability is related to porosity and moisture diffusion, but describes how well a material will permit the passage of moisture vapour.
Consider the difference between a plastic coat and a Gore-Tex coat; walk up a hill in a plastic coat and moisture will start to condense on its interior. This is a good analogy to describe the difference in vapour permeability between lime and cement.
Hygroscopy in Lime Mortars
Hygroscopy describes the ability of a material to take in moisture from its surroundings. Its relevance in this context is mostly in terms of moderating internal humidity and reducing the risk of condensation within a building.
As stone and cob buildings have a high thermal mass, their walls are more prone to condensation when heating is not used in a sensible manner, i.e. maintained at a constant rate.
Modern paints and plasters have a low hygroscopy, so if moisture vapour condenses on them it sits on the surface and forms condensation. Condensation promotes mould, fungi, and other organisms deleterious to human health.
Lime has a relatively high hygroscopy, so any moisture condensing on the surface will be absorbed and then evaporate when the relative humidity of the air drops.
This is a very brief and simplified account of what constitutes breathability. It is suitable to give a basic understanding of what is meant by the term, though it is by no means comprehensive. Suffice to say that lime has a track record of over two thousand years as a binder and weatherproofing agent. When it comes to managing the movement of moisture and moisture vapour through an old building it is the sensible choice in the long term.