‘Breathability’ isn’t a strictly defined scientific term. It’s a broader term used to describe a range of characteristics within materials. The most relevant of these for building materials are porosity, moisture diffusion, vapor permeability, and hygroscopy.
In this context, these characteristics relate to a material’s ability to handle moisture: its capacity to manage the movement of liquid water and water vapor, and its ability to permit evaporation. When it comes to breathable wall systems, lime has more desirable characteristics than cement in these areas, making it a suitable choice for pre-1900 buildings.
Porosity in Lime Mortars
Porosity refers to the nature and connectivity of the tiny voids present within a material. A material can be porous but not permeable if these voids aren’t interconnected and of a suitable size to allow the passage of liquid water. In lime mortars, the pores are connected, allowing the passage of liquid water. This enables water within the wall to move to the exterior and evaporate.
Moisture Diffusion in Lime Mortars
While water can penetrate both lime and cement mortars, lime allows for quicker evaporation. If you were to saturate a cube of each and place them in the same conditions, the lime cube would dry faster. This is moisture diffusion, the material’s ability to lose moisture through evaporation.
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Vapor Permeability in Lime Mortars
Vapor-permeable materials allow the passage of water vapor. This is crucial for old buildings to absorb and release moisture. Materials like masonry paint and waterproof cement renders have low vapor permeability, while soft lime mortars have high permeability, helping to regulate internal humidity and prevent damp and mould.
Hygroscopicity in Lime Mortars
Hygroscopicity refers to a material’s ability to absorb and release moisture from its surroundings. This is important for moderating internal humidity and reducing the risk of condensation.
Stone and cob buildings, with their high thermal mass, are prone to condensation if not heated consistently. Modern paints and plasters have low hygroscopicity, trapped moisture in the wall and encouraging condensation and mould growth on the wall surface. Lime, on the other hand, has higher hygroscopicity, absorbing moisture and releasing it when conditions change.
Disclaimer
This is a simplified overview of breathability. While it aims to provide a basic understanding, it should be regarded as a brief and simplified introduction. Lime has a long history of use as a binder and weatherproofing agent. When it comes to managing moisture and vapor in old buildings, it remains a sensible choice.
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