Why do I need to use lime?

1. Lime-based mortar is more flexible and less brittle.

If there is building movement, lime mortar will eventually re-seal the cracks in the mortar. Having a softer mortar means soft bricks and stones won't be forced to break. The mortar is the "sacrificial component" of any masonry wall.

2. Lime-based mortar is breathable. This means that moisture that gets in the wall (from damp conditions inside or outside) will escape rapidly. Lime mortar will draw water out of the masonry and allow it to evaporate, reducing dampness and the risk of spalling in freezing conditions.

3. Lime-based mortar is long-lived. Lime mortar that is made from 98% pure calcium will last decades.

4. Lime-based mortar is softer. This means it can be removed easily without damaging the bricks or stones. Masonry bonded with lime can, therefore, be reused, which is great for sustainability in construction.

5. Lime-based mortar is more environmentally friendly as it sequesters a high percentage of the carbon dioxide released through its production during the process of carbonation. It is also produced at lower temperatures than cement and, therefore, produces 20% less carbon dioxide in the first instance. 

7. Lime-based mortar is more aesthetically pleasing.

8. Lime-based mortar has a soft, natural, traditional appearance.

9. Lime-based mortars are set more slowly. This means they can be reworked for up to 24 hours.

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Lime's ability to deal with moisture is one of the key issues:

Most materials are vapour permeable. Not many are capillary active. What this means is that moisture in the form of water can move through the material. Modern buildings rely on damp-proof courses and cavity walls to essentially make a raincoat for the house. Traditional buildings have solid wall constructions that absorb water when it's raining and release it again when it's dry. The thickness of the walls prevents moisture from reaching the internal plastered surface before it dries out again. When cement-based materials are used on traditional buildings moisture gets in (in part, as a vapour from inside) but if it condenses within the wall then it cannot get back out. This is the case with cement pointing. Cement render is even more insidious because if cracks open in the render, wind-driven rain can get behind it and accumulate. 

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A benefit of lime plaster is that it functions in much the same way as the external wall surface. It can take on moisture when conditions are particularly humid internally, and release it again when conditions are drier. This goes a long way to managing internal moisture levels. In an ideal world, and when environmental conditions allow for it, moisture within the building will travel through the wall and evaporate to the outside air.

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These factors are the basis behind lime being termed 'breathable'. It must be noted however that in the context of historic building conservation, breathability refers specifically to the movement of both vapour and water. Materials that allow only the passage of vapour would not be considered 'breathable' when used in traditional building environments.