The use of Lime in building materials has a long history, dating back to the times of the Roman Empire, when lime was used for construction purposes such as mortars and render finishes. It continued to be used until the nineteenth century until Portland Cement and other cements came onto the scene, when the use of lime-based materials began to decline.
Lime is still frequently associated with vernacular buildings within areas in the south of England along The Jurassic Coast, and in places such as Bath and the Cotswolds. Lime has also recently experienced a bit of a revival in modern building and construction for use in modern renders.
So why would you want to use lime as a component in your render finish for your EWI system?
The Use of Lime in Building Materials
Lime is considered to be a ‘healthier’ building material in comparison to Gypsum and cement-based products. This is because quicklime absorbs carbon dioxide during the setting process of carbonation – otherwise known as the lime cycle. Essentially, the lime cycle is the hardening process of lime mortar and lime wash when water evaporates from lime putty and the lime reacts with the carbon dioxide present within rain water.
During this repeated cycle, the lime experiences a repeated chemical change until finally it is converted back to calcium carbonate (which is basically the original limestone). This is a slow process, but essentially what this means is that your lime mortar will eventually get stronger as time goes by. This process creates the oldest and most flexible and breathable form of lime.
Another reason that lime is frequently used in mortars and building materials is due to the fact that lime is a caustic. This primarily means that it has disinfectant qualities – lime mortars, renders and washes have been used to create hygienic and comfortable surfaces for buildings for thousands of years.
Lime can also be produced on a small scale; it can be produced in small quantities to meet primary demands, therefore saving energy and resources. With reference to the use of lime in the Roman era, and the gradual hardening of lime during the lime cycle – lime-based renders are also durable and have historically stood the test of time, which means that reproduction of materials for repairs is less necessary. It’s therefore a favourable option for those who are environmentally conscious but also want a durable render facade.
Lime-Based Renders and EWI systems
Lime as a component in building materials adds the benefit of breathability and vapour permeability; the greater the amount of pure lime in the building materials, the better the breathability. Because lime is porous, it absorbs and releases humidity (it breathes), therefore helping to maintain the thermal comfort of a building. This makes it fantastic for older buildings by allowing the building to breathe, and also in external wall insulation systems.
Lime-based render is an excellent addition when used in external solid wall insulation systems. This is because the breathability of lime means that it can prevent ingress of moisture, which as we know would disrupt the effectiveness of the system. This is because any kind of moisture content prevents the insulation material from retaining heat properly.
Due to its small particle size, lime can fill minute voids within a surface, which makes it a great adhesive in comparison to cement which has large particles. Due to this, lime also binds gently to background materials, allowing for flexibility and crack resistance – although lime-based renders are more likely to develop fine, hairline cracks in comparison to larger cracks within cement-based renders.
When installing lime-based render, it cannot be applied during freezing temperatures, as this will delay the carbonation process and the render can take up to a month to properly set which may cause the render to fail. Lime-based render must also be treated with a breathable finish such as lime wash or silicone paint to protect the underlying render.
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