salt levels external walls

The new EWI Pro Heritage range is a lime-based system, with solutions for varying salt levels in external walls. The Lime Harling is ideal for substrates with low/medium salt levels, and the NaCl (as the name suggests) is best for those with high levels of salt. But how does salt affect a substrate, and why is it necessary to take salt levels into consideration?

Salt Levels in Heritage Buildings

It’s important to take salt levels into account when working with heritage properties. This is because the property has been around much longer, and so the salt will have accumulated to a greater amount than a new build. Heritage properties are also typically made of more natural materials, such as sandstone and limestone, which are more susceptible to salt erosion.

Salt levels in heritage buildings occur as the result of several chemical reactions, however, the most common cause is with the formation of carbonic acid which is formed when rainwater reacts with carbon dioxide.

The result is an acidic rain that breaks down and dissolves the calcium carbonate (salt) in lime-based stones and mortars, disrupting the bind of the material. If a sandstone building was built with a mortar containing calcium-carbonate, the calcium carbonate within the mortar can be dissolved and eroded. Limestone and sandstone are particularly porous materials, so the water containing the calcium carbonate (salt) moves through the external walls by capillary action, depositing the salt in different pore spaces.

Eventually, the pores within the external walls become filled with salt and the cycle of dissolving and depositing puts pressure on the walls. The salt generally makes itself known with the presence of efflorescence on the surface, which is essentially just salt deposits on the face of the wall. 

Cementitious renders and mortars are an inappropriate material to combat the effects of salt because trapping the salt behind the render will just cause further erosion of the building structure. Lime-based renders and mortars are a softer material, allowing the salts to push through. The point of this is that when the salt escapes from the substrate and settles on the surface of the render, it’s imperceptible to the eye and is far less harmful than if it were trapped inside the walls.

Rendering Walls that Contain a High Level of Salt

Rendering a salty substrate requires careful attention. The level of salt first needs to be established before choosing the materials; substrates with a relatively low level of salt require the use of the Heritage Lime Harling. This is a preparation layer before the Heritage Lime Basecoat and Heritage Lime Renders are applied.

Substrates with a medium/high level of salt will need the Heritage NaCl. This acts as a basecoat, so once applied the Heritage Lime Render can go straight on top. The Heritage Lime Plaster would also be an ideal solution for internal walls as a finishing coat on top of the EWI-269 Lightweight Basecoat.

Salt in Buildings: An Overview

  • Acidic rain causes salt deposits to form in porous building fabrics such as limestone.
  • Excess salt causes erosion and structural damage.
  • Lime-based renders are ideal for heritage buildings because lime is porous and allows the salt to escape rather than trapping it within the structure where it causes more damage.
  • Heritage Lime Harling is ideal for substrate preparation in cases where there is low salt content.
  • Heritage Lime NaCl is for cases where the substrate has a high level of salt, it is essentially a specialist basecoat whereby a lime render is applied directly on top of it.
  • Lime plasters for the internal of buildings are also ideal for heritage buildings.

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