Wind driven rain is a major problem especially in coastal areas, as it can saturate the building fabric, leading to staining, spoiling of internal decorations, even to structural damages long-term.

Salts dislodged by driving rain from the fabric can be deposited by water onto internal surfaces leading to efflorescence and crumbling of the wall fabric.

Current (Modern) Solutions to Driving Rain

In order to protect old walls from driving rain, a common solution is rendering the wall fabric with a waterproof render

The fabric of older buildings – typically the ones built before the 1930s – often contains a significant amount of moisture, which is kept in check by ongoing natural evaporation. In order to keep moisture levels low, the render used for the waterproofing of old buildings must allow this evaporation, i.e. it must be breathable.

Because waterproofing and breathabilty are two opposing characteristics which are technically difficult or expensive to achieve, in the process of waterproofing breathability is often overlooked or sacrificed. This oversight results in the gradual build-up of moisture inside the wall fabric, leading to (often undetected) major dampness problems in old buildings.

Due to its widespread use and low cost, one of today’s most common rendering materials is cement – a material that offers good weather protection, but due to its low porosity and high density is also non-breathable.

Some of the reasons why cement-based renders should not be used in older buildings are:

  • Non-breathable: they don’t let moisture freely evaporate from the underlying (damp) wall fabric, resulting in an excessive build-up of moisture leading to long-term dampness problems.
  • Too hard: they stress the underlying fabric, resulting in detachment of the render which can damage the softer building fabric.
  • Too brittle: being inflexible, ongoing vibrations and building movement crack cement renders. Once their waterproofing ability gets compromised they let rainwater in, becoming part of the dampness problem. 
  • Poor heat insulator: being a non-porous, dense material, cement is a poor thermal insulator, resulting in condensation and mould problems.

The Building-Friendly Solution for Old Buildings

The ideal solution to the problem of driving rain in older buildings is a rendering system that is both waterproof and breathable.

The technology of a traditional rendering system that fulfills both of these criteria originates from ancient Rome. Being outstanding architects and builders, the Romans have discovered that by adding volcanic soils and other minerals to lime, they can significantly alter its properties, especially its mechanical strength and water resistance, while retaining its breathability. 

The materials most commonly added to lime were pozzolans (volcanic soils or rock fragments) and cocciopesto (milled bricks or terracotta fragments). These reacted chemically with the free lime, forming water resistant compounds. Such mortars were able to harden quicker not only in the presence of water but even underwater in the total absence of air, and they are known as hydraulic mortars.

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It is important to understand that the degree of porosity and breathability of lime plasters is primarily determined by the properties of lime and not by its hydraulic additives, an important factor being the firing temperature of lime. By firing limestone at low temperatures (at around 850-900 °C), the breathability of lime is retained, while the carefully selected hydraulic additives make the mortar fully waterproof.

These are different from today’s NHL mortars, whose much higher firing temperatures (around 1200 °C) impair their breathability.

These lime mortars have stood the test of time, being extensively used by the Romans in very demanding environments including sewers, ports, spas and aqueducts. They have also been widely used in Venice, well suited to the humid and aggressive environment of the Venetian lagoon.

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