Re-Rendering Old Buildings

Solving Wind Driven Rain

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

Salts dislodged by driving rain from the fabric can be washed and deposited on the internal surfaces of walls leading to efflorescence and crumbling of the plastering.

Rendering old buildings is one of the solutions against wind driven rain.

Current (Modern) Solutions to Driving Rain

In order to protect old walls from driving rain, a common solution is the rendering of 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 renders used for the waterproofing of old buildings must permit the evaporation of moisture, as such they must be breathable.

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

Due to its widespread use and low cost, one of the most commonly used render materials today is cement – a material that promises good weather protection - however in older buildings it also comes with some notable drawbacks.

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

Chemical incompatibility with old buildings: cement contains many chemical additives and salts (up to 10%). In the presence of moisture, these salts from the cement continuously migrate into the masonry¹, steadily increasing its salinity, leading to spalling, crumbling of the underlying wall fabric and the detachment of the cement render. A hollow-sound render, when tapped, indicates this detachment.
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.
Hard and brittle: being dense, rigid and inflexible materials, ongoing vibrations and building movement make the cement crack. Once their waterproofing ability gets compromised, cementicious materials let rainwater in, leading to additional dampness problems over time.
Poor thermal performance: being a dense material, cement plasters in general are very poor thermal insulators, causing condensation and mould problems.

Lime-based breathable render alternatives exist that can solve all water penetration problems fully in a building-friendly way with no drawbacks, keeping the wall fabric breathable long-term.

The Lime Alternative

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 fulfils 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.

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.

hdr — Roman volcanic sands and ashes added to lime by the Romans

Waterproof and Breathable Lime Plaster - How Does It Work?

By adding volcanic sands and ashes (natural pozzolans) to the lime, the Romans have developed a wide range of lime plasters for a very wide range of applications - including many waterproofing applications which today are solved with modern Portland cement. Some of these Roman lime plasters were able to set underwater, being suitable for very damp environments. The longevity of these plasters was also outstanding, many of them lasting for centuries.

How can a lime plaster be waterproof and breathable at the same time? Modern research² gave us a better understanding on how these plasters work. It all has to do with the pore structure of the material.

Normal lime plasters are macroporous, which means they have relatively large pores. Looking at their pore size distribution, most pores in air-limes are between 0.1 to 1.0 μm, with some larger pores in the 5 to 50 μm range.³

lime-pore-distribution-core-conservation — Lime mortar pore size distribution, allowing both vapours and liquid water

Liquid water can only circulate through pores larger than 0.2 μm in diameter. Water can't penetrate pores smaller than 0.2 μm. Due to their pore distribution, normal lime plasters allow the passage of both liquid water and vapours.

The addition of volcanic ingredients to lime, depending on the type and quantity of volcanic materials mixed, can change the pore size of lime plasters. The right mix and quantity of volcanic ingredients can reduce the pore size under the critical 0.2 μm threshold, making some of these lime plasters completely waterproof while retaining their breathability.

Thus Rinzaffo MGN is an open-pore microporous (small pore) lime plaster that acts as a breathable water and salt filter. Its small pores stop liquid water. Its open pore structure allows the wall fabric to breathe. Its small open pores are impervious to salt penetration and crystallization, being free of crumbling and decay. This results in an extremely long service life of this plaster (several decades) even in very damp and demanding environments (e.g. basements), lasting much longer than any NHL plaster or other waterproofing, dehumidifying or restoration plasters operating on other principles.

coreconservation dampness natural pozzolan59 — Volcanic lavas (volcanic pozzolans) are highly porous and breathable materials

The MGN Lime Rendering System

Normally, the rendering applied on an old building consists of a main render coat - cement, NHL or lime - applied directly onto the wall fabric, with an optional finishing on top.

The MGN lime rendering system, that "ticks all boxes" - breathability, water and salt resistance, moisture control, long life expectancy - takes this concept further, by applying the following 2 plaster coats:

Base coat: the Rinzaffo MGN Roman breathable lime waterproofing base coat. This protects the wall from wind driven rain penetration, keeping the underlying wall fabric dry while allowing it to breathe. Being salt-resistant, it also protects the main render coat from premature decay from salts.
Main coat: a main lime render coat (Sanacolor 2000 MGN), available in 24 colours including white. This material is commonly used as the finishing material, however a dedicated lime finish can also be added on top (Rasante B2007 MGN) if needed.

This concept is very similar to the current lime rendering concept, except it extends that it by replacing the cement or NHL base with a much more breathable Roman waterproof, salt-resistant base coat. Both materials being heritage-friendly, they can be used on any historic building.

The rendering concept is detailed below.

rendering-common-core-conservation — Common lime rendering

rendering-improved-core-conservation — The improved long-lasting lime rendering solution

Benefits

Here are some of the key benefits of the MGN Roman Lime Rendering System:

Breathable
Tried-and-tested traditional lime rendering technology
Roman lime waterproofing coat protects the fabric against driving rain
Manufactured of extremely high quality certified raw materials
Contains no cement nor chemical additives
Simple application: premixed formula, just add water
Maintenance-free
Coloured in mass in 24 colours, no repainting necessary
Does not fade or change its colour over time, being coloured with natural earth pigments
Ages beautifully, giving patina to the building
Suitable for the most demanding conservation jobs

For more information, please see below a list of recommended MGN lime plasters for the implementation of this solution, as well as technical pages, projects, image galleries and videos related to this solution and materials.

References

1
Vecchio F: Evaluation of mortar performance for maintenance, conservation and restoration interventions - Master's thesis, Polytechnic of Turin, April 2019. https://webthesis.biblio.polito.it/10439
2
Arizzi, A. & Cultrone, G. Aerial lime-based mortars blended with a pozzolanic additive and different admixtures: A mineralogical, textural and physical-mechanical study. Constr. Build. Mater. 31, 135–143 (2012). https://doi.org/10.1016/j.conbuildmat.2011.12.069
3
Santos, A. R. L. dos, Veiga, M. do R. da S., Silva, A. M. dos S. & Brito, J. M. C. L. de. Tensile bond strength of lime-based mortars: The role of the microstructure on their performance assessed by a new non-standard test method. J. Build. Eng. 29, 101136 (2020). https://doi.org/10.1016/j.jobe.2019.101136

References

1
Vecchio F: Evaluation of mortar performance for maintenance, conservation and restoration interventions - Master's thesis, Polytechnic of Turin, April 2019. https://webthesis.biblio.polito.it/10439
2
Arizzi, A. & Cultrone, G. Aerial lime-based mortars blended with a pozzolanic additive and different admixtures: A mineralogical, textural and physical-mechanical study. Constr. Build. Mater. 31, 135–143 (2012). https://doi.org/10.1016/j.conbuildmat.2011.12.069
3
Santos, A. R. L. dos, Veiga, M. do R. da S., Silva, A. M. dos S. & Brito, J. M. C. L. de. Tensile bond strength of lime-based mortars: The role of the microstructure on their performance assessed by a new non-standard test method. J. Build. Eng. 29, 101136 (2020). https://doi.org/10.1016/j.jobe.2019.101136

Technical Guide - Download

Download the technical guide of this solution in PDF format.

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Recommended Products

Here are the typical recommended materials / products for this solution. Other plaster variations are possible as we have different types of main coats (normal or thermal) and finishes (smoother, grainier, coloured etc.) depending on your needs or application. Please get in touch to discuss additional options.

Rinzaffo

Roman Waterproofing Lime Base Coat

Base

Waterproofing

Sanacolor 2000

White / Coloured Decorative Lime Render for External Use

Main

Completed Projects

Here are some of our projects using this solution:

External Lime Rendering of an Old Farm House

Residential

Photo Galleries

Here are some photos demonstrating this solution. Click on any image to open the photo gallery.

Lime Rendering an Old Farm House

Some walls of this old farm house have been re-rendered with the Sanacolor 2000 MGN lime render.

Plaster Application

Here are some key application points about the application of each plaster, as well as links to the full application guides. Additional documents can be found on the individual plaster pages.

Rinzaffo

Base, Waterproofing

Application Guide

Clean the masonry: the plaster must be applied on a cleaned and uniformly wet substrate. All crumbling and loose parts must be removed by brushing so the masonry is free of dust, salts and oils. If possible, also wash the walls with a pressure washer; this will clean and wet the walls in one go. Close larger holes with lime mortar and pieces of bricks.

Must be the first coat: the Rinzaffo MGN plaster must be the first coat on any wall. It should not be applied on top of other parge coats or plasters because when those fail the performance of the Roman base coat will also likely to be affected.

No salt-inhibitors nor PVA bonding agents should be used before the application of the plaster. The plaster bonds well on its own.

Masonry paints should be removed (which occasionally can be challenging) so the plaster can firmly adhere to the underlying stones or bricks. Remove at least 50% of the existing paint – the more is removed, the better.

Mixing: mix the material with clean tap water only without adding anything else (no other material or additive), until a homogenous, creamy-consistency mix is obtained.

Wet the wall fabric abundantly before applying the plaster, as well as in-between each subsequent coat. Lime plasters need moisture as they set slowly in a damp environment. If the walls are already damp, there is no need to wait to become drier, you can proceed with the application of this plaster.

Level uneven surface first: very uneven walls (e.g. stone walls or crumbling old brick walls) are recommended to be patched up and levelled first before the application of a continuous coat, to ensure the consistency and required thickness of the base coat.

Application: apply the plaster in 10 mm coats.

IMPORTANT: Respect the thickness: do not under-spec the material. Apply min 10 mm (1 coat) for above ground level walls. Apply min 20 mm thickness (in 2 coats) for underground or extremely damp or salty walls. If more than one coat is applied, embedding a 10 x 10 mm fibreglass mesh is recommended between the coats – a standard practice in the industry.

IMPORTANT: Close all pores, no matter how small. Treat and apply the plaster as a tanking-grade material. Attention should be paid to compact it on the wall closing off all pores, no matter how small, leaving no gaps or holes where salts or liquid water could come through. Once an area has been completed, recheck that here are no missed holes, not even small ones.
The plaster application video below explains the concept in detail.

Light key: give the plaster a light key using a wet brush. Do not cut into the material with the edge of a trowel.

IMPORTANT: Dark patches. Allow the plaster to dry for 48 hours. Ideally, the whole surface should dry out uniformly to light brown, however you might notice some dark or damp-looking areas exhibiting surface condensation. Dark areas indicate insufficient thickness of the plaster in raport to the amount of moisture behind it. This occurs in areas where the underlying wall fabric is very damp, the intense evaporation causing surface condensation.
The fix is easy: apply extra material over such dark areas increasing the thickness of the plaster, closing off all pores. The increased plaster volume dilutes the vapour flow allowing the surface to dry.

Additional coats can be applied in further 10 mm increments. Use an embedded fibreglass mesh for extra reinforcement over the recommended thickness.

Application conditions: ambient and wall temperatures must be between +5 to +30°C during application. Surfaces should be protected from rain and humidity until they have completely dried (approx. 3 – 10 days depending on weather conditions).

IMPORTANT: Please watch the plaster application video below before applying the material. Unmute the video if it plays without sound.

Sanacolor 2000

Main

Application Guide

Clean the masonry: the plaster must be applied on a cleaned and uniformly wet substrate. All crumbling and loose parts must be removed by brushing so the masonry is free of dust, salts and oils. If possible, also wash the walls with a pressure washer; this will clean and wet the walls in one go. Close larger holes with lime mortar and pieces of bricks.

Mixing: mix the material with clean tap water only without adding anything else (no other material or additive), until a homogenous, creamy-consistency mix is obtained.

Application: apply the plaster in 10 mm coats.

Recommended thickness: the recommended application thickness is about 20 mm.

Additional coats can be applied in further 10 mm increments. Use an embedded fibreglass mesh for extra reinforcement over the recommended thickness.

Finishing: apply the last 5 mm of plaster as finish, creating the desired texture. Alternatively a fine plaster can also be applied, if needed (Rasante B2007 MGN) is recommended.

Various textured finishes can be achieved, depending on the finishing technique used:
- Coarse finish: by finishing the surface with a sponge or wooden trowel.
- Smooth finish: by using a stainless steel trowel, compressing and smoothing the semi-dry surface, the sand granules are pushed into the material, resulting in a smooth finish.
- Washed finish: by finishing the surface with a sponge trowel. After the surface has hardened, the surface is washed with water and blotted with a sponge to bring out the aggregate, its specific texture and colour.

Painting: as this is a coloured plaster, it does not need painting. However, it can be painted with a breathable mineral paint. Wallpapers and modern emulsion petrol-based paints, with no or limited breathability, should be avoided.

Application conditions: ambient and wall temperatures must be between +5 to +30°C during application. Surfaces should be protected from rain and humidity until they have completely dried (approx. 3 – 10 days depending on weather conditions).