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Waterproofing Old Walls

Stopping Liquid Moisture
Table of Content — open

There are genuine applications where liquid water has to be kept out of certain areas and waterproofing old walls becomes a necessity. However, old buildings also need to breathe, which can appear a contradictory requirement to waterproofing old walls. Let's explore this topic in more detail.  

Water ingress can take many forms, the most important ones being:

  • Roof leaks
  • Excessive rainwater or wind driven rain
  • Chimney dampness 
  • Dampness in wall sections slightly or partially underground
  • Waterproofing damp cellars or basements
  • Water splashback at the base of the walls

In such cases, due to their properties, regular lime plasters do not provide a robust enough, long-lasting solution. As a result, waterproofing jobs on old buildings are often performed with modern materials such as tanking slurries, bituminous tanking materials, plastic membranes, just to name a few. Although these perform well in keeping liquid water out, when used in older or listed buildings designed to breathe, they also come with important drawbacks or trade-offs

Modern Waterproofing Materials in Old Buildings

Modern waterproofing materials have been designed in line with modern building philosophy: keep all moisture out. As a result modern waterproofing materials are completely waterproof, stopping both liquid water and water vapours. Although this is a desirable behaviour in many applications, in older buildings this behaviour is not ideal.

Unlike newer buildings, older buildings have been designed to be vapour permeable (breathable). By design, a well maintained fabric stops liquid water penetration but allows vapour waters to penetrate. This way the wall can regulate its own moisture content and keep the fabric dry (within limits). 

Modern waterproofing materials being non-vapour-permeable (non-breathable), by blocking the movement of vapours, they permanently change the moisture dynamics of old walls, resulting in some very undesirable consequences long term.

Here are some of the most notable drawbacks of modern waterproofing materials when used in damp old buildings:

  • Non-breathability: non-breathable waterproofing materials lead to moisture accumulation in the old wall fabric. As the moisture content of the fabric increases, moisture starts spreading and shifting in the walls, being pushed up or sideways into areas where there was no moisture present prior to the waterproofing / tanking intervention.
  • Cement - Chemical incompatibility with old buildings: cement based materials (e.g. waterproofing plasters, tanking slurries etc.) contain many additives. One of the common additives is gypsum which is added in non-negligible quantities (3-5% by volume) to cement to make it dry slower and stay workable for longer. Gypsum being a salt (Ca SO4) chemically attracts moisture, slowing down evaporation thus extending the drying time of cement.
    However, the salts present in cement, now in ongoing contact with the underlying damp fabric, often cause irreversible damages to old masonries. In the presence of moisture, the different chemical properties (different pH value) of cement and the wall fabric results in ongoing salt migration from the cement into the masonry, steadily increasing the salinity of the wall fabric.1Vecchio 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
    Changes in ambient temperature and humidity makes the salts crystallize at the cement-masonry interface, leading to the detachment of cement, as well as to the crumbling of historic fabric under the cement. This detachment and crumbling is often mistakenly attributed to the hardness of cement, rather than to its inherent salinity and associated electro-chemical phenomena.
  • Cement - Rigidity, lack of flexibility: cement plasters and slurries being dense, hard and rigid materials, they accommodate building movement poorly, developing cracks. Rainwater penetrates into the wall through the cracks, leading to additional moisture problems over time. 

So the ideal waterproofing solution for old buildings would be some selectively waterproof material that can stop liquid water while allowing water vapours to pass. In other words, it should be waterproof and breathable at the same time. 

These requirements, at first glance, seem to be contradictory - however the Romans figured out a simple, workable solution that just does that, the solution also being very building-friendly.

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 research2Arizzi, 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 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 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

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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.

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Volcanic lavas (volcanic pozzolans) are highly porous and breathable materials

The Right Waterproofing Solution for Old Buildings

The practical waterproofing of old walls with lime that "ticks all boxes" - breathability, water and salt resistance, moisture control, long life expectancy - consists of the application of the following 3 plaster coats:

  1. Base coat: the Rinzaffo MGN Roman waterproofing salt-resistant lime base coat
  2. Main coat: a main lime plaster coat, which can be a regular or thermal lime coat
  3. Finishing coat (optional): a good quality lime finish

The recommended materials are listed below. This concept can replace modern non-breathable waterproofing and tanking materials, being a listed building and heritage-friendly solution, that keeps old buildings breathable.  

waterproofing old walls internally
Internal waterproofing of old walls with lime
waterproofing old walls externally
External waterproofing of old walls with lime

For more information, please see below a list of technical pages, projects, image galleries and videos related to this solution.

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

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.

Completed Projects

Here are some of our projects using this solution:

More Information

Here are some related pages with additional technical information, giving you a more in-depth understanding of this topic.

Photo Galleries

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

Showing items: 1 - 3 of 3 total.
The Crypt Waterproofing Project

Waterproofing the service area of an old crypt with traditional Roman lime waterproofing and Cocciopesto plasters. On some parts of the room the MGN Lime-Pozzolanic Tanking Slurry System has also been applied to reinforce the waterproofing in critical areas.

Flood Proofing a Flooded Extension

This old cottage has been extended a few years ago, receiving a new extension. During flooding the waterproofing of the newbuilt extension did not hold, resulting in the flooding of the building. Using the Roman lime materials the waterproofing has been reinforced, preventing future flooding. 

Farm House Conversion And Thermal Insulation

This old farm house has been converted to living accommodation and thermally insulated using Roman lime materials in a sympathetic way.

Videos

Here are some videos related to this solution. Please unmute the videos when playing them.

Showing videos: 1 - 6 of 8 total.

Any Questions? Need Technical Advice?

If you have any questions about a project, a problem, a solution, or any of our plasters - please get in touch.

We understand that each project is unique. Using the contact form below feel free to ask us any questions. Give us as much detail as you can about your project so we can get back to you with more relevant answers. 

Are you in the process of buying / selling the property?
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Testimonials

Here are some client testimonials:

cottage

I agree, I can see with my own eyes how the Rinzaffo is dealing with moisture. I’m actually amazed by the product. It’s a difficult product to work with initially, but we have to understand that this is not the finish. I have seen with my own eyes how it is dealing with moisture. There are a couple of walls that I can see need 3 or 4 mm extra.

To breathably tank a building like Romford farm is unknown. I must admit I was dubious at the start, but open to the idea. There is no modern building products that I am aware of, or am used to that could possibly be an alternative.

I can see with my own eyes and other senses how the dampness in that building is disappearing the more work that we do. … You have to understand …. I entered that building early on…. It was rotten with moisture. I would say inhabitable. Day by day it gets better.

Builder, Romford Farm

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

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Base, Waterproofing

  • 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.

Cocciopesto Deumidificante

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Main

  • 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.
  • Additional coats can be applied in further 10 mm increments. Use an embedded fibreglass mesh for extra reinforcement over the recommended thickness.
  • Finish options: main lime coats can be optionally left without finishing. To have a finished surface any MGN finish can be applied: Calcina Fine MGN (white lime finish), Intonachino Arenino MGN (coloured lime finish) or Marmorino MGN (Venetian Marmorino finish).
  • 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).

Calcina Fine

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Finishing

  • 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. 
  • Recommended thickness of the finish: 3-4 mm. For best results, the finish should be applied in two “half-coats” of 2 mm each, with a fine 3-4 mm fibreglass mesh embedded in-between. The mesh makes the finish more flexible, minimizing the appearance of fine cracks.
  • 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 breathable lime plaster, wall surfaces should 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).