Chimneys can be problematic areas of an old property, chimney dampness being a common problem, for several reasons. Chimneys are:
As a result, here are the most common chimney problems.
Rainwater can easily find its way down and into the chimney stack as a result of wind driven rain. Rainwater can find its way into the chimney through the cracks of the mortar joints or through the flashing.
To minimize the rainwater ingress, chimneys can be capped.
Rainwater ingress not only leads to unsightly damp patches on walls along the chimney flue but the chimney soot washed down by the rainwater causes far more damages to the masonry than water.
Chimney salts can cause some of the most mysterious dampness problems on chimney walls. These can include discoloration, mysterious hazy damp patches, paint damages, the bubbling and crumbling of plaster which can even occur on internal walls or walls with closed off or decommissioned chimneys, leaving old building owners puzzled about the source of these problems. Chimney damp patches may come and go - they fade during the warmer summer months then suddenly reappear in full force after some intense rain.Â
In such cases, most of the time the poor pointing, wind driven rain or chimney leaks are suspected to be the cause of these problems, however a thorough inspection of the chimney (pointing, chimney stack and flashing) reveals nothing out of the ordinary. Sometimes, in a desperate effort to solve the problem, the chimney stack is completely removed, yet the dampness problems reappear after some time even if the chimney stack is no longer present.
The source of these problems is not caused by water ingress, but by the electro-chemical effect of salts. It is not a water ingress problem but a salts problem.
Here is what happens: chimneys are typically used for decades or centuries to burn logs or coal. During the burning process soot is formed, a fine black powder that is deposited along the chimney flue. Due to ongoing temperature and humidity changes, as well as rainwater leaks, the soot migrates into he mortar, finding its way near the surface, causing yellow, brown or black discolouration along the chimney walls.
But there is more to it! Salts in contact with water form ions, a fancy name for electrically charged molecules. Salt ions can attract and trap humidity (water vapours) from the environment, accumulating significant amounts of moisture. Moisture accumulation by salts results in hazy damp patches on the surface of the walls. Moisture tends to accumulate along the saltier areas of the plaster, as dampness is held in place by the electrical charges of the salt ions,
Once salts find their way into the building fabric, they cause a number of irreversible changes. During the winter period when the ambient humidity is high, salts liquefy and the damp patches expand. During the summer, when the ambient humidity is lower, salts tend to slowly dry out and the damp patches fade. However, during drying the salts also solidify and crystallize, expanding in volume 5-10 times. This volume expansion during drying leads to the physical breakdown of brick, stone or plaster resulting in crumbling.
Here are are some examples of walls affected by soot and salts.
Salts cause major problems to old buildings, being the main cause of historic fabric loss. Research experiments have conclusively demonstrated that crumbling is not caused by high humidity but by salts. You can read more about the various problems caused by salts in old buildings here.
Near the base of old chimneys, rising damp is one of the most common problems. Because old chimneys also contribute to the structural reinforcement of old walls, they can have a considerable size, thickness and volume. Sitting on the damp ground, they are affected by ground moisture which over time leads to the development of rising damp.Â
The thicker and the more voluminous a masonry is, the more water it can store, as highlighted by a prominent British research paper.
Solving chimney dampness problems is not complicated with the right materials and know-how.
Solving rainwater ingress is quite straightforward. A thorough chimney inspection can usually locate the points of water ingress and fixing that will resolve the problem.
Solving salts related problems is usually done by replastering.
The modern solution to this problem is replastering the affected walls with a sand-and-cement plasters and a gypsum finish, as nowadays most builders use modern materials. This solution has an apparent workability as cement plasters can withstand the effect of salts for longer than lime plasters. However, cement being a non-breathable material it actively contributes to the build-up of moisture inside the masonry leading to all sorts of dampness problems longer-term - hence cement should NOT be used in older buildings.
In order to keep the wall fabric breathable, replastering with lime is a much better option. However, regular lime plasters in a damp and salty environment are often not resilient enough to withstand the effect of salts long-term, resulting in their premature breakdown.
Dealing with high humidity and the destructive effect of salts requires special lime plasters. These are ancient plaster mixes initially used by the Phoenicians and Greeks, then rediscovered and perfected by the Romans throughout the centuries. Being outstanding architects and builders, the Romans have observed that adding to lime certain volcanic sands and ashes (also known as natural pozzolans - highly porous and breathable lava materials), these can significantly alter the properties of lime while retaining its breathability.
Through the skilful use of different types of volcanic sands and ashes, the Romans have created many types of lime mortars with outstanding properties - some of them becoming waterproof or able to set underwater, or becoming highly resistant to salts resisting seawater.
These volcanic lime mortars have been extensively used by the Romans in very demanding environments including sewers, ports, spas and aqueducts and they have survived for many centuries. They have also been widely used in Venice, as they perform extremely well in the humid and aggressive environment of the Venetian lagoon.
Traditional pozzolanic Roman mortars differ significantly from today’s modern NHL (Natural hydraulic lime) mortars.
NHL mortars - a mix of lime and clay - need to be fired at higher temperatures than normal (air) limes for the lime and clay to react. The higher firing temperatures and more heat partially closes off the pores of NHL plasters, reducing their breathability. Thus, NHL plasters trade-off breathability for improved water resistance.
Roman mortars use lime and volcanic materials. These are mixed at room temperature without heat as the volcanic sands and ashes have already been pre-burnt by the volcano. This keeps the pores structure open resulting in high breathability. The volcanic ingredients make the plaster highly resistant to dampness and salts, making these Roman mortars perform extremely well in damp and salty environments.
The commercially available material today is the Rinzaffo MGN lime plaster. It comes in 25 kg bags with all necessary ingredients pre-mixed, requiring only water. Once it hardens it becomes waterproof, extremely resistant to all salts (chlorides, nitrates, sulphates), yet thanks to its porous volcanic lava ingredients, it stays breathable allowing the wall fabric to breathe.
This plaster - depending on where it's used - performs several important functions:
This lime plaster is made exclusively of natural materials. It contains no chemicals nor additives. All prime materials as well as the final product is certified.
Thus, the Rinzaffo MGN plaster is a lime plaster that acts as breathable water and salt filter. Due to its small pores it blocks liquid water. Due to its open pore structure allows the wall fabric to breathe. Having no salt content of its own, it does not introduce salts into the masonry. Being waterproof, it becomes impervious to salt penetration and crystallization, resulting in an extremely long service life even in very demanding and damp environments (e.g. basements).
To resolve the damp patches and discolouration of chimney areas permanently, the following renovation schedule is recommended:
The renovation process is outlined below.
Rising damp can be addressed in a number of ways, depending on the age, type and construction of the building, as well as the severity of rising damp. These aspects are discussed in detail on the rising damp solution page.
For more information, please see below a list of recommended plasters for resolving chimney problems, along with a list of technical pages, projects, image galleries and videos related to these solutions.
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.
Here are some of our projects using this solution:
Here are some related pages with additional technical information, giving you a more in-depth understanding of this topic.
Here are some photos demonstrating this solution. Click on any image to open the photo gallery.
The Salt Warehouses of Venice (Magazzini del Sale) date back to the beginning of the 15th century. They were built to store a very precious trade commodity: salt. Situated across 9 large halls, the salt warehouse could hold up to 4500 tons of salts.
As a result of its location (Venice) and its use (a salt storage) it is probably the most salty building fabric in the world. The only lime plaster capable of withstanding such an extremely salty environment is the Rinzaffo MGN Roman salt-resistant base coat. This plaster is also gentle to the historic fabric – when it reaches its end of its life, it comes off gently without damaging the underlying historic (in this case the nearly 600-year old) wall fabric.
Here are some videos related to this solution. Please unmute the videos when playing them.
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.Â
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.Â
The plaster application video below explains the concept in detail.
