How Does Residual Moisture In Masonry Walls Affect Its Quality?
Today, we share the first post in a four-part series on building decay and deterioration, opening with the topic of residual moisture trapped within historic brick and masonry wall assemblies.
The photograph above shows a brick wall with deteriorated plaster removed, exposing the inner substrate brickwork. The disintegration of the plaster is indicative of long-term moisture exposure between the plaster and brick layers. The brick surface is weathered, displaying characteristic efflorescence associated with deteriorated and moisture exposed masonry. Efflorescence is a result of hydrated soluble salts, within the masonry, that have migrated through the masonry due to moisture movement.
The outline of today’s article follows:
The topic of moisture trapped between brick masonry walls and plaster internal wall coverings is a common topic and source of questions for our clients and customers.
Summary of water entry points through masonry
Masonry walls are susceptible to water entry through several mechanisms, including capillary action, cracks, voids, and porous materials. Capillary action, a prominent cause of moisture infiltration, occurs when water is drawn into the fine pores and capillaries of the masonry material due to surface tension and molecular attraction. This phenomenon allows water to move upward against gravity, leading to water accumulation within the masonry wall.
Double wythe masonry exterior walls with a plaster finish can be over 9-inches thick. Although historic masonry elements generally have higher perocity then modern or contemporary Portland type machinery or motor, the historic motor will often dry very slowly. It can be a bit counterintuitive, naturally people think of higher curiosity surfaces or substances having the ability to dry faster, however lime mortar can dry out extremely slowly. Compared to Portland type mortars, lime mortars have an open structure, which increases the porosity but also increases the amount by which water is entrenched and embedded deep into the structure of the mortar.
This issue comes up frequently when talking about brick pointing, tuckpointing and repointing. Unlike Portland cement, which forms interlocking crystalline structures during curing, lime mortar remains in an amorphous state. This lack of crystalline formation contributes to the slower drying time. Lime mortar has a more open-pore structure compared to modern cement-based mortars. This allows for better vapor transmission and moisture movement, making it more breathable. As a result, moisture within the lime mortar takes longer to evaporate or migrate out of the material.
The image above shows the long term effect of moisture trapped in a plaster and brick wall assembly, emphasizing the need for preservation and conservation to protect the historic masonry from further decay. You may notice the cellulose paper face of a wallboard in the discombobulated hodgepodge of materials. The picture above shows so much at once because you can see the exposed face of the substrate brick covered in efflorescence with gaping and deteriorated lime mortar joints, the interstitial materials, and the undemolished wall all in one view.
If you look closer, in the photo below, layers of gypsum at the surface with yellow-brown. We are more focused on the issues of hydrated plaster and water trapped between solid double (or triple, in some cases) wythe masonry walls and plaster which is applied directly to the face of the brick masonry wall. In this case, that sandwich of materials is present in this example, but there’s an extra element which is common in all types of historic buildings in Washington, DC: drywall or wallboard or plasterboard was applied on top of the plaster in layer decades after the original construction.
The yellowing and browning of paper facing on drywall when exposed to moisture or water is primarily due to the same process of lignin degradation and oxidation that occurs in regular paper. Drywall, gypsum board, and plasterboard (as explained, all different names for the same type of material) are made of a core of gypsum, a type of mineral like asbestos but possibly less hazardous, sandwiched between two layers of paper. The paper facing supports the shape and form to the gypsum core and gives the drywall its smooth surface.
When drywall is exposed to moisture or water, the following sequence of events takes place:
1. Water Absorption
The paper facing is porous, allowing it to absorb water readily. When the drywall comes into contact with moisture, the paper absorbs the water like a sponge.
2. Hydrolysis and Lignin Degradation
As water is absorbed into the paper, hydrolysis occurs, breaking down the bonds between the molecules in the lignin present in the paper fibers.
3. Oxidation and Yellowing
The oxygen dissolved in the water reacts with the broken-down lignin molecules, causing them to undergo oxidation. This oxidation process leads to the formation of chromophores, which are responsible for the yellowish coloration.
4. Further Degradation and Browning
With continued exposure to moisture or water, the lignin degradation and oxidation continue, leading to further darkening of the paper, eventually turning it brown.
All of the details of the yellowing / browning process above do not list the growth of mold, and you can clearly see, especially in the first photo, there are black-ish spots on the outermost face of the wallboard. That black-isk discoloration is most likely mold or fungus growing in the paper face of the drywall. Although paper comes from trees, the structure of the cellulose fiber of paper is less dense than wood fiber and therefore a perfect breeding ground for mold, when wet. Cellulose is a natural polysaccharide, a complex carbohydrate, found in the cell walls of plants, therefore one of the most abundant organic compounds on Earth.
When the cellulose paper face of drywall becomes wet, mold spores that are naturally present, abundant even, in the air can settle on the moist surface and start to grow. Once the mold spores find a suitable environment with organic material and moisture, they rapidly multiply and form visible patches of mold on the paper facing.
The historic Washington, DC neighborhoods are filled with structural brick framed buildings. However, these buildings are generally over 100 years old and in need of particular care and upkeep, by specialists who understand historic masonry preservation. Deterioration invariably leads to deterioration and the detrimental effects of water infiltration. Moisture can seep into old masonry walls through various entry points, leading to structural damage and decay over time. In this article, we will delve into the challenges posed by moisture in masonry, the traditional historic masonry wall assembly composition, and the factors affecting the rate of moisture movement. We will also explore techniques for preventing structural damage and decay, along with monitoring and assessment methods for historic masonry upkeep and preservation.
Capillary action in masonry walls
Capillary action can draw moisture through masonry walls. Capillary action allows, or causes, water to migrate through seemingly solid materials. Capillary action is the movement of liquids through narrow spaces, like thin tubes or porous materials, due to cohesive and adhesive forces. Molecules at the liquid’s edges stick to surfaces (adhesion), while intermolecular forces within the liquid pull it upward (cohesion), defying gravity in capillaries.
The water entry, and related damage caused by capillary action in historic lime mortar brick masonry arises from the same process that enhances its durability. When moisture infiltrates the deteriorated, and tgerefore overly porous, mortar joints, in need of repointing or tuckpointing, capillary action draws moisture into the lime mortar joints, initiating a cycle of absorption and evaporation. Over time, this incessant moisture movement leads to further erosion and decay within the mortar, weakening the entire masonry structure, in a vicious cycle, at an increasing rate. Additionally, freeze-thaw cycles in colder climates exacerbate the problem. The repeated expansion and contraction due to freezing and thawing further deteriorate the mortar, causing cracks and disintegration. As a consequence, the structural integrity and weather-defensive shell of historic buildings can be compromised, necessitating preservation efforts, more extensive than just repointing with compatible lime mortar and addressing sources of water ingress. In this example, extreme damage to the interior plaster resulted from unmitigated exterior masonry joint deterioration. This problem could have easily been stopped, right in its tracks by proactive brick tuckpointing.
As a related consideration, but a side note, the issue of capillary action is also a factor, or force, in hydrostatic pressure in underground water movement, including at basement and foundation ground water leakage. When the surrounding soil or ground is saturated with water, it exerts pressure on the lower portion of the wall, facilitating capillary rise. This phenomenon can result in dampness and damage to the interior surfaces of the structure, compromising its stability and aesthetics.
Traditional historic masonry wall assembly composition
Historic masonry walls were constructed using lime mortar, which has been widely favored for its ability to accommodate slight movements without cracking. The wall assembly typically consists of bricks or stones joined together with lime mortar. Lime mortar allows some breathability, permitting moisture to escape, thereby reducing the risk of trapping water within the wall.
Challenges with moisture in lime mortar
While lime mortar offers certain advantages (re-calcification, bond, workability, lower embodied energy), it is not impervious to moisture-related challenges. Over time, lime mortar will deteriorate, leading to gaps and cracks that allow water infiltration. Additionally, soluble salts present in the mortar and masonry can exacerbate moisture-related issues. The build-up of these salts can cause efflorescence, a white crystalline deposit, further deteriorating the masonry’s surface.
The pictures below show an early CMU foundation in Northeast Washington DC. The foundation was built at an interesting time, just before the advent of modern liquid applied foundation waterproofing polymers, but after the beginning of the use of Portland and lime combined mortars.
Similar to the problems experienced in the wall shown above with residual exposure to excess moisture, historic lime binder brick mortar faces similar challenges.
Plaster directly attached to brick surfaces
In historic masonry structures, plaster was often directly applied to brick surfaces without a vapor-permeable barrier. This practice inhibits moisture evaporation, leading to increased dampness in the walls and potential damage to both the masonry and the plaster.
The adjacent picture shows an interior plaster brown-coat wall covering. This plaster is applied directly to the historic common brick wall substrate. When moisture enters through the open or permeable mortar joints of a brick wall, in this case: a double whythe brick composition, the moisture is then trapped within an assembly of plaster and brick. The moisture will continue to permeate and move across the assembly, but it moves slowly.
Factors affecting the rate of moisture movement
Several factors influence the rate of moisture movement in masonry walls. These factors include climate conditions, such as rainfall and humidity levels, as well as the type and porosity of the masonry materials used. Additionally, the presence of cracks, gaps, and capillaries within the masonry greatly impacts the rate of moisture infiltration.
Entropy and building decay
To safeguard historic masonry walls from moisture-related structural damage and decay, a proactive approach is essential. Implementing proper drainage systems around the foundation, repairing cracks and voids, and improving surface runoff can help mitigate water infiltration. Moreover, modern conservation techniques, such as the application of breathable sealants, can enhance the moisture resistance of historic masonry without compromising its original character.
Monitoring and assessment techniques
Regular monitoring and assessment are crucial for identifying early signs of moisture-related issues in historic masonry. Visual inspections, moisture meters, thermal imaging, and data loggers can be employed to detect moisture levels and potential problem areas. These techniques enable timely interventions and preventive measures to protect the masonry from extensive damage.
Historic masonry upkeep and preservation
Preserving historic masonry requires a delicate balance between conserving the original materials and incorporating effective modern conservation practices. Repointing with compatible lime mortar, plaster repair using breathable materials, and controlled dehumidification are some of the strategies employed for the upkeep and preservation of historic masonry walls. A well-executed preservation plan ensures that the architectural integrity and historical value of the structure are retained for future generations to appreciate.
To properly maintain, repair, and care for these historic buildings, a knowledge, interest and understanding of historic building principles is required. Here in Washington DC, historic masonry buildings are extremely expensive and the amount of financial loss caused by improper repointing and low quality construction is staggering. However, in addition to the direct financial value of the property, there is also a cultural loss when historic buildings are damaged. By comparison, consider neighboring poor cities, when historic buildings are damaged, it’s not just the loss of value to the property owner, there’s also a loss to all inhabitants and visitors of a city, present and future, who care about architecture, history, and culture.
We encourage all of our clients, and all readers of this article and to our blog in general, to prioritize the historic built environment of Washington DC and neighborhoods such as Capitol Hill, Dupont Circle, and Georgetown and become educated on on the difference between proper historic preservation versus improper work which leads to significant damage to the historic fabric of a building.
From a conservation and preservation perspective, several approaches can be taken to improve conditions related to deteriorated historic brick masonry. Primarily, lime mortar brick joints and low temperature fired soft red clay bricks should be inspected and checked on a routine maintenance schedule, either seasonally or at least annually. If brick masonry is kept in good condition, the life of embedded wood elements can be significantly extended. Hire a professional contractor which specializes, understands and appreciates historic construction elements and buildings.
You can learn a lot more on our blog. Feel free to check it out. If you have questions about the historic masonry of your building in Washington DC, contact us or fill out the webform below and drop us a line. We will be in touch if we can help.
