Managing Moisture in Masonry
Protective coating’s role in stopping moisture infiltration in masonry buildings
There are certain things in life we can be sure of. The sun will rise in the east and set in the west; winter follows summer; and you will be required to pay taxes. We can add one more certainty to the list: Buildings will leak.
The architecture of our buildings embodies our heritage and is a reflection of our values. As you tour the great cities of the world, you can read their stories in the skylines. Masonry buildings comprise a significant portion of the built environment; its design flexibility, durability and beauty have made it the choice of design professionals. Masonry is one of the most durable methods of construction. The proof of this is evidenced on nearly every street corner around the world.
A lifetime of experience in the construction industry can be summed up in a single statement: Buildings leak for a number of reasons. While it is true that properly designed and constructed masonry buildings are effective in managing and protecting a building from moisture, the reality is that few buildings are constructed without defect. Protective wall coatings often are the first line of defense in correcting problems and preventing moisture ingress.
The Benefits of MasonryThe benefits of designing with masonry are numerous. Masonry offers versatility and beauty with an endless selection of color, shape and texture. The Pyramids of Giza speak to masonry’s durability. These materials are fire resistant and sound resistant. All of these benefits combined with a long lifecycle make masonry the material of choice for many design professionals.
The Challenges of MasonryWhile masonry provides many benefits, it is not without its issues. Most professionals agree that moisture is the biggest challenge when building with masonry. Clip joints, missing flashing, obstructed cavities and missing design details are a few of the factors that can contribute to moisture penetration. These issues can be compounded by settlement cracking, leading to efflorescence, spalling face shells, corrosion of reinforcing and damage to the occupied spaces within the building.
Protection starts with designWhen it comes to protecting buildings from moisture, redundancy in design is a good thing. There are two primary types of wall design utilized in masonry buildings today: cavity wall and single wythe construction. The goal of both designs is to protect the building from moisture; however, the degree and method of protection differs with each approach.
Cavity Wall (rain screen)
- Double layer of protection
- Built in redundancy
- Moisture management
- More expensive
- Single layer of protection
- Relies on perfection
- Moisture barrier
- Less expensive
Protective Wall CoatingsProtective wall coatings are effective tools in the fight to protect buildings from moisture. Generally, wall coatings are defined as high build coatings (10-20 dry mils) versus thin film paints (3-5 mils). The difference in film build is primarily related to higher solids and polymer type, allowing for thicker film build without cracking of the coating. The benefits that high build coatings provide over paints are numerous. These benefits include:
- Properly applied high build coatings provide waterproof protection to masonry buildings.
- Properly applied high build coatings provide protection from carbonation of mortar as well as chloride ion ingress.
- High build, elastomeric coatings have the ability to bridge moving cracks up to 1/16 inch.
- Thicker film build allows for the encapsulation of aggregate for aesthetics and masking surface imperfections.
- Today’s high build coatings formulated for use on masonry substrates can tolerate the naturally higher pH of masonry substrates better than most paints.
- High build coatings provide equal or better aesthetic and color range compared to typical paints.
- High build coatings are more durable and will physically last longer than thin film materials.
Coating Repairs That Last
A primary suspect in moisture infiltration is mortar – either because of cracking joints, settlement, or clip joints. Regardless of the cause, damaged mortar should be repaired before the application of a wall coating. Cracks larger than 1/32 inch, signs of rust bleed, or efflorescence are indicators of problems. Additionally, mortar should be struck concave and smooth. Raked or beaded joints should be avoided on exterior masonry. Damaged masonry units should be repaired or replaced using compatible materials.
With both cavity walls and single wythe walls, drainage is an important element of a properly functioning wall. Even with the best design and workmanship, moisture can find its way into the wall section. Flashing and weeps are a critical component of any wall system. Damaged and missing flashing should be replaced. Care should be taken to ensure that weeps are not coated over or obstructed.
Other areas that can contribute to moisture ingress are sealant joints, penetrations, window perimeters and parapet caps.
Selecting the Right MaterialNot all materials are appropriate for all substrates or applications. Masonry and concrete masonry are unique substrates with unique requirements. Efforts to save money often go unrewarded when projects need to be recoated because an inexpensive paint was used in place of a protective coating (and subsequently fails).
When selecting a material, it is important to choose one that is formulated for use on concrete, masonry and concrete masonry. Masonry substrates can be high in pH, which can cause certain types of materials to fail prematurely. Using a material that was not formulated for use on masonry can lead to premature color fade, chalking and delamination of the coating.
Vapor permeability also is an important attribute of a wall coating. While preventing moisture from getting into the substrate, a protective wall coating also must allow moisture within the substrate to escape. A coating functions much in the same way our skin functions, it is waterproof while still allowing our bodies to perspire. When selecting a material, it is important to evaluate the test data provided by manufacturers and verify that the mil thickness used to report permeability is the same mil thickness that is recommended for the application of the coating. Reporting a high permeability value is meaningless if the recommended coating application is significantly thicker.
Once a material has been selected, it is a good idea to have a mock-up installed on the building. Mock-ups provide validation of the compatibility of the coating with the substrate and provide an example of workmanship. It is important that the mock-up be installed in an area that is representative of the substrates to be coated and be allowed to remain until the end of the project. The same crew that will be working on that particular project should install it. The mock-up allows the owner to evaluate color and texture, and becomes the standard of performance for the project. Mock-ups protect the owner, the contractor, the design professional and the manufacturer. Memories quickly fade, but mock-ups are the ultimate arbiters.
Successful InstallationAs with most construction materials, preparation is 90 percent of the battle when it comes to successful wall coating applications.
During the assessment phase, a thorough inspection of expansion joints, penetration and window perimeters should be conducted. Damaged sealant should be removed and replaced. Care should be taken to ensure compatibility of the sealant with the wall coating to be applied. Parapet caps and walls should be inspected and repaired as necessary. Parapet caps should extend three inches down the face of the masonry and should be caulked to prevent wind-driven rain from being blown up the wall beneath the cap.
All efflorescence, mold, mildew and rust stains should be removed before application of a coating. Cleaning methods will vary based upon the nature and age of the substrate. For best results, consult with a cleaning products manufacturer for recommendations appropriate for the specific project. As a general rule of thumb it is best to use the gentlest method of cleaning possible, taking care not to damage the substrate. Once clean, allow the wall sufficient time to dry before the application of a wall coating.
The application method will vary based upon job site conditions. Wall coatings may be applied by brush, roller, or airless sprayer. Regardless of the application method, it is important that the material be installed in a uniform manner. The most common application issue related to high build coatings is uniformity. When properly applied, a high build coating provides a continuous layer of protection over the exterior of the building. Pinholes, holidays and skips are tantamount to having holes in the bottom of a boat. These voids can allow moisture to penetrate the wall system, leading to the formation of efflorescence and eventually delamination of the coating, exposing more of the substrate.
Uniform film thickness also has an impact on aesthetics. Uneven application of a wall coating can lead to variance in sheen and color. A common problem in the application of a wall coating is lap marks. For a uniform appearance, it is important that the coating be back rolled in one direction. Failure to do so can result in a texture effect commonly called “zebra striping.” This is similar to the effect created when grass is mowed in two different directions.
Finally, one of the most important factors in a successful coating application is the applicator. It is important to select a qualified contractor who has experience installing high build coatings and is familiar with the nuances of masonry buildings. Good communication between the contractor, design professional and coating manufacturer will contribute to a successful installation.
Pulling It All TogetherThe benefits of designing with masonry are numerous. The enduring beauty of masonry buildings can be found on nearly every street corner around the world. While it is true that properly designed and constructed masonry buildings are effective in managing and protecting a building from moisture, the reality is that few buildings are constructed without defect.
Success begins with preparation: repairing damaged mortar, masonry units, flashing and sealant. Material compatibility and vapor permeability are important performance attributes. Mock-ups provide assurance to the owner and protection to all parties involved on the project. Finally, uniform application is critical to the proper performance of a wall coating. Together with proper repairs, protective wall coatings can provide the first line of defense in correcting problems and preventing moisture in masonry buildings.
Originally published in Masonry magazine.
About the Author
Christopher Perego is the market segment manager for building restoration for BASF Construction Chemicals, LLC. - Building Systems.