Fixing Leaky Chimney Systems (Part 1 - Water Intrusion)

This is the first in a series of three articles about water remediation and professional chimney technicians. In this article, we will concentrate on water remediation in masonry chimneys and the types of water repellents available to treat your customers’ water remediation issues. 

Water Intrusion

Most experts agree that the durability of an external masonry chimney depends primarily on its ability to resist water penetration. The most important factors that influence whether a chimney will resist water intrusion are chimney design, material selection, and the quality of construction and workmanship. We can affect these factors only when we are called upon by the homeowner or circumstances to tear down and rebuild a chimney. In the vast majority of cases, we are in a position to remediate the water 
intrusion without the luxury of tearing down and rebuilding the chimney properly.

To treat water intrusion, we first need to understand water intrusion. Although brick and 
mortar appear to be solid and relatively dense, a closer inspection will reveal that they are actually very porous. In fact, they are a network of interconnected pores called capillaries, which circulate water by means of suction. These capillaries are too small to be seen clearly by the naked eye, but can absorb and circulate large amounts of water.

The same capillary action that allows a masonry chimney to become saturated also allows it to dry itself. As the sun warms and wind blows over the surface 
of the masonry, the water that was trapped in the capillary structure moves to the surface in vapor form and is wicked away. This process is continual, until all the moisture evaporates and leaves the masonry dry, or another event wets the structure. If the chimney never completely dries between wetting events, the pore structure can be overwhelmed, and the result is water entering the living space. This is usually when we are called in by the homeowner, who is surprised to learn their chimney is the culprit!

Water Repellents

Making an informed decision about the type of water repellent to use can mean the difference between a satisfied client and an irate one. There is a vast array of choices available to us, so which should we choose? A discussion of the basic classes and types of water repellents will take a lot of the guesswork out of the equation and help you navigate to the correct product. 

There are two classes of colorless water repellents for masonry: film formers and penetrants. As the names imply, film formers produce a continuous film on the surface of the masonry. Penetrants penetrate up to 1/4 inch into the masonry structure and do not form a film on the surface.

Solvent vs. Water Based

Additionally, water repellents can come in either solvent- or water-based forms. The water or the solvent is merely a carrier to deliver the water repellent. Some locales may restrict the use of traditional solvent-based water repellents due to the amount of volatile organic compounds (VOC) that they release into the atmosphere. Most manufacturers now produce a VOC compatible version of their solvent-based water repellents which are legal to use in those jurisdictions. I often field the question, “Why would I want to use a solvent-based water repellent? Are they better than the water-based products?”

The easy answer is no. In the high 90% of issues you will be remediating, a water-based water repellent will perform extremely well. However, most water repellents cannot be applied when temperatures are expected to drop below freezing. Solvent doesn’t generally freeze and dries quicker than water, so if you have an application where you expect the temperature to drop below application guidelines before it dries, a solvent-based product might be a good choice. Additionally, if you are treating a surface that may have been treated with an unknown product, a solvent-based product can provide a better chance to penetrate it. While water-based water repellents can be used for most applications, there are wide ranging differences in their compositions and performance. Many of them are latex-based, providing complete films to resistant water despite claims that they are breathable.

Film Forming Water Repellents

In many cases, use of a film forming water repellent can accelerate the type of damage you are trying to stop. If a masonry chimney becomes wet, and the vapor is blocked from being wicked off the surface through use of a film forming water repellent, the moisture can condense into a water droplet as the temperature drops, and – as the temperature continues to fall – expand as it forms into ice crystals, resulting in spalling or delaminating.

Areas that do not have multiple freeze-thaw cycles are also not immune to damage. As water moves through the brick and mortar, it picks up mineral and metallic salts in liquid form. With a penetrating sealer, we can see this as efflorescence as it was wicked to the surface. Film formers can trap these salts inside the brick causing the same effects as spalling caused by freezing water.

Film formers generally have less than half the expected life span of penetrating water repellents. Some of this is due to chemical composition, and much is due to UV degradation. Film formers sit on the surface and therefore are continually subjected to 
damaging UV exposure. Despite these deficiencies, some municipalities have begun using these coatings to combat graffiti. 

Penetrating Water Repellents

There are six main types of penetrating water repellents: siloxanes, silanes, silicates, 
methyl siliconates, RTVs, and blends. Penetrating water repellents achieve their repellency by coating the pore structure below the surface. They change the contact angle at which water reacts to the pore, changing it from a positive pressure (suction) to a negative pressure (repellent). Due to their silicone structure and because they work below the surface, the UV radiation from the sun does not break them down nearly as quickly as a surface film former. In addition, because they coat the pores of the 
substrate rather than filling them, they have extremely high vapor transmission rates, which allow them to breathe normally and dry at a natural rate.

Siloxanes
Siloxanes achieve excellent penetration due to their small molecular size. They bond with silica and alumina in brick and mortar in the presence of UV light from the sun to form a permanent chemical bond with the substrate. Water-borne siloxanes can be applied to damp surfaces and have a long life of 10 years or more. They exhibit very good vapor transmission capabilities and water repellency.

Silanes
Silanes have a smaller molecular size than siloxanes and have excellent penetration. Because they are also resistant to UV radiation from the sun, they enjoy  a lifespan of up to 10 years. Some water-borne silane-based products may be applied to damp 
surfaces as well, but they are less effective on lower alkaline surfaces such as brick because they require alkalinity in the substrate to cure. They are best suited for horizontal, alkaline surfaces such as concrete drives and walkways. They exhibit very good vapor transmission capabilities and water repellency.

Because both silanes and siloxanes react with the silica in masonry, care must be exercised during application to ensure that there is no overspray on glass surfaces (which is almost entirely silica) as it can be difficult to remove if it is allowed to cure before wiping it off the surface.

Silicates
Silicates are commonly used to restore deteriorated masonry and to consolidate natural stone by stabilizing the substrate. They are not commonly used with brick. They exhibit poor vapor transmission capabilities, poor water repellency, and have a five-year lifespan.

Methyl Siliconates
Methyl siliconates are sometimes injected into brick masonry to form a barrier to damp rising. An alkaline solution reacts with the silica in the substrate in the presence of carbon dioxide to form a water repellent surface. They exhibit good vapor transmission capabilities, fair water repellency, and have a five-year lifespan.

RTVs
RTV stands for Room Temperature Vulcanization. It is a silicone rubber that contains 
petroleum distillates that cure at room temperature. When cured, it is elastic and can bridge small gaps. It is commonly used as an anti-graffiti coating and forms a skin if over-applied due to large molecule size. They exhibit good vapor transmission capabilities, good water repellency, and have a five to 10-year lifespan.

Blends
Blends can be made by combining any of the above penetrating water repellents. For instance, blending siloxanes and silanes can make a very effective water repellent for horizontal cementitious applications. Field mixing is not recommended as correct proportions are extremely important to the finished product and in a blend, the final product takes on some properties of both repellents, somewhat modified.  These more formulated products often provide the best performance properties, especially where multiple substrates and non-alkaline substrates are encountered.  Blends can incorporate more reactive polysiloxane materials to improve performance on more neutral substrates like brick, low surface tension components to wet out previously treated substrates, amino functionality to obtain higher water beading and resist mold and mildew formulation, and no or low surfactant levels for early water resistance, and solvent free to have low VOCs complying with environmental regulations.

Conclusion

By understanding the process by which masonry wets and dries and the differences in classes and types of water repellents, you should be able to make an informed decision regarding how to best remediate your customer’s water penetration issues.