Why Sealants Fail and How To Avoid It.

Summary: Sealants fail. They fail adhesively, cohesively, or because the substrate failed. Select the correct sealant, prepare the substrate, and install the sealant correctly to avoid sealant failure. Remember, sealants, like your teeth, can be temporary – if you don’t take care of them, they will go away.

Importance of sealants

It wouldn’t take you long as a building enclosure practitioner (Enclosurist) to learn that sealants are everywhere.

Considering how often sealants are used, I would argue that sealants are the most important component keeping water out of our buildings today. Sealants are also one of the most overlooked components when it comes to design and installation.

Where are sealants used?

Sealants are literally used everywhere; between metal panels, at roof penetrations, at wall penetrations, at expansion and construction joints, at window perimeters, in structural glazing, above (and below) relieving angles, even between the concrete pavers and your foundation wall. 

Why are they so often used? The reason is simple.

Sealants offer a relatively inexpensive solution to the complex problems associated with keeping water and air where they belong. By squeezing a backer rod into an opening and covering it with sealant, you’ve just managed to stop air, water vapor, liquid water, and the client’s eyes from moving through the opening.

Sealants fail

Unfortunately, even though sealants are great, they also fail. 

When exposed to the damage functions (to be explained in a future post), sealants deteriorate over time. The rate of deterioration depends on several factors, including the type of sealant and environmental factors. 

Since sealants are commonly installed between dissimilar materials, many sealant joints are subjected to movement. This can further contribute to its failure.

Ask any experienced Enclosurist. They would most likely be able to tell you of at least a few projects where failed sealants contributed to the premature failure of building enclosure components.

How do sealants fail?

In this section, we will take a high-level look at how sealants fail.

For a more detailed discussion, refer to our resources at the end of the article.

Sealants generally fail in three manners. These are:

• Adhesively,
• Cohesively, and
• due to Substrate failure

Adhesive failure is when the sealant detaches from the surface to which it was attached, thereby breaking away from the surface. Factors contributing to this type of failure include improper surface preparation, joint movement exceeding the sealant movement capability, or improper bead configuration (more on this later).

When sealants fail cohesively, the sealant fails pulls apart while remaining attached to the substrate. This is usually caused by not properly mixing a multi-component sealant, selecting the wrong sealant for the application, improper bead configuration, air bubbles in the sealant during the application, or joint movement exceeding the joint’s movement capacity.

Lastly, sealants can fail due to the failure of the substrate. This means that the sealant remained intact, but a piece of the substrate has split or broken off. This typically happens because the substrate has not been properly prepared before the sealant was applied. In most instances, sealants will fail before the substrate does unless deteriorated or weak substrate conditions were present during sealant installation.

Can we avoid sealant failure?

Yes and No.

Although sealants fail, there are specific ways to greatly reduce, if not eliminate, the potential for premature sealant failure.

Note that I used the word premature. 

That is because all sealants have a lifetime and will eventually deteriorate due to exposure to the elements, movement, or a host of other factors. It is common to include sealant maintenance as an item on the building enclosure maintenance recommendations to clients. (The truth is that sealants are rarely maintained and only receive attention when the damage resulting from failed sealants become evident).

How to avoid sealant failure

At this time, I’ll use the age-old Enclosurist cop-out and say, “Refer to the Manufacturer’s Instructions.”

Let’s assume you don’t have those handy; what are the general things you need to consider?

There are 5 main factors to consider when thinking about sealant joints. We plan to do a more detailed post on sealant joint design, but for now, remember the following:

• Correct sealant material selection
• UV and weather resistance
• Anticipated movement
• Surface preparation
• Shelf life
• Sealant geometry

Correct sealant material selection: Sealants need to be compatible with the adjacent materials existing and anticipated. For example, certain sealants can be painted while others can not. Certain sealants deteriorate underwater while others do not. Selecting the correct material is critical.

UV and weather resistance: Related to the question about material characteristics is the question about resistance. Certain sealants are more resistant to UV, pollutants, ozone. Select the sealant suitable for the unique environmental challenges.

Anticipated movement: How much movement will this joint see? Is it limited to a fraction of the joint width, or will the sealant experience significant stress and strain? Will the movement be seasonal or occur daily? 

Surface preparation: Sealants will typically not adhere to dusty or oily surfaces. Certain metals and plastics can also be problematic. Properly cleaning substrates is important. Additionally, certain sealants can stain substrates, so substrates may need to be protected.

Shelf life: Most sealants have a set shelf life. Using expired sealants may greatly impact the long-term performance of the sealants. In addition to shelf life, be aware of a sealant’s pot life if mixed on-site. Pot life refers to the time during which the sealant needs to be installed after being mixed. 

Sealant geometry: Since sealants need to move it is important to use bond breaker tape to allow the sealant to attach only in areas where attachment is necessary. Sealants attached on three sides are more likely to fail than sealants attached on two sides. The two sided attachment allows elongation better than a three sided attachment.

Each of these topics deserves its own article, and we hope to expand the sealant section of the BuildingEnclosureBlog in the future. Should you be interested to learn more, refer to the helpful resources at CR Laurence. Specifically, this article that served as one resource during our research.