The purpose of this article is to briefly discuss common sealant failures and the installation considerations for restoration and maintenance projects. The replacement of your building’s sealants should be treated as a scheduled maintenance item, just like an oil change on your car. Sealants are used in exterior wall construction to prevent the passage of liquids, light, or gases between two materials despite differential movement between those components. Even the best barrier, drainage, and diversion systems will not work if the sealant joints are not properly designed, configured, and installed. The majority of sealant failures are due to faulty joint installation or the use of the wrong sealant.
Sealants are one of the most important and widely used weatherproofing materials, and are also one of the most commonly misused products. Of all of the factors which affect the performance of a sealant, installation is the most crucial. Below is a brief list of common types of failures:
Cohesive: Cohesive failure occurs when the sealant loses its ability to elongate under expansion either due to age or due to improper installation. Improper configuration can cause a properly adhered sealant to tear itself apart. This is especially true in the case of three-sided adhesion or with sealant which is applied too thickly.
Adhesive: Adhesive failure occurs when one or both sides of the joint loses adhesion to the substrate. This can be caused by improper joint preparation, configuration, or tooling. For example, a joint in which the adhesion plane is not free of debris will be susceptible to poor adhesion.
Three-Sided Adhesion: When two substrates move, the sealant acts elastically to bridge those two materials. If the sealant is in contact with a third surface, it cannot properly move and will remain inelastic, causing either adhesive or cohesive failure or both.
Improper Backing Material: Improper sizing and inconsistent depth of backer rod are common causes of joint failure. If the depth is not controlled, the geometry or “hour-glass shape” of the joint is unable to be managed.
Improper Tooling: Joints can fail if proper tooling methods are not used. Common failures of improper tooling include joints which are not installed to correct geometric dimensions based on the type of joint configuration (i.e. fillet, bridge, concave, convex, etc.). Soaps or solvents should not be used as a tooling aid because they will interfere with the sealants adhesion.
Loss of Sealant Properties: Sealants can weather and lose some of their performance characteristics by reversion, hardening, or dry-rotting. For example, urethane sealant installed can revert if subjected to continued water immersion.
Successful installation depends on several steps, including joint preparation, priming when required, proper geometry, mixing, application, and tooling of the sealant. The following items should be considered prior to installing sealants:
Temperature: All manufacturers provide recommended installation conditions, including the temperature of both the air and the substrate to which the sealant is being applied. Suggested installation temperatures are typically between 50-80 degrees Fahrenheit, depending on the type of sealant being used. Always consult the manufacturer’s standard data sheets.
Surface Preparation: The substrate must be structurally sound, dry, clean, and free of any impurities. Primer (if required) should be mixed or prepared in small quantities in strict accordance with the sealant manufacturer’s recommendations and applied only to the face of the joint substrate. The primer should be applied prior to the joint backing to prevent contamination of the backing. It is a good practice to perform adhesion tests.
Backing Materials: Backer rod, bond-breaker tape, or other type of release mechanism must be installed to prevent three-sided adhesion and to control the geometry of the joint. The size and depth should be maintained consistently. If using backer rod, the material should be 25% wider than the joint, set to a depth ½ the width of the joint, and should not be compressed, punctured, or squeezed during installation.
Geometry: There are numerous sealant details but all of them should address the same principle: avoid three-sided adhesion. The best performing butt joints are tooled over round backer rods to an hourglass shape. The hourglass shape relieves strain at bond lines by lowering the peak stress and shifting the peak stress to the body of the sealant material and away from the bond lines. Joints are most effective when the sealant depth is half of the joint width and less than or equal to ½ inch.
Proper tooling: Proper tooling ensures the adhesion of a sealant to the joint substrate by the use of mechanical pressure. Tooling should always be done with a dry tool without the use of tooling aids, which can affect adhesion and may also promote dirt retention on the sealant by slowing the curing time. Always avoid using soaps or solvents when finishing joints.
For more information about how to begin your sealant restoration project, please contact Trisco Systems, Inc. at 419-339-3906.
Information for this article was obtained from Sealant, Restoration, and Waterproofing Institute’s publication: “Sealants: The Professional’s Guide”, as well as their Technical Bulletin #7 “Sealant Types and Uses”. Conproco’s “Dictionary of Moisture Protection and Restoration”, 2008 Edition, was also referenced.