By: Orion Doscher
Sealant, commonly referred to as caulk, fills transition gaps between similar or dissimilar materials. It’s used to keep air and water out, and to allow for the expansion and contraction of building materials. Having the proper sealant correctly installed significantly contributes to the watertight integrity and overall lifespan of a façade system.
The two commonly utilized types of sealant for exterior are polyurethane and silicone. Their primary difference is that polyurethane is organic, and silicone is an inorganic material. This directly affects how they react to exterior conditions, and thus their lifespans. The organic polyurethane is sensitive to the ultraviolet (UV) light from the sun, and will break down with exposure to UV rays. Since silicone is not significantly affected by exposure to the sun, silicone sealant will last approximately 20 years, as opposed to the typical 5 to 10-year life cycle of polyurethane sealants. Due to the difference in longevity, the cost of silicone sealants is typically higher than that of polyurethane.
Choosing which type of sealant to install involves analyzing the transition areas where sealant will be installed and keeping in mind the maintenance plan that will be put into effect. In the long run, it may be more cost effective to install silicone sealant one time, versus installing polyurethane twice. On the other hand, longer lasting silicone is not always right for the application. Since they have greater flexibility and movement capability and can expand up to one-and one-half times the size of the original gap, polyurethane sealants are often recommended for larger transition gaps. Hybrid sealants that optimize the strengths of each type are also available.
Once the most appropriate sealant for a façade system is selected, it’s critical that it be properly installed. The goal is to achieve a strong two-point adhesion over a gap by properly preparing the surface, using backer rod and tooling. A sealant with a concave geometry at a material gap and an expansion or control joint provides an hour-glass shape for optimal performance. It is also important to ensure that the sealant is installed at an appropriate temperature. Drastic differences between the internal temperature of the building, and the temperature on the exterior can lead to blistering. In cold weather, heated air inside the building will try to escape at the gaps where sealant is being applied, resulting in air pockets in the sealant.
Surface preparation is key, especially when replacing sealant. Previously installed sealant must be completely removed and the joint to be sealed must be thoroughly clean and dry. Any remaining sealant, especially if it’s a different type than what’s being installed, can allow moisture to collect at the edges of the joint and jeopardize the adhesion of the sealant at the entire gap. Moisture can also lead to blistering, as the water present within the joint can evaporate and form air pockets. Sealant will often present symptoms of premature failure within a few days after installation. There are cleaners and primers available for surface preparation and pull tests can be performed to ensure that installed sealant is well adhered.
Closed cell backer rod is a cylindrical, flexible, polyethylene foam used to fill the gaps between building materials and serves as a bond breaker. Backer rod should be sized at 25% larger than the gap to ensure it can maintain placement during the installation of the sealant. Sealant will only bond with the materials on either side of the gap, and not with the backer rod. The cylindrical shape of the backer rod allows the sealant to maximize the bonded surface area on each side of the gap, while reducing the cross-section width in the middle of the gap. By reducing the cross section in the center of the gap, the sealant becomes more flexible and allows for more expansion and contraction. Backer rod should be installed so that the depth of the installed sealant is one half the width of the gap; the depth should also be uniform. It’s also important to ensure there are no penetrations in the backer rod, and that it’s not punctured or cut during installation. The backer rod acts as a barrier preventing air flow from the inside of the building through the joint where sealant is being installed. If the backer rod is punctured, it can also lead to blistering. Again, blistering will usually present itself within a few days of installation.
Tooling provides an aesthetic component of the sealant installation process as well as an hour glass shape for optimal performance during expansion and contraction; however, incorrect tooling can also lead to premature failure of a sealant joint. In the past, it was common to perform “wet tooling.” A tool dipped in soap and water would be run down the joint. The soap and water would keep the tool clean and prevent the sealant that was being installed from sticking to the tool. The problem with wet tooling is that the slick soap and water mixture created a film over the sealant that interfered with the curing process. So, while this installation method is sometimes easier for on-site workers, it can lead to premature failure of the sealant. Tooling of a joint should be performed in one motion using a dry tool. Tooling will not only make the joint look neat, but can also contribute to the functionality of the joint. If the depth of the sealant is significantly more or less than half the width of the surface adhesion at the 2 points of contact, then that may lead to cohesive failure. Cohesive failure occurs when the sealant joint fails at the center of the sealant due to improper elongation and contraction. The concave profile created by tooling can contribute to maximizing the bonding surface area on the sides of the joint and reducing the cross section in the middle of the joint, similar to the cylindrical profile of the backer rod.
Transitions between building materials are always vulnerable locations for water and air infiltration. Correct sealant that is properly installed on a façade system, prevents both water and air from entering the building. Sealant should not be installed at all joints, however. For example, due to its impermeability, sealant should not be installed above flashings because it will not allow water to escape from the interior of a wall system. The proper selection and application of sealants is vital for a sustainable and watertight façade assembly.