Window sealants play an important role in maintaining a building’s integrity by providing a protective barrier against external elements. They’re essential for weatherproofing, as they help seal gaps and cracks around windows, preventing drafts, rain, and wind from penetrating the interior space. They also contribute significantly to energy efficiency by reducing heat loss in the winter and keeping cool air inside during the summer, which lowers energy consumption and utility costs. Furthermore, window sealants prevent moisture infiltration, which is vital for avoiding mold growth, wood rot, and structural damage caused by prolonged exposure to humidity and water intrusion. By ensuring a tight, long-lasting seal, high-quality window sealants enhance the durability, comfort, and overall performance of a building.
Types of Sealants
There are a number of window sealant types: silicone, polyurethane, acrylic latex caulk, butyl rubber, and hybrid (Silane-Modified Polymers or SMPs). Each has pros and cons and is best suited for particular applications.
SILICONE SEALANTS
Pros
Silicone sealants offer excellent flexibility and durability, are resistant to UV rays, weather, and temperature extremes, and have a long lifespan of 20 years or more.
Cons
The disadvantage of silicone sealants is that they have poor adhesion to some surfaces, like wood and masonry, they’re not paintable, and can be expensive.
Applications
Silicone sealants are best used for glass-to-metal, glass-to-glass, or glass-to-ceramic applications and exterior windows where flexibility and UV resistance are needed.
POLYURETHANE SEALANTS
Pros
Polyurethane sealants offer strong adhesion to a variety of materials, including wood, metal, and masonry, are paintable after curing, and provide good durability and flexibility.
Cons
The downside of polyurethane sealants is that they have a shorter lifespan compared to silicone, typically 5 to10 years. They’re also sensitive to UV exposure and may degrade over time unless protected. In addition, they take longer to cure.
Applications
Polyurethane sealants are recommended for wood and masonry windows and for applications requiring high strength and adhesion.
ACRYLIC LATEX CAULK
Pros
Acrylic latex caulk is paintable and easy to apply, is cost effective, and is low VOC and easy to clean up with water.
Cons
Acrylic latex caulks have a downside in that they’re less flexible and can crack over time, they’re not as durable in extreme weather conditions, and provide poor adhesion to non-porous surfaces like glass.
Applications
Acrylic latex caulk is best for interior window trims and frames and where painting is required.
BUTYL RUBBER SEALANTS
Pros
Buty rubber sealants offer excellent adhesion to various materials, are highly water-resistant, and remain flexible in extreme temperatures.
Cons
The disadvantage of butyl rubber sealants is that they’re stringy when applied, which can make them messy. In addition, they aren’t paintable, and can shrink over time.
Applications
Butyl rubber sealants are best suited for skylights and roofing windows.
HYBRID SEALANTS (SILANE-MODIFIED POLYMERS OR SMPS)
Pros
Hybrid sealants combine the best properties of silicone and polyurethane and are made with silane-modified polymers, also known as SMPs. They provide strong adhesion and UV resistance and are paintable and low VOC.
Cons
The disadvantage of hybrid sealants is that they are more expensive than traditional sealants and have limited availability in some markets.
Applications
Hybrid sealants are best for high-performance exterior window sealing and applications requiring durability and paintability.
Other Considerations
Preparation
Window sealant application — especially exterior application — is more sensitive to environmental factors than many other construction tasks. Surfaces to be sealed need to be clean, dry, and free from dust or grease for proper adhesion. This can take careful planning and coordination to ensure that materials kept on site are properly stored and protected from the elements and that sealing is done when the temperatures and humidity are in the proper range. Before sealing takes place, surfaces should be inspected and cleaned if necessary to ensure proper bonding. The time spent properly prepping can save considerable time and expense in preventing callbacks.
Color Options
Silicone, polyurethane, acrylic, and butyl sealants are available in only a limited number of basic colors. Hybrid sealants, however, have many more color options allowing for better aesthetics.
Application
Some sealants need a primer for proper adhesion to certain substrates. Joint depth should also be considered. Proper joint width-to-depth ratio is generally 2:1, which allows for optimal movement. For deep gaps, backer rods help control sealant depth and allow for adequate movement.
Applying during hot weather may make the sealant runny, or during cold weather may make it stiff and difficult to extrude. Either of these situations may make application ineffective. In addition, applying too little or too much sealant can cause flaws in the bead, and improper tooling of the bead can leave air pockets or gaps. Omitting backer rods to fill in deep joints can require too much sealant, which can result in reduced flexibility.
Curing
Some sealants require specific curing conditions. Applying a long-curing sealant during hot temperatures may make moisture escape by bubbling up through sealant, causing holes. Extreme temperatures and high humidity can cause longer cure times, which can reduce bonding. Dirt or dust in the air can collect on a wet bead before it skins over, so it’s essential to ensure that the air is free of contaminants until skin-over takes place.
Conclusion
Sealing windows is an important task that needs proper preparation and execution to ensure the job is done correctly. This includes careful selection of the sealant for the specific application and that it’s applied under the right conditions to meet code requirements and to ensure maximum performance and lifespan of the windows. RB
