Contractors and designers frequently specify metal building materials for their superior protection against the elements.
A continuous layer of protection for a weatherproof building envelope
By Arnold Corbin
While metal panels comprise much of the air and water barrier of a building envelope, to achieve a continuous layer of protection one vital component cannot be overlooked: proper installation of the appropriate sealant.
Selecting the Correct Sealant
When specifying a sealant for a metal construction application, the best practice is to follow the metal building component manufacturer’s recommendations, especially to attain weathertight warranties that may be available. Not all sealants and sealant manufacturers are created equally.
When a sealant is inadequate, significant failures can compromise the barrier resulting in water leaks, frost and ice accumulation, and air and dust infiltration. To ensure a weathertight barrier, professionals should understand the sealant options available, as well as the performance capabilities of each. Generally, three major types of sealants are used in metal building applications: butyl, silicone and polyurethane. Additionally, polyether is a newer type of sealant that is gaining popularity.
Butyl Sealants
Butyl sealants are non-curing and remain flexible after installation, allowing for dynamic joint movement. Available in both tape and pumpable grades, pumpable butyl sealants are commonly applied to panel seams in the factory, while butyl tape is more frequently applied in the field to seal static joints. The flexibility and self-healing properties of butyl sealants allow panels to expand and contract without compromising sealant integrity, making butyl ideal for use inside joints or panel seams. However, butyl sealants are not UV resistant and should not be used in applications where the sealant is exposed to sunlight because the product will deteriorate leading to sealant failure. When properly installed in appropriate applications such as in standing seam roofs and metal panel joints, butyl sealants offer durable performance and can provide a long life span, often 25 years or more.
Polyurethane Sealants
Polyurethane sealants are curing sealants that offer excellent adhesive qualities, as well as UV resistance and long-term durability. Typically applied in the field, polyurethane sealants are available in a pumpable, gun grade.
With strong adhesive properties, polyurethane sealants are commonly used to adjoin dissimilar materials such as metal trim to a concrete foundation. In roof applications where there is a risk of standing water, polyurethane sealants provide a long-lasting, watertight seal. Unlike butyl sealants, polyurethane is UV stable and available in various colors, making it an ideal sealant for exposed applications. However, polyurethane sealants are not self-healing like butyl sealants and can easily fail when subjected to excessive shear movement.
Silicone
Silicone is a pumpable sealant known for its ease of application and clean up. Silicone sealants typically cure within 15 to 20 minutes after application, so they must be installed in the field. Frequently used inside window applications, silicone is long-lasting and can withstand a wide range of temperature conditions. Silicone is noncombustible, making it the standard sealant for fire-rated assemblies. In addition, it is USDA compliant and commonly used inside food-processing facilities. However, silicone’s bonding properties are not as strong as other types of sealants and can fail under excessive shear force. Like polyurethane, most silicone sealants are not paintable but are available in a variety of colors and are UV stable.
Polyether Sealants
Polyether sealants combine some of the best qualities of polyurethane and silicone, although they are not yet widely used in the industry. With similar curing times to polyurethane sealants, polyether is a field-applied, gun-grade sealant. Polyether sealants have strong bonding properties like polyurethane but are more flexible like silicone. Available in a wide variety of colors, polyether sealants are a good choice for exposed applications where the sealant is color matched to the exterior.
With a variety of sealants available, contractors should carefully evaluate all product qualities to select the sealant that best meets the performance requirements of the project. Relative to overall construction costs, sealants are a minor expense, typically less than 1% of the cost of a building. While it may be tempting to cut costs on a less expensive sealant, it is not worth the risk of thousands of dollars in damage and repairs from sealant failure. By following manufacturers’ recommendations and selecting the appropriate sealant for the application, you can avoid sealant failure and achieve a long service life on metal roof and wall projects
Arnold Corbin is technical services manager for Metl-Span, Lewisville, Texas. To learn more, visit www.metlspan.com.
