Caulks and sealants are used in metal construction to fill gaps and cracks. They are a barrier to prevent the passage of air, water, moisture, gas, noise, dust and smoke. Generally representing less than 1 percent of a building's cost, they are extremely important to the water/airtightness of the building. For this reason, correct selection based on properties and applications is important to the weathertightness of the building envelope.
Which caulk and sealant will fill your inside straight?
Caulks typically are associated with filling gaps that do not experience much expansion or contraction, and are used to prepare for painting. They are rigid and inflexible. In metal construction, caulks are used on the interior filling gaps between drywall, windows and trim, or casework before paint is applied.
“The word caulk is an old boat building term and is sometimes used by manufacturers as a general purpose term for acrylic materials with little or no movement capability,” says Jason Bakus, vice president of Sealex Inc., Harbor Springs, Mich. “Acrylic latex caulks are normally paintable with water-based or solvent-based paints, but are not generally used in metal construction due to the amount of shrinkage they experience as well as their tendency to crack over time. Most manufacturers refer to their elastomeric sealing products as sealants.”
Sealants play a vital role in metal construction and act as a seal between metal and other exterior materials to form a barrier for infiltration or exfiltration of moisture, air and airborne particles.
“Sealants are used when there is a high likelihood of expansion, contraction or movement between the metal substrates and are designed with polymers allowing flexibility,” says Dr. Roger Moore, director of marketing and product support manager, Novagard Solutions Inc., Cleveland. “Sealants like Novaflex metal roof sealant serve in a number of roles in metal construction including structural glazing as well as any application where a seam between two metal substrates come together and a flexible watertight seal is required.”
Sealants are used in many different applications in metal construction ranging from standing seams, metal end laps, roof penetrations and curbs to expansion joints, roof to wall transitions, roof steps and height changes, ridge expansions, gutter seams and many others.
Metal panels create a couple of distinct challenges for sealants. “First, metals expand and contract with changes in temperature, so joints with metal panels definitely experience dynamic movement,” says Bee Miller, manager of market development, Architectural Specifications, Franklin International- Titebond Adhesives and Sealants, Columbus, Ohio. “Additionally, because many of the most common paints and surface coatings in the industry are designed to last for a long time, some of the specialty coatings can be challenging to bond to and maintain a strong bond with.”
Silicone sealants provide excellent joint movement capabilities (ASTM C920, Classes 25, 35, 50 and 100/50) up to 50 percent. Silicone sealants offer superior adhesion to common building material substrates including glass, aluminum, wood, steel, vinyl and masonry. However, a primer is recommended for use on some substrates, particularly cementious substrates. “Documented studies on the long-term performance of silicone sealants have been published by the major manufacturers and indicate performance in excess of 20 years in terms of resistance to moisture, oxidation, high temperatures and UV exposure,” says Moore. “These products are not recommended in high-traffic areas where abrasion may cause degradation of the surface. Application temperatures of silicone sealants far exceed those of inferior polymeric sealants and often range as low as -20 F to +140 F and also have large service temperature ranges such as -40 F to +400 F.”
Most silicone formulations are not paintable; however, modified silicone sealants may be paintable. Silicone sealants may stain some porous substrates such as concrete and some natural stone substrates. Silicone sealants are well known for their ease of application and clean up. They are typically 100 percent solids, or non-solvent type, and easily meet VOC environmental regulations, are not flammable and clean up easily after use.
Pre-cured silicone sealants-or silicone membranes as they are sometimes called-are commonly used in a variety of metal building applications. “Because of their ultra-high movement capability (200 percent plus) and unique properties, they are used for sealing high movement areas on metal building applications such as expansion joints, roof to wall transitions, roof height change details, joints between new and existing buildings, ridge applications and others, including a variety of repair applications,” Bakus says. “Pre-cured sealants are bonded to metal and other substrates using a separate adhesive and require no fasteners.”
In the past, there has been some concern with compatibility of some silicone sealants with metals such as Galvalume and galvanized. “This issue was with acetoxy (acid-based) cure products, which are no longer used in metal construction applications,” Bakus says. “Neutral cure silicone sealants have been used in metal construction applications for many years and do not pose corrosion problems with these metals.”
Not used as commonly as silicones, polyurethane sealants offer superior joint movement capabilities and have good adhesion to most common building substrates. Polyurethanes for the metal building industry are generally one-component, moisture-cure sealants designed to skin and cure rapidly. Premium polyurethanes are specified due to their superior UV resistance and long term durability.
Builders should select a polyurethane with a minimum of 50 percent joint movement: +/- 25 percent. “Selection of polyurethane can depend on substrate,” says Mark Platz, industrial business manager, ITW Polymer Sealants North America, Irving, Texas. “With today’s specialized coatings, not all polyurethanes adhere the same to all surfaces. While some can adhere to wet surfaces or even underwater, others may require a primer or pretreatment depending on the substrate. Premium polyurethanes remain flexible with life expectancies reaching 20-plus years depending on exposure to extreme elements. They cure to a tough, durable, elastic consistency with excellent cut-and-tear resistance, come in a variety of colors, and most are paintable.
“With service temperatures from -40 F to 200 F and elongation availabilities of 500 to 600 percent polyurethanes are frequently requested by the metal building contractor. Polyurethanes are available in gun grade (cartridges) and can be non-sag or self-leveling. They are easy to tool for an aesthetically clean finish. When applied in close proximity, neutral cure silicones can prevent polyurethanes from curing. This problem does not exist if either product is allowed to cure prior to application of the other.”
Typically polyurethanes exhibit good compatibility with the metal and masonry surfaces; however, “They should not be used in structural glazing applications with contact with glass,” cautions Moore. “These sealants can be formulated to give aboveaverage UV resistance and may be paintable. Some formulations contain solvent, and shrinkage due to solvent evaporation must be taken into consideration.” Some health professionals recommend wearing respirators during application.
The most common solvent-based synthetic rubber sealants are acrylic. They are most often used in perimeter sealing or other low joint movement applications. They may need special handing due to flammability and they have environmental considerations. “Solvent-based sealants typically have good durability and can be applied at below freezing temperatures,” says Miller. “They are typically paintable but only after a seven- to 14-day full-cure period. They are flammable in the wet state, can be difficult to tool due to short open times, and can produce a significant odor during application and cure time.”
Non-skinning, non-drying (Butyl) sealants are the primary sealant in standing seam roofs and the joints of insulated metal panels. Designed to stay soft and flexible, they ensure a positive seal when jointing roof or wall panels. They should exhibit a non-stringy consistency with easy cut-off characteristics for clean application.
“Butyl sealants are easily pumped into the female leg of standing seam roof panels and are compatible with all current types of paints and coatings used by today’s rollformers,” says Platz. “This product does not cure like standard pumpable sealants, allowing for movement, self-healing, and can offer a life expectancy equal to that of the roof system. This sealant requires an application temperature range of 10 F to 120 F and a service temperature range of -60 F to 200 F. This is a non-curing sealant, therefore it is not paintable, is supplied in white or off-white (color) and is used in conjunction with mechanical fasteners.”
Within the past few years, hybrid sealants such as modified silicone (MS) (silyated polyether) and SPUR (silyated polyurethane) have come onto the construction market and claim the best qualities of both silicone (UV resistance) and polyurethane (paintability). Hybrid sealant use in metal construction is minimal at this time, but is growing.
“Common hybrids used are tested to withstand +/-50 percent expansion/contraction,” says Miller. “Hybrid life expectancy is generally very good. Appearance and UV resistance are very good, and they are typically paintable if discoloring occurs from weathering over time. They handle and tool nicely and have low to no odor.”
When selecting caulks and sealants, evaluate all performance characteristics to determine the optimum sealant against the cost. Discuss your sealant applications with the manufacturer to determine the best product for each application as there is no one product for all applications. Compatibility with the substrate may require different curing mechanisms.
Mark Platz, industrial business manager, ITW Polymer Sealants North America, Irving, Texas