For Want of a Nail: The Importance of Sealants
by Larry Livermore
Sealants represent a very small part of the cost of a wall system, but they play a major role in its performance. They are keys to properly-installed fenestration products from single-family homes to high-rise curtainwall systems.
However, unlike a one-size-fits-all approach for many materials, using just any old caulking doesn’t even approach fulfilling the need for proper sealing. The designer or specifier is challenged with a wide choice of sealants for a specific application.
The Selection Process
Selecting the right sealant for a given application involves several considerations. It must provide a weather-tight seal that remains intact despite movement of the joint due to thermal differentials, windload and building movement. In some glazing applications, it must also be capable of absorbing stress from external sources without transferring that stress to the glass. Meanwhile, it must withstand the environmental onslaughts of water and ultraviolet radiation and retain its properties as it ages. Consideration must be given to the type of substrate to which the sealant must adhere (a primer coat being necessary for some), the type and configuration of the joint to be sealed and the compatibility of the sealant with other materials.
Sorting Them Out
Perhaps the most critical of the performance characteristics that must be considered is a sealant’s adhesive strength, cohesive strength, percentage of recovery (after deformation), tensile strength and durability under the effects of weathering. The importance of adhesive and cohesive strength is self-evident. Other characteristics include hardness, thin film integrity, permanent set and compression set, peel adhesion, yield strength, slump, sag, low-temperature flexibility, ultraviolet and water resistance.
AAMA JS-91 identifies sealant categories as low, medium or high performance, with the classification indicating the capability of the sealant to handle joint movement as follows:
• Low Cyclic Performance Sealants. Low cyclic performance sealants, or caulks, are those with the least ability to withstand movement. They are used in relatively stable joints (those likely to experience less than ±5 percent joint movement)—typically referred to as “painters caulk” for sealing around door and window frames in single- and multi-family residential buildings. Generic materials used for these include drying oil-and-resin-based compounds, bituminous-based caulks and mastics and some polybutene compounds.
Expanding aerosol polyurethane foams, which are also used in residential or light commercial projects to seal the joints between the rough opening and the window frame, could be considered part of the low cyclic performance category. Care must be taken to select a foam with an appropriately low “pressure build,” so that its expansion as it cures will not cause deflection of the window frame and hinder proper operation or create gaps that actually degrade window performance. Low pressure foams should be used to provide an air seal.
• Medium-Performance Sealants. These are sealants having a joint movement capability of ±5 percent to ±12.5 percent. Included in this group are the acrylics (solvent and waterborne), butyls, neoprene, nypalon and other synthetic resin compounds, etc.
• High-Performance Sealants. High-performance sealants are those which have a movement capability of ±12.5 percent to ±25 percent. It is this class of sealants that is used far more extensively than any others in architectural grade and curtainwall applications because it can accommodate significant amounts of movement in wall joints. The types included in this category are silicones, polyurethanes, polymercaptans, polysulfides and some solvent release and waterborne acrylics.
High-performance sealants are especially required for architectural-rated fenestration products, such as those designed for exposure to higher windloads as experienced in multi-story applications.
Fortunately, guidance is available for the selection of sealants. AAMA offers several publications about sealants, which can be accessed by visiting the
AAMA Publications Store at www.aamanet.org.
Larry Livermore serves as technical standards manager for the American Architectural Manufacturers Association in Schaumburg, Ill. He may be reached at
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