The sealants used to provide weather protection in masonry expansion and control joints form the final link in the cladding envelope. Successful movement joints that are watertight and durable require proper joint design and construction; adequate joint sizing; appropriate product selection; and careful sealant installation. The choice of sealant material should be based on performance, aesthetics, and durability. Only high-performance elastomeric sealants, such as polysulfides, urethanes, and silicones, should be used in masonry control and expansion joints. These sealants are covered by ASTM C 920 Standard Specification for Elastomeric Joint Sealants.The movement capability of sealants is closely related to modulus of elasticity, defined as the ratio of the force (stress) needed to elongate (strain) a sealant to a certain point. Many sealants are available in single-component and multi-component formulations. Single-component sealants are supplied ready for application and require no mixing. Multi-component sealants are mixed at the jobsite just before application. To maintain movement capability over an extended service life, sealants must have good elasticity in cyclic movements. Elasticity is a measure of the sealant's ability to recover its original shape after elongation or compression. Recovery for good quality elastomeric sealants may range from 75 percent to 95 percent or higher. Potential joint sealant problems include chemical deterioration from ultraviolet radiation, surface discoloration, and lack of substrate compatibility. Polysulfide, urethane, and silicone sealants each have advantages and limitations that make them more or less suitable for any given application. ASTM standards should be used to specify sealants because they establish minimum acceptable criteria for different types. Within each generic sealant type, individual sealant formulations vary from one proprietary product to the next. To help facilitate product comparisons on an apples-to-apples basis, Sealant Waterproofing & Restoration Institute (SWRI) has instituted a sealant validation program. To get a product validated, the sealant manufacturer submits a request to an independent laboratory, which tests the product for compliance based on three key performance criteria. If the product passes all the tests, the laboratory submits the results to SWRI, which then grants a certification of validation to the manufacturer.