Technical Procedures Disclaimer
Prior to inclusion in GSA’s library of procedures, documents are reviewed by one or more qualified preservation specialists for general consistency with the Secretary of Interior Standards for rehabilitating historic buildings as understood at the time the procedure is added to the library. All specifications require project-specific editing and professional judgement regarding the applicability of a procedure to a particular building, project or location. References to products and suppliers are to serve as a general guideline and do not constitute a federal endorsement or determination that a product or method is the best or most current alternative, remains available, or is compliant with current environmental regulations and safety standards. The library of procedures is intended to serve as a resource, not a substitute, for specification development by a qualified preservation professional.
We’ve reviewed these procedures for general consistency with federal standards for rehabilitating historic buildings and provide them only as a reference. Specifications should only be applied under the guidance of a qualified preservation professional who can assess the applicability of a procedure to a particular building, project or location. References to products and suppliers serve as general guidelines and do not constitute a federal endorsement nor a determination that a product or method is the best alternative or compliant with current environmental regulations and safety standards.
This guideline includes general information on the characteristics and common uses of sealants and identifies typical problems associated with this material along with common causes of its deterioration. For guidance on replacing deteriorated sealant, see 07900-03-R.
Characteristics of sealants:
- Typically made of synthetic elastomeric materials.
- May be single-component (no mixing required), or multicomponent (mixing required).
- Good adhesion
- Good cohesion
- Good elasticity
- Good weathering characteristics
- Common types of sealants include silicone, polyurethane, polysulfide, acrylic, latex and butyl-based.
- Typical historical and current uses for sealants include:
- Prior to 1950, oil- and resin-based caulks were the most common building joint sealants; typically used in bearing masonry construction; these were not considered elastomeric sealants due to their limited movability.
- More elastic sealants were used after 1950 due to the popularity of curtain wall construction, which tends to move more than masonry construction.
- Polysulfide sealants: First widely used elastomeric sealant.
- Silicone sealants: First developed in the 1950s as two-component products; first silicone building sealant (one component) was developed by Dow Corning about 1960; typically used for nonporous surfaces with a high factor of movement; common in metal and glass cladding systems.
- Silicone sealants were also used for structural joints in storefronts in place of mullions; this was common in the 1960s.
- Butyl sealants: available in construction in the mid-1950s.
- Acrylic sealants: Available in the 1920s; first acrylic sealant for buildings developed by Tremco Manufacturing Company in 1958; typically used in small-scale construction under conditions requiring limited movement.
- Urethane sealants: Typically used in joints requiring abrasion resistance; first ones were multicomponent types; typically used for porous surfaces with a high factor of movement such as cladding joints.
- Latex sealants: Available in the 1960s; typically used in light building construction and residential construction under conditions requiring limited movement.
- Isobutyl-based sealants: Typically used for glazing joints.
- Sealants are commonly used in joints between individual stone or metal panels, between stone panels and flashing, at expansion and coping joints in masonry, around window and door openings, and in joints at horizontal surfaces.
- Butt-joints are the most commonly used, but other types include fillet joints, lap joints, glazing beads, and glazing heel beads.
Natural or inherent problems:
- Staining: Common with silicone sealants; visible as dirt on the sealant, or staining on the adjacent masonry where the sealant's plasticizer has migrated into the substrate.
- Weathering: Deterioration of sealants can be caused by prolonged exposure to water, ultraviolet light, and freeze- thaw cycles; evidence of weathering appears in the form of chalking, discoloration, cracking, or softening.
- cohesive failure: this means deterioration of the internal integrity of the sealant; cracking parallel to the interface of the joint is an indication of this type of failure
Vandalism or human-induced problems:
- Loss of Adhesion: When the sealant separates from the substrate; this may be caused by the presence of coatings or contaminants that prevent proper adhesion; adhesion is also a problem - especially with silicone sealants - if exposed to prolonged periods of wetting; poor adhesion may also result from poor surface preparation, incompatibility of substrate with selected sealant, or incompatibility of old sealant with new sealant - understanding of the substrate and sealant properties is essential to sealant performance.
- Inappropriate Choice of Sealant and Improper Joint Design: Correct installation of the bond-breaker tape or compressible foam backer rod is important in preventing the sealant from adhering to the sides of the joint; temperature is also important when installing sealant - it shouldn't be too hot or too cool, otherwise the width of the joint will not permit the installed amount of sealant to accommodate expansion and contraction.
- Uncured Sealant: Most common in multicomponent urethane sealants; sealant that is uncured is often due to incomplete or improper mixing of the sealant components, or from using materials that have outlived their shelf life.
- Bubbling and Blistering of the Surface: Common in single- component and multi-component urethane sealants; in single- component types, this can caused by curing at high temperatures or high humidity; in multi-component types, this can be caused by curing at high temperatures, which affects both the cure and durability of the sealant.