Historic Preservation - Technical Procedures
Sealants: Characteristics, Uses And Problems
20Th Century Building Materials (Ed. Tom Jester, Nps)
Thermal And Moisture Protection
Sealants: Characteristics, Uses And Problems
SEALANTS: CHARACTERISTICS, USES AND PROBLEMS
This standard 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
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
- 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
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.
VANDALISM OR HUMAN-INDUCED PROBLEMS
- 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.
- 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.
- 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
- 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
- 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.
END OF SECTION