DESIGN/SEALANTS
Figure 3.5.3 shows three examples of poor joint design. In the first case the sealant is not tooled to
the proper depth, the sealant bead is too thick and the bead width-to-depth ratio is too low. In the
second example, there is no backer rod, no support to tool the sealant against, and the bead is too
thick. In the third case there is no backer rod so the sealant is bonding against the back of the joint,
resulting in so-called three-sided adhesion. This will result in cohesive and/or adhesive failure of
the sealant.
The location of the sealant joint within the wall is an important design and installation consideration.
Locating the sealant joint at the exterior subjects the sealant to the most extreme environmental
conditions and the largest differential movements. If the sealant joint is located inward, it is
protected from most of the environmental extremes and is subjected to smaller differential
movements. Also, interior sealants can be installed from inside the building simultaneously with
erection of panels on upper floors and under more severe weather conditions than exterior sealant
application. The application of an interior sealant can be complicated by the location of columns,
beams and floor edges, and these interferences must not be overlooked in the design phase.
UNACCEPTABLE
Sealant not tooled properly
No backer rod
No backer rod
Sealant bead too thick
Sealant bead too thick
Sealant bonding to back of joint
Wrong width-to-depth ratio
No support to tool against
Joint cut too shallow
Figure 3.5.3 Poor Joint Designs (Schroeder and Hovis)
Most guidance on the design and installation of sealant joints contained here and elsewhere
concerns simple horizontal and vertical joints and does not generally address more complex joint
configurations. These include intersections of horizontal and vertical joints, doglegs and other
transitions. The lack of adequate design details for these complexities are a common source of
performance problems due to the unusual stresses and movements that occur at these locations.
An adequate joint design must include details for all joints, intersections between joints and
locations where joints terminate at other envelope components. Without the provision of design
details at all locations, the sealant installation at irregular joints is left to the mechanic in the field.
PAGE 3.5-7
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