The performance of sealant joints depends on several design issues including the configuration of
the joint itself and the selection of the sealant and backup materials.
The basic objectives of building joint design are to provide a seal that prevents rain penetration,
excessive heat flow, water vapor migration, and air leakage. As discussed in the section Design/
Rain Penetration, rain penetration can be prevented with a perfect seal, i.e., the face seal approach.
Creating such a perfect seal in the often harsh environment of the building skin is difficult, and in
fact it is not necessary. Instead, the joint can be designed to control the forces causing rain
penetration using a pressure-equalized joint design. This latter approach actually prohibits the use
of an air seal at the wetted plane. An air seal is still necessary to control air leakage, but it is
located inward of the wetted plane so that air pressure, the major force causing rain penetration,
can be controlled. Locating the air seal inward from the exterior also protects the sealant from
environmental stresses. Heat flow at joints is generally small due to the small cross-sectional area
involved and can be reduced using a dead air space or some insulation material. Water vapor
diffusion can generally be ignored, especially if there is no exterior seal at the joint. In heating
climates, however, severe vapor condensation can occur if interior air leaks into cold spaces in the
joint. In cooling climates, condensation can occur if humid exterior air leaks through the joint and
contacts cold interior surfaces.
The two basic considerations in sealant joint design are the determination of the expected
dimensional movement of the joint, and the geometry and configuration of the joint. The basics of
sealant joint design and movement are covered well by O'Connor. This document describes the
various performance factors that must be considered in joint design and provides three sample
calculations of joint width. For aesthetic reasons, designers may prefer to limit the width and
number of sealant joints without proper consideration of whether the resultant joint design will be
effective. In some cases the joints are made excessively wide to make up for their insufficient
number, or there are enough joints but they are made as small as possible. In either case, the joint
design will be inadequate and, as a result, the building will create more joints by cracking or worse,
i.e., walls will bend, joints will be crushed, or curtain wall fasteners or masonry ties will be sheared.
In order to design a sealant joint, one must determine the expected movement of the joint given the
numerous factors affecting this movement. O'Connor discusses these factors, including thermally
induced movement, structural loading and construction tolerances, and how they must be
considered in designing sealant joints. Basically, one must consider each of the various
performance factors and determine the required joint width and expected range of movement.