Page 255 - Construction Waterproofing Handbook

5.14 CHAPTER FIVE
dual-seal joint system or providing diversion protection in the substrate in addition to the
sealant joint detail. Double protection joint detailing includes
● Double sealing
● Secondary seals
● Binary systems
● Joint protection systems
● Substrate diversions

Double Sealing
Many construction cladding materials used today are permeable to water. Concrete, precast
concrete, masonry blocks, and brick all allow water or vapor to enter directly through the
building cladding and bypass sealant joints. In addition water enters through substrate
cracks, defective mortar joints, and other envelope voids. In many instances, considering
that field construction is not a perfect science, sealant joints may simulate two sponges
sealed together.
When water bypasses a joint through a substrate it travels transversely on a path of least
resistance. Water then collects at backs of substrate breaks or joints (usually where a
sealant joint is installed), finding a path into the interior drawn by the difference in air pres-
sure between interior and exterior. This leakage often appears as joint leakage when in fact
it is due to substrate permeability (Fig. 5.17).
Therefore, it is often prudent to double-seal exterior joints. The secondary joint effec-
tively seals interior areas from water intrusion, bypassing initial sealant joints. If accessi-
ble, second joints are sealed from the exterior (Fig. 5.18), but they can be sealed from
inside the structure (Fig. 5.19). In both cases, joint design should include allowances for
drainage of moisture that passes the first joint, back to the exterior, by installing flashing
and weeps. Double-seal designs should not include materials that are sensitive to negative
moisture drive, which is present in these applications.
Double sealing has several advantages beyond those derived from waterproofing. This
joint design stabilizes air pressure in the space between sealant beads, thus eliminating
positive vapor transmission into a building by air pressure. The interior bead also stops
vapor inside a building from entering cladding where it may condense and cause damage,
such as spalling or corrosion, to building components. This double-sealant installation also
serves as an energy conservation method by effectively eliminating uneven air pressures
that cause airflow into or out of a structure.
Design of inner sealant beads is controlled by design of exterior joints. Since both sides
of a joint movement will be equal, the same material should be used on both. Using
sealants with low movement capability on interior sides leads to ineffectiveness in pre-
venting water and air transmissions.

Secondary Seals
When envelope components are involved that are not permeable, such as glass or metal,
sealant joints can be designed using secondary seal systems to act as a backup barrier to
the primary joint sealant material. The secondary seal should be designed to act as a