Page 250 - Construction Waterproofing Handbook

SEALANTS 5.9
material applied in a joint. It also provides a surface against which uncured sealant mate-
rial can be tooled to force it against both sides of the joint for proper installation.
Sealants do not adhere to the backer rod, only to the joint sides. Three-sided adhesion
(two sides and bottom of joint) places too much stress on sealant material in movement
(elongation), causing tears that result in cohesive failure. See Fig. 5.10.
Backer-rod is round extruded foam that is installed into joints by using a much-larger-size
rod diameter than the joint width. After being fitted into the joint, the rod expands tight against
the sides of the joint to permit the application of sealant over it. To operate in this manner, the
rod material used should be a minimum 25–50 percent larger than the joint width.
In joints not sufficiently deep for installing a backer rod, a bond breaker tape should be
used, Fig. 5.11. For cant or fillet-shaped joints, either tape is used or some manufacturers
now produce backer rod that comes in half or quarter-round shapes to facilitate the proper
installation of sealant materials in these types of joints, Fig. 5.12 and Table 5.2. Half-round
backer rod should be 20–40 percent larger than the joint opening width. Quarter-round
material is used on cove joints, see the installation section and detail in Fig. 5.53, on whose
horizontal substrate meets a vertical substrate.
There are four basic types of backing systems for sealant application:

● Closed-cell backer rod
● Open-cell backer rod
● Dual-cell backer rod
● Backer tapes

CLOSED-CELL BACKER ROD

Closed-cell backer rod is a cylindrical closed cell polyethylene foam. It is extruded to
eliminate any open cell structure that can permit moisture or air transmission between
the cell structures. Closed cell backer-rod is recommended for joints susceptible to the
presence of moisture prior to joint sealing such as horizontal joints.
If moisture accumulates in the backer rod (possible with open-cell materials), it will
prevent the sealant from curing, as moisture remulsifies sealants. Closed-cell rod is not
susceptible to moisture, but it can not be used with air-cured material since it prevents cur-
ing the unexposed backside of the sealant material.
Closed-cell backer rod materials can cause sealants to improperly cure and “out-gas”
if the closed-cell structure is inadvertently damaged during installation. Should the baker-
rod be cut improperly or otherwise damaged and sealant then applied over it, the dam-
aged cell release entrapped air into the uncured sealant. This causes blistering in the
sealant during the curing process, Fig. 5.13. Whenever closed-cell material is used, appli-
cators should never use sharp applicator tool that can puncture the cells and cause this
out-gassing problem.
Nongassing formulations of closed-cell backer rod are available, manufactured from
polyolefin material. Such formulations claim to prevent damage to sealants if accidentally
punctured during installation.