Breaking the Bonds of Bad Sealant Jobs

Seals at window openings and other penetrations need to be done right the first time. Are your seals failing because of the most common application error—forgetting the bond break?

It's great they remembered the backer rod in this attempt to seal the joint, but with the depth of the sealant so skinny compared to the width, it's probably going to fail. Read on to learn why. 

NOTE: Read this whole series here.

Continuous air and water barriers are essential to healthy and high-performing buildings, but making these barriers truly continuous is more than just slapping on some building paper. It requires meticulous detail work. Sealants—properly applied—are a key part of that.

Sealants are liquid-applied substances tooled to a concave surface shape, with “edge bonding” to each substrate. In the case of air and water barriers, they connect one field of the wall to another or to the component in the penetration—the window, the pipe stack, the duct, etc.

Drawing a bead on proper joint sealing

Essential to any sealant application is a backer rod or bond breaker tape. These ensure that:

• adhesion is between the substrates only (no perpendicular stress from the back of the joint to weaken the focus on the connection between the substrates)

• the sealant is supported on the back side as tooling exerts pressure on the sealant

• the sealant bead is well-proportioned (ratio of width to depth of 2:1)

Backer rods come in various diameters so that they compress about 25% of their cross-section into the gap.

Open-cell backer rods have the advantage of “breathing,” allowing curing to the backside of the joint, and are not affected by any puncturing that might result during tooling. By contrast, closed-cell backer rods, if punctured during tooling, can offgas and create bubbling in the sealant. Closed-cell backer rods don’t absorb water, while open-cell ones do. A third type of backer rod is the “hybrid” bi-cellular backer rod; it does not outgas when punctured and only takes up moisture at cut ends.

Use the type of backer rod recommended or required by the sealant manufacturer.

Conventional moving weatherseal.
Photo Credit: Dow Corning Americas Technical Manual, page 56

Bond breaker tapes are particularly well suited to closed joints (no gap), where the two substrates are perpendicular to each other. Omitting bond breaker tape is incredibly common in this configuration—even though it is this perpendicular, or “fillet,” configuration that places the most stress on the sealant with any joint movement.

You can’t mechanically support a sealant joint the way you can a tape or membrane; the adhesion of the sealant to the substrates alone is what you are counting on.

The corner bond break relieves the stress on the non-compressible sealant bead.
Photo Credit: Dow Corning Americas Technical Manual

Performance testing

There are a slew of ASTM standards to consider for liquid sealant performance. The best guidance on this comes from the National Institute of Building Science (NIBS)
 Whole Building Design Guide (WBDG) Web Resource Page on Joint Sealants (pdf). This document also has useful sample specification information.

Just as important, though, is performance field-testing of sealants. Dow Corning’s 2011 Technical Manual (pdf, needs email address to proceed) includes a “Standard Field Adhesion Test.” Although clearly best suited for the commercial construction context, the concept and techniques can be applied to residential construction.

Field adhesion test.
Photo Credit: Dow Corning Americas Technical Manual, page 73

Seal the deal: Three tips for putting it all together

• Use liquid sealants as part of air and water barrier systems in exposed applications where they can be inspected, repaired, or replaced.

• Field-test sealants in addition to applying standardized tests in specifications.

• Use manufacturers that integrate their products into continuous barrier systems; one of note is the Tremco Engineered Transition Assembly. This approach fully specifies the air and water barrier system for commercial building assemblies, including: an ASTM test (E2357) for airtightness that includes a window in the assembly; project management and pre-construction meeting checklists; and a one-of-a-kind warranty against air and moisture infiltration.

NOTE: Establishing barriers at the cladding level of the building assembly is considered a “face-sealed” approach only appropriate for precipitation exposure of 20 inches annually, or less. And the appropriate approach for face-sealed, surface level barriers is the “weeped”, two-stage sealant system describe below.

Rainscreen Joint.
Photo Credit: United Professional Caulking and Restoration