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?
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.