How you secure two planes together—such as fiberboard siding and walls, or window flanges and wallboard, for example—can have a big impact on the performance of the building envelope. So making sure your sealant bead is right can be critical. In this article, OSI Application Engineer Paul Majka explains how to lay a proper bedding sealant bead and why it’s important.
When you’re laying a bedding bead, you’re essentially applying a gasket. So your bead should be shaped like a gasket—rounded, like an O ring. Some installers will flatten the bead by pushing the tip of the gun into it too much. It’s an easy mistake to make if you aren’t an expert in sealant application. But it can really undermine the seal’s performance.
Bedding beads are laid in bedding joints, which is a type of sealant joint formed when the surfaces of two planes need to be bonded and sealed together.
A bedding bead is actually doing double duty. It’s both sealing and bonding two substrates together in a cladding assembly. Sealant, rather than adhesive, is used here to not only adhere the two materials together but also seal against the water and moisture.
Apply a 3/8” inch rounded bead to one of the substrates. Compress both substrates together to bond. Avoid excessive movement between substrates during compression. This will break down the sealant’s capacity to maintain a gasket seal.
Because bedding beads are typically concealed behind various cladding materials such as trim blocks or window or door flanges, it’s usually easier to spot a poorly applied bedding bead than a proper one. A proper one may just show some slight sealant oozing from behind a window or door flange. But an improper bedding bead will show smearing or uneven clumping around the edges of the flange. In these cases, there’s a good chance that the ability of the sealant to deliver the performance needed to protect and secure the building envelope has been compromised.
This sealant application is typically not exposed to sun but can be subjected to water, air, heat and cold. It is important that the two joint substrates are compatible with the sealant for performance. If there is any disparity here the joint will be compromised and fail in performance against these elements.
