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In Friday Fax #809 back in April we answered the question of whether damming foam is required at the top of a vertical FST Duct Sealant installation, or is gravity enough to keep the expanding FST in place. The answer is, yes, it is required because it restricts the foam expansion, creating critical compression that forces the foam laterally against the duct walls, increasing adhesion. It also forces the foam into every nook and cranny around installed cables. This improves water and gas blocking, and pull-out strength. The top damming wrap is needed for an optimal seal. Now we have a new, closely related question: "Can other materials be substituted for the foam damming strips to block the expansion and create this critical compression?" The reason for the question might be the unavailability of damming strips at an installation. Examples of substitute materials might include solid duct plugs of various composition, balled up rags or paper towels, etc. The answer to this question is, no, other materials should not be substituted. Why? Light, porous materials may not create enough restriction and compression, while solid materials trap expansion gas. This is a no-no. The foam damming wraps supplied in FST kits (and available separately as catalog #FST-DAM) restrict expansion appropriately, which is to create the ideal compression while allowing the necessary amount of gas to escape. Do not recommend substitute damming materials. Note that the answers to both questions apply equally to SolidSeal Duct Plug for communications conduits. Be ready for this question from the field, and make sure your customers have adequate quantities of damming strips. |
