LBNL Report Number
This report summarizes the results of two high temperature longevity tests conducted by the Energy Performance of Building Group (EPB). The first test involved the aging of common "core-to-collar joints" of flexible duct to sheet metal collars, and sheet metal "collar-to-plenum joints" exposed to continuous 200°F (93°C) circulating air. The second test consisted of baking duct tape specimens in a constant 212°F (100°C) oven following the UL 181B-FX "Temperature Test" (Underwriters Laboratory 1995) requirements. The longevity of the sheet metal collar-to-plenum joints was tested based on the 10% of unsealed joint leakage criteria (established in previous work (Walker et al. 1998 and 1999, Walker and Sherman 1998, and Sherman et al. 2000)). Since an unsealed flexible duct joint can have a variable leakage depending on the positioning of the flexible duct core, the longevity of the flexible duct joints could not be based on the 10% of unsealed leakage criteria. Nevertheless, the leakage of the sealed specimens prior to testing, could be considered as a basis for a failure criteria. Visual inspection was also documented throughout the tests durations. The flexible duct core-to-collar joints were inspected monthly, while the sheet metal collar-to-plenum joints were inspected weekly. The baking test specimens were visually inspected weekly, and the longevity was judged by the observed deterioration in terms of brittleness, cracking, flaking and blistering (the terminology used in the UL 181B-FX test procedure). The baking specimens were ranked in terms of these deterioration criteria. The deterioration in the flexible duct core- to-collar joints was also ranked following the same criteria. This study concluded that the duct tape performance in sealing joints depends on the joint’s geometrical dimensions; it gets worse as the number of dimensions required to describe the joint increases (1-D to 3-D). 1-D joints are the easiest to seal with duct tape, and thus the least to experience failure. However, this type of joints is rarely seen in field installations. 2- D joints, such as the flexible duct core-to-collar joints tested in this study, are less likely to fail than 3-D joints, as the shrinkage could have a positive effect in tightening the seal. 3-D joints, such as the collar-to-plenum joint, are the toughest to seal, and therefore failure most rapidly. This is essentially caused by the shrinkage of the duct tape backing that results in the peeling of its rubber-based adhesive off the sheet metal fixture. The baking test results showed that the failure in the duct tape joints could be attributed to the type of combination of the duct tape and the material it is applied to.