Discussing Ducts Types and Tips
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00:14 Black flex duct vs. silver flex
04:02 Duct sweating
06:53 Preventing duct sweating at connections
08:24 Duct liner best practices
09:14 Vapor barriers
10:30 Panduit straps vs. silver tape
11:51 Contact cement vs. silver tape
13:21 Outward-cinching stapler use
14:26 Overview of tips
15:09 Running flex ducts properly to prevent compression
We occasionally come across black flex in Florida, which isn’t the same as trailer flex. The black flex is water-resistant and tends to handle moisture a bit differently than a silver flex duct, but it is sometimes erroneously replaced with silver flex or used as trailer flex. The silver flex duct is reflective and tends to reflect heat from the roof decking, whereas the black flex absorbs that heat and results in greater thermal gains inside the duct. Although there is variation between manufacturers, silver flex tends to be a bit more durable than black flex. However, black flex is FAR less prone to sweating than silver flex.
Flex ducts tend to sweat at connections, where the insulation has been compressed, and wherever it touches another surface (like a truss or another duct). Condensation occurs at those locations because they tend to be cooler than surrounding areas (and are more likely to reach the dew point). The ducts and air handlers can also sweat if they’re not touching but merely shade one another.
When you’re making a connection, make sure you understand that condensation is most likely to occur at that point. Be sure to seal it completely and let the mastic (duct sealant) dry on the collar before applying the outer liner to it. Whenever you must use wet mastic, it’s best to keep mastic tape on hand to use as a backup option. It’s also worth remembering that a Panduit strap is just a mechanical connection and won’t stop leakage.
Compression can also cause duct sweating, so address points of potential leakage and make sure you’re pulling the outer lining properly (without bunching the insulation). However, you need to roll the edge of the lining a bit to maintain a vapor barrier on the outside; that vapor barrier must be completely intact to prevent water vapor from making it into the ductwork. The vapor barrier should be against the duct board and not be pulled back or compressed in any way. (Panduit straps can contribute to compression. Silver tape is generally a better option, especially if you use denatured alcohol and apply it smoothly with a tape squeegee.)
Spray glue and other contact adhesives/cement are good for short-term contact, but it dries out and becomes brittle over time. So, we prefer silver tape to contact cement, spray glue, and fab tape. Outward-cinching staplers can be used to attach staple flaps.
Overall, when you fabricate connections, you have to pay attention to the duct type (and material), proper strapping, and how tightly the flex duct is pulled. Having tightly pulled flex duct reduces sweating and turbulence. Ideally, flex duct should only exist in straight runs; we can rely on metal fittings if we need to make an angle.
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