What is the long-term durability of flex duct?
lance_p
| Posted in Energy Efficiency and Durability on
The Unico ductwork in this other thread:
didn’t survive too well in that application. What about the more common forms of insulated flex duct being used? Are there any products in particular that have shown to be worse than others? Any utter failures?
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I can’t speak to the specific type of flex duct shown in that thread you referenced, but I can tell you that plastic flex duct will always degrade more quickly than metal flex duct. I avoid the use of flexible plastic ducting for this reason. In terms of long term durability, the best will be the corrugated metal flex duct, followed by the foil-type flex duct, and the plastic flex duct will be quite a bit less durable than the metallic types.
I suspect the reason the flexible ducting fails in this application is due to the moving, potentially hot, air degrading the polymer or depleting the plasticizers more quickly than if the plastic was used in a non-airduct applications.
Bill
Thanks for your thoughts, Bill. Are insulated versions of the corrugated metal ducting available, or do they require post-insulating? I would like to keep as much of our ducting insulated as possible, especially ventilation returns from bathrooms and supply ducts for HVAC.
Ventilation returns worry me in particular as cool metal ducts flowing warm humid air from showers will be sure to end up with condensation inside and I would want to make sure corrosion didn't compromise them. Perhaps PVC would be a better choice for bathroom exhausts.
I’ve seen pre-insulated foil duct, but the corrugated metal stuff I think you’d have to insulate yourself. You can get insulating sleeves for the duct where you just pull the duct through the sleeve. The sleeves are easier than the wrap if you just have a short run. If you’re doing the work yourself, it’s probably best to use the heat materials the first time and not worry about doing it again some day in the future.
I’ve never used PVC for air ducts. I’d be ok with it for a bathroom exhaust fan, but I don’t think I’d use it for forced air ducting.
Bill
I like the idea of using traditional flex duct for the first ten or so feet from a mechanical system just for its noise absorbing and vibration isolating properties. Also, if it's going to be accessible in a basement ceiling the risk is low, in my opinion.
Going up through walls and snaking through the second floor is where I get nervous since it's an immense job to replace at that point. You're pretty much tearing the house apart to gain access.
You really shouldn’t be using flex duct inside of walls. The big application for the stuff is above drop ceilings in commercial buildings between a main plenum and the vents in ceiling tiles. In residential construction, it’s probably most common with bathroom fans and exhaust for dryers and water heaters.
Flex duct should really only be used where absolutely necassary for HVAC applications. Flex duct has much more flow restriction compared to rigid ducting. I’d never run flex duct inside of walls unless I has to fish a run for some retrofit application. In new construction or large renovations you are far better off using rigid ducting as much as you can.
You can put sound baffles inside of rigid ducting if you need to attentuate sound. I’ve rarely seen it done, but it is possible. It’s common in generator rooms and large mechanical rooms so commerical contractors are likely to be more familiar with it. The sound baffles look like airfoil-shaped vanes made of perforated sheet metal and filled with fiberglass or mineral wool. They are quite effective.
Bill
Flex duct can last pretty much "forever," even in attic environments. The reasons that it gets replaced are generally more related to poor installation practices and maintenance than material limitations.
"Poor installation practices" include running flex duct across catwalks and other areas where it becomes damaged, and all of the terrible things that we talk about here and that Alison Bailes discusses at length in his blogs.
"Maintenance" for me has more to do with the air handler and filtration system than the ducts. If you mess up the air handler or have unfiltered returns, you get a lot of dust in your ducts. It's much harder to clean flex duct than rigid sheet metal. If you have dusty ducts and they get wet because of condensation in a cold attic or crawl space, you've got a rich organic soup that cannot ever be properly cleaned. So you replace them.
Caveat: In the 1980's and 1990's, there was a type of flex duct, mostly with a grey plastic sleeve. The sleeve was not UV stabilized and was sensitive to even tiny amounts of UV light. I guess the manufacturers thought they were being installed inside buildings so UV stabilization was unnecessary. In any case, even the light that sneaks through a ridge vent was enough to turn the sleeve into Swiss cheese. The lifespan of that stuff was never more than 20 years.