More Musings of an Energy Nerd
If your home has a forced-air heating and cooling system, it probably has leaky ducts—a fact has been confirmed by countless researchers over the years. Fortunately, many U.S. jurisdictions have adopted more stringent building codes in recent years, and these code changes have nudged HVAC contractors (at least in some communities) to pay more attention to duct sealing and duct leakage testing. Because of these code changes, anecdotal evidence suggests that the ducts installed in new American homes aren’t as leaky as they used to be. Most existing homes, however, still have leaky ducts.
If you are an energy-conscious builder, you probably already know your duct basics:
- Duct leaks are very common; in many homes, duct leaks are responsible for significant energy losses.
- For ducts located in an unconditioned space—for example, an unconditioned attic or crawlspace—leaks in the supply system will depressurize a house, while leaks in the return system will pressurize a house. Either condition can cause problems.
- Duct leaks outside of a home’s thermal envelope waste more energy than duct leaks inside a home’s thermal envelope.
- Even if ducts are located inside of a home’s thermal envelope, duct leaks can still connect to the outdoors. For example, supply system leaks in a ceiling between the first and second floors of a two-story home can pressurize the joist bay, forcing conditioned air outdoors through cracks in the rim joist area.
- It’s much easier to seal duct seams during new construction than in an existing house.
Characteristics of a good duct system
A good duct system:
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7 Comments
Thanks Martin for another good article! Uninsulated, leaky ducts in unconditioned attics and basement are notorious 'vampires' that waste energy and drive up HVAC costs for homeowners. Even conscientious HVAC installers often fail to seal ducts when they run inside joist bays, thinking, "That's fine - it's inside conditioned space!" even though this also wastes energy and drives up costs. As you noted, I have also found plenums with significant leaks along unsealed seams and boots. HVAC installers also frequently 'insulate' plenums by wrapping them with shiny bubble wrap, e.g. Reflectix, which provides almost no R-value. Foil backed fiberglass blankets are the best option, and the foil surface should be sealed with foil tape on the joints.
Three suggestions:
(1) High quality foil tape, e.g. Nashua 322, is a good solution for sealing and UL listed, much easier to apply than mastic, and the only real solution to seal flex duct connections with the poly 'tube/ and embedded wire. We also seal the poly vapor barrier around flex duct insulation. You need to apply it carefully, of course, pressing wrinkles flat etc. It also sticks tenaciously, as I found when I had to disassemble and change duct connections sealed with foil tape. (Note: Installers should NEVER use silver fabric 'duck' tape, which fails over time.)
(2) Straight duct runs should be insulated flex duct, to reduce noise in the system.
(3) Turns / elbows should always be hard metal, and the joints should be sealed - even though the joints are supposed to be 'air tight'. Unscrupulous / untrained HVAC installers often simply bend flex duct - which can sharply reduce pressure. See the examples in the attached photos.
I hope this is helpful.
Flex duct should be avoided, as it's garbage. It's not durable and will need to be replaced every 15 to 20 years.
Surprised a HVAC person would recommend flex duct.
I'm not a trained HVAC engineer but those I know, and almost every one of the many professional presentations I've seen on HVAC/duct design has said that flex duct should be avoided.
Though last year I did see a presentation at Building Science Corp's annual conference (aka "Summer Camp") that did side-by-side studies of identical houses, one with flex duct and one with rigid duct, and though they expected the rigid duct to perform much better, they were essentially equal in performance. They still recommended rigid duct for long-term durability but were surprised that the flex duct did so well.
There's also many different kinds of flex duct.
Sorry, but high quality flex duct, e.g. from GAF, is not "garbage" and is durable. You are probably referring to the first generation of flex duct in the 1980s and 90s, which was not UV stabilized and degraded even with the small amount of light from a ridge vent. Plus, problems with flex duct are almost always due to bad installations - e.g. bends, twists and turns, poor support, etc - as shown in the photos I attached above.
I'll grant you that today's higher quality flex duct can perform well, IF installed properly. However, nothing beats rigid duct work for durability and ability to be cleaned.
Agreed that flex duct CAN be installed and maintained to work properly, and like you mention can help with acoustics for less than the cost of a duct muffler/silencer, but in the real world it has nowhere near the average service life of rigid ductwork.
A couple of things not mentioned by others:
Rats and mice can VERY easily chew through flex duct, not only causing leakage and disgusting smells from their feces and urine, but it's also a vector for transmissible diseases like hantavirus. Though I don’t doubt it's possible, I have yet to see sheet metal ducts with holes from pests.
Even if the ductwork is installed properly, trades coming in later phases of construction and particularly low paid workers and inept homeowners after occupancy can much more easily damage flex duct. The miserable Terminix tech, cable guy, or anyone else making $22/hr crawling through a 130 degree attic will happily destroy your ductwork if it means getting out of there sooner.
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