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Another Duct System No-No: The Unsupported Butt Joint

Butt Joints Without Rigid Connectors In Flex Duct Can Lead To A Lot Of Duct Leakage.

HVAC installers and I have a little game of hide-and-seek that we play. Of course, there are the obvious problems I find in duct systems, like the ductopus, ducts installed right up against the roof deck, and duct leakage galore. But there’s one duct problem that I have to hunt for.

When I go into an attic for a big, spread out house that’s served by one system, I look for long runs of flex duct. Flex duct, you see, comes in bags that contain sections 25 feet long or less. Houses can be 100 feet wide sometimes, so this can lead to the object of my search.

When I’m checking out the long runs of ducts, I look for places where the foil insulation wrap has foil tape all the way around. If the house has a radial duct system (the ductopus), it’s not uncommon to find two pieces of flex duct that have been spliced together – the dreaded butt joint.

When I find one, I walk up to it and give it a squeeze. What I’m hoping to find is that the butt joint is difficult to deform because the installer inserted a sheet metal connector into the two separate duct sections and then fasteners and lots of mastic to hold it all together. What I too often find instead is that the butt joint has as much give in it as the the rest of the flex duct.

The photo at the top is one such butt joint. When I squeezed it, I could tell there was no connector, so pulled back the insulation. What you see is that the installer simply taped the two pieces of flex duct together and then covered it with insulation.

This type of connection may pass a duct leakage test initially, but, especially in a hot attic, that tape will eventually fail. When it does, the duct leakage will be sky high.

Here are my recommendations:

  • Don’t install single runs of flex duct that two or more pieces are so long that they need to be patched together.
  • Use a trunk-and-branch duct system to help keep runs of flex duct short.
  • Always use a metal butt joint connector if you don’t follow the advice above.


Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He also has a book on building science coming out in the fall of 2022. You can follow him on Twitter at @EnergyVanguard.


This Post Has 7 Comments

  1. Allison . . . you might want
    Allison . . . you might want to rethink your nomenclature in this article before the notion that raters go around squeezing “unsupported butt joints” turns into the top hit on 
    Unless your wording is a clever strategy to get as many hits as a Swedish housekeeper in a French hotel.

  2. Although there’s no hard-and
    Although there’s no hard-and-fast rule, I would be suspicious of any flex run that’s long enough to require a joint. Unless the flex is completely stretched out tight, and supported without any sags, friction losses for 40′ or 50′ of flex will go through the roof. 
    Wonder what would happen if builders decided to pay mechanical subs according to delivered airflow, based on third-party design and verification?

  3. Ed: I’m
    Ed: I’m always open to increasing awareness of the EV blog and bringing in new audiences. But we’re not gonna promise complete satisfaction if a search on ‘squeezing butts’ is what brought them here. 
    Christopher: I’ve read that zip ties also don’t do so well in hot attics. 
    David: Yeah the friction losses would be way too high, but who measures airflow anyway? It’s not just really long runs that get butt joints, though. It seems that installers do this also when they’re trying to use up the pieces they have left (or whatever’s nearby). 
    If they got paid by percentage of design airflow that gets delivered, I think that would be great. If they did it by the total cfm’s, there’d still be the built-in bias toward oversizing.

  4. We are considering a Pulte
    We are considering a Pulte built home in Arizonia and were wondering how it woud be p0ssible to have duct systems varified after tings are closed up Pulte talks highly of quality control

  5. Ferd: Is
    Ferd: Is the home you’re considering an ENERGY STAR qualified home? I know Pulte does a lot of ENERGY STAR, but I don’t know if all of them are.  
    If so, it’s been through a third party verification process by a certified home energy rater who did an inspection of the house before the drywall went in and another when the house was finished. The inspections included looking at the ductwork for problems and testing it for leakage. There’s no guarantee that the rater used on the house you’re looking at caught everything, but an ENERGY STAR home should have fewer problems than one that hasn’t had any third party inspections and testing done.  
    If the home you’re considering was part of a sampling process for ENERGY STAR verification, it may or may not have been inspected or tested. It would be part of a batch of up to 7 homes, one of which gets rated.  
    If the house hasn’t been tested, you could always hire someone to come in and test the duct system(s) for leakage and measure the airflow and static pressure.

  6. Ferd, you might want to ask
    Ferd, you might want to ask Pulte to see results of the duct leakage test. You can contact me off-list if you’d like help interpreting the results. 
    Energy Star does not (currently) require air balancing, and I’m not aware of any production builder who has this done. It’s a big issue, because even conscientious builders like Pulte have no idea if the system is delivering correct amount of air to each room.  
    You can pay a third-party verifier to test the air balance against the room-by-room load calculation (presumably Pulte can provide this report). I’m sure Pulte would be very interested in the results,especially if you’re paying! 
    The National Comfort Institute is a good source for locating an air balance technician. Just make sure s/he is certified for air balancing. 

  7. I appreciate that EV is
    I appreciate that EV is talking about butt joints instead of twittering pictures of other bits like some Congressmen…

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