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Community and Q&A

How do I fix this duct nonsense?

EngineerNate | Posted in Energy Efficiency and Durability on

I’m in the process of redoing the attic access to my second floor attic. I was up there doing conduit to rewire the upstairs air handler and upstairs lights (I wanted minimal extra holes in the structure, conduit let me get everything from the panel to the attic in one shot) and the poor execution by the previous owners got to me so I expanded the scope of my project.

So far I’ve framed in the opening I want and made a 10″ high insulation dam. I may add additional dam height (ply or foam board) in a more removable fashion, I was concerned about actually being able to crawl over the lip if I went any higher.

Before and in progress shots:

As you can see, before it was just a ragged hole. They sawzalled through the ceiling structure. To close it up, they just stuck a sheet of duct board over it and screwed it to what was left of the ceiling. The closet would get roasting hot in the summer. In the winter it’s fine because it’s sucking up all the warm air from the house!

I also ran conduit for a home networking project, if anyone is wondering after looking at the pics. That’s the big box in the foreground.

Anyway, I have a plan to insulate and seal the attic access with a new hatch. Where I need help is those ducts. I don’t think it’s practical to run them in the conditioned space. Limitations of a 94 year old house. That said, flex through a ragged hole in the ceiling is obviously not right. Is there a product specifically for making a clean penetration though drywall? What would be the most energy conscious way to fix this without a full reconfiguring of the system?

Only two of the runs enter the attic. Two in the photo terminate into the hall outside the closet (duct going to the left) and into the room next door (duct doing the boa constrictor move around the front). Those are in the conditioned space already.

Any input is truly appreciated.


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  1. Expert Member
    BILL WICHERS | | #1

    Do I see an extension cord in a piece of liquidtite conduit in that first pic? That’s a no-no.

    Regarding the ducts, just extend the rigid plenum up through the ceiling with a flange to seal the drywall. All those twisty flex ducts will work MUCH better if you get them straightened out as much as possible too, or just replace them with rigid ducting if you can (vastly better performance).


  2. Expert Member
    NICK KEENAN | | #2

    That's not bad compared to the crap I had to deal with today. I kid you not, one flexible duct was suspended from the ceiling by a USB cable tied around a drywall screw. The real crime was the installer had been skimpy with the duct tape and the sheet metal screws. It did make disassembling them easy though.

    The black cable in the liquidtite looks like Cat5 networking to me.

    As Bill said, extend the rectangular duct up through the ceiling. Then you can seal it and insulate around it. Make sure to be scrupulous both with sealing the ducts with foil tape and insulating them.

  3. EngineerNate | | #3

    Thanks guys. Is continuing to use duct board my best option? I've heard mixed things about it's effect on air quality.

    Cord is Cat6 cable. It's since been removed anyway as I decided to do custom length cables instead of premade. I made sure the stuff I bought was riser rated.

    I've got an electrician friend I'm running all the electrical decisions by and my electrician is going to give it a look over and do the hookup at the panel for me. The liquidtight is only going to have low voltage in it. It was just easier to run and terminate than standard flex and pulling in it is easier. All of the power wiring is THHN stranded in 10 and 8 gauge.

  4. EngineerNate | | #4

    I can see the confusion about the cord now. There's an unplugged LED work light cord hanging out the attic right next to it.

  5. Expert Member
    BILL WICHERS | | #5

    Note that you’re not supposed to mix data and power cabling in the same conduit. Not sure if you’re doing it or not, but wanted to let you know.

    You may find sealtite easier to work with than liquidtite. Sealtite has a flexible plastic covering over flexible metal conduit instead of the all-plastic conduit liquidtite is. Sealtite is more flexible and a lot easier to pull wire through. I much prefer sealtite to liquidtite conduit.

    I would use steel sheet for the plenum and tack some rigid insulation (polyiso probably) to the outside. You can get some sizes of steel ductwork premade, but it’s bit very expensive to have an HVAC contractor that has a ductwork shop make some pieces up for you if you need a custom size.


  6. EngineerNate | | #6

    I could probably buy a section of oversized metal duct if a standard one won't work and cut sections out of the rectangle and rivet/aluminum tape to the needed size.

    I guess I'm using sealtight then. I've been calling it metallic Liquidtight. I learned from experience that it's far superior for anything longer than an equipment whip. Keeps ground continuity as well which is nice on the LV stuff for shielding.

    The two are in separate runs. The power wiring is all EMT. The sealtight was just a labor saving choice for the less critical data wiring. If one of those gets crushed somehow the house doesn't burn down.

  7. user-2310254 | | #7


    One of my new systems needed a custom air handler. The HVAC contractor simply took his dimensions to a local sheet metal shop.

  8. EngineerNate | | #8

    Might be worth at least getting a quote for a new plenum from an HVAC guy. I have a couple that have been recommended to me locally. That way if I try to get a plenum made up myself at a sheetmetal shop I have a reference for cost.

  9. bennettg | | #9

    Idea: Get a "standard" metal plenum that fits your air handler supply opening, install take-offs for the two ducts that exit through the walls in it and attach standard Nx8" rectangular duct sections appropriate for the airflow to go up through the ceiling and cap it off. Attach the two remaining take-offs to the sides of the rectangular duct before the end. Install balancing dampers. Some take-offs have them already. Insulate the plenum and duct.

    Hopefully you can do your takeoffs so that the flex is a straight shot.

    Some sizes of both metal plenums and half-section rectangular duct are available at the box stores.

    While you're at it, do you have a large enough filter grille/slot?

    1. EngineerNate | | #10

      The filter is a 20x20x1, it pulls from the stairwell on the other side of the same wall the duct on the left exits. Not a fan of having the return and that duct so close together but not sure there's a fix for that at the moment.

      Can you explain the balancing dampers a bit?

      I wouldn't mind installing a deeper filter holder to use thicker filters to get more filtration capability without restricting the airflow.

  10. Expert Member
    BILL WICHERS | | #11

    Balancing dampers allow you to adjust the airflow to each individual duct so that you can get the right amount of airflow to each room/vent. The “right amount” is usually whatever is needed to result in even temperatures throughout the area of the building being served.


    1. EngineerNate | | #12

      Gotcha. I need those in the downstairs system. The airflow is super uneven room to room.

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