I think the duct is actually fine. The duct connects to a register that is shared by two rooms. When heating, one room is gets hot, while the other is cool (not too cold). I think the problem is either insulation or windows. The hot room has 1 window, the cool room has 3 windows. I've put film over the widows, but it's still cold. Next I'll try blankets.


My house was built in the 1930s, I assume these ducts were installed when the home was built. This duct goes from the supply plenum in the basement to the 2nd floor master bedroom. The best I can measure, it's 10" x 4" .

If you look in the photo below, the duct work is comprised of several small sections of metal. They look to be about a foot long each, with some overlap between sections. Note: the light source is a ribbon of LEDs that I lowered down the duct.

My concern is that the metal is buckled in several spots and is coming away from the wall, and I'm concerned that it's leaking air pressure. Presumably, the outside of the duct is the wall studs and lath and plaster.


  1. Should I actually be concerned about the buckling metal? Is there really that much air pressure being lost?
  2. Is there anything I can do to repair this?

Repair Ideas

I had a couple of ideas of some repairs.

  1. I could try to reattach the metal to the walls with sheet metal screws from the inside. This would require drill extensions and an 90 degree adapter ... and it would probably be pretty difficult to do.
  2. I could try to drill through the walls on the outside of the ducts and secure the metal with toggle bolts.
  3. I could try to fill the gaps with spray foam gap filler, but that could backfire if it expands and pushed the ducts out further.
  4. I could try to run flex ducts inside the old duct work, or maybe duct board since there aren't any rectangular duct products.

enter image description here

  • 1
    Are you having a specific problem, like trouble cooling/heating an area of your house? It's definitely a good find and something that could be problematic, but you have to consider the time/expense needed to fix it versus the overall impact.
    – Steven
    Aug 10, 2015 at 18:28
  • Specifically, the upstairs is not as cool as the downstairs ... but that could be for any number of reasons I suppose. Generally, I'm working on sealing and insulating my ducts. I could measure air temperature from the start and stop of the duct, don't really have a tool to measure airflow. Aug 10, 2015 at 18:49
  • 3
    I can't imagine it would be losing much pressure unless there's another opening into the stud bay somewhere. In my area, a lot of the houses of that era used unlined stud bays for ducting when they were upgraded to forced air (and before the building code caught up). I'd put it on the list of things to do next time you open the wall, and in the meantime just seal around where it enters the cavity.
    – Comintern
    Aug 10, 2015 at 23:58
  • 3
    If your problem is that the upstairs isn't as cool as the downstairs, you might need a return air path from upstairs; many older houses that were built without central air only have return air intakes on the lower floor. Considering that these ducts run through a wall cavity primarily in conditioned space, I don't think improving their seal will change things appreciably and probably isn't worth the effort unless you have the wall open already. Aug 11, 2015 at 20:34
  • 3
    @WalterStabosz that's not necessarily bad or unusual. If you also have a return downstairs, try closing it. This will force the system to pull more return air from upstairs where it is hotter. You can also try closing some of the supply vents downstairs. Note that this all presumes you can do this without causing undue strain on the system, i.e. closing some of these off will not cause your fan to overwork itself. Aug 13, 2015 at 12:51

2 Answers 2


As has been well stated in the comments thread above, your solution may stem more from tuning the HVAC system and your home than repairing this minor obstruction. Chances are you won't see much benefit from a time-consuming repair.

The suggestions given are good:

  1. Examine the overall airflow paths for the space. Close heat supplies and returns to force more heat to the cooler areas and draw more air from them.

  2. Consider the passive effect of open doorways, etc., and make adjustments to how you use them.

  3. Consider a duct booster fan to pull more return air through the existing upstairs return. This can probably be installed in an accessible location near the furnace.

Any home needs seasonal tuning. You may need to reverse these changes for the heating and cooling seasons.


This may sound obvious, but why don't you just cut a section of the plaster out on one side. Remove and replace the duct and then patch the section with new plaster? It's really not that big of a job.

  • Valid suggestion, but I'd have to cut through the plaster and lath (no small task). Also the duct runs vertically through two floors. Dec 2, 2015 at 18:08
  • I'm upvoting this because this is the correct answer. I don't why it was downvoted. It's a dirty, dusty, messy job, but this is what you should do. Lathe is easy to cut. Wallboard is cheap.
    – ssaltman
    Dec 2, 2015 at 18:46

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