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The ductwork in our 30 year old house has started coming loose where it comes up through the floor. It's nailed to the subfloor, but in several places, the nails have come loose, leaving gaps of an inch or two between the duct and the subfloor. This lets bugs into the house and probably isn't too good for our heating and air efficiency.

What's the best way to re-hang the ductwork and remove these gaps? How much do I need to worry about breaking something while repositioning the 30 year old ducts?

More information

The portion of the duct that goes to each supply vent is hung by one or two nails each on the left and right sides. The nails have either come loose from or gradually tore through the plywood subfloor and fell out, or the nail gradually tore a hole through the duct (so it's still in the subfloor and the duct is hanging beneath it).

I hope that I'm using these terms correctly. Here are some photos.

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Note the nail hanging on the left after it came out of the subfloor. There's another nail on the left (not visible) that's still in the subfloor, but the weight of the duct pulling on the nail apparently caused a vertical tear through the duct, letting it fall. There's a 3/4" vertical gap on the left between the duct and the subfloor and a smaller gap (but still enough to let in a draft) at the top where the duct no longer fits properly because it's hanging off the left.

alt text

Another supply vent. The nails have fallen on both sides, so this duct is apparently supported by nothing in particular, leaving about 2-3" vertical gap all around. The subfloor looks pretty worn, so I'm not sure that nailing it on each side as was done before would work very well.

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2  
Can you post a photo? I'm having trouble picturing where exactly the gaps have opened up. –  Mike Powell Sep 7 '10 at 2:25
    
On a side note, be vigilant in checking for asbestos duct insulation before proceeding with duct work. It was prevalent in older homes. –  Mike B Sep 11 '10 at 21:48
    
And if you're sure there's no asbestos, vacuum out that duct...looks half clogged. –  Alex Feinman Oct 15 '10 at 19:56

2 Answers 2

If you don't mind a little extra thickness, what about hanging some L brackets over the edge of your finish flooring, and then screwing the ducts to the L? Then, you can put the duct cover back on right over the lip of the L, covering it up.

Bad ASCII art time!

   --+       +--  L brackets
==== ||    || ====Floor
====  |    |  ====Subfloor
       \  /  
      |DUCT

I have no idea how you'd hold the duct in place while screwing it, mind you, unless you can get access from below. Maybe you could glue a rope to the inside of the duct and have someone yank it back up into place.

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This is what I was thinking as well - only rather than putting L brackets on, just take the ends off the existing ductwork and replace them with new ones (and cut the end of the new ones to make flanges) –  Eric Petroelje Oct 15 '10 at 20:58

On your bottom photo can you just put some screws in the back? For other vents do something similar? Or get out a handy tube of silicone or liquid nails and go crazy.

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