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roberthall 12-09-2006 10:37 AM

Wet insulation in flex ducts
Hi Y'all,

I live in NC where summers are very humid. I have found that the insulation wrapped around the flex duct airway and covered by an outer plastic covering is fully saturated with water causing the flex duct to sag between the supports. I have had to cut open the outer cover to let all the water drain out. When inspecting the interior of the floor boot inside the house I can see moisture condensing on the inside surface of the chilled metal boot. What puzzels me is how does the moisture get into the insulation. And of course, how to prevent it.

Can anybody throw some light on this issue? Any ideas would be appreciated.

Christopher 12-09-2006 10:52 AM


It appears that the outer jacket of the flex duct was not properly sealed at both ends, allowing moisture intrusion during cooling cycles. I believe at this point you have no alternative other than replacing the duct. When buying the flex duct get only the best - reflective outer jacket, R value 6 minimum.

1. Secure inner duct.
2. Tape outer jacket in place.
3. Seal taped outer jacket joint with a mastic such as:


roberthall 12-09-2006 12:18 PM


Thanks for your reply. I take it that the outer jacketand the insulation shoud be cut so that they are longer than the inner duct( with the coiled spring wire) so that the outer jacket and insulation can be secured further up the boot than the inner jacket which will be secured at the bottom of the boot. I find it very difficult to get the inner duct over the sheet metal of the boot. 6 inch duct for 6 inch opening at the bottom of the boot?

Thanks again.

Christopher 12-10-2006 10:12 AM


Ordinarily, flex duct is connected to collars of the same size. Since the collars are made for flex duct, it is an engineered system and all the components fit together easily. As for 6" metal duct, 6" flex duct is the correct size. If you are unable to slip the inner duct onto the metal by rotating the coiled spring hose (inner duct) while slipping it on, you may need to first crimp the metal duct like one end of prefabricated rigid metal duct sections. The home centers sell a tool for making the crimp. Be sure to secure the inner duct with tape.

As for the cut length, all components are typically cut to the same length. When connecting to a metal duct section, it may be advantageous to extend the cut of the outer jacket. Provided that the flex duct is not taut (there should always be a little slack is total length), you can always pull on it thus shrinking the length of the secured inner duct a bit.

Be careful not to puncture either the inner duct or outer jacket. This will result in moisture intrusion and you will be back to square one.


roberthall 12-10-2006 11:17 AM


I'll try the "twist and push-on" technique. I guess that opens the wire somewaht. I'll be under the house today installing new duct.

Thanks again for the suggestion and advice.


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