Should I leave the old insulation on my ducts, when I add new insulation?

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  #1  
Old 06-16-20, 11:40 PM
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Should I leave the old insulation on my ducts, when I add new insulation?

I removed my entire duct system (6" rigid round) due to rust holes, no tape/mastic anywhere, and other poor installation issues. I brushed/washed the interiors out, and will be using mastic to seal every seam/joint.

The insulation on it is 30+ years old... (faced fiberglass) some is tattered and torn and needs replaced, while some is still intact. Is there a reason why leaving the old insulation on wouldn't be a good idea?

I've read where some have said it's ok to leave the old on, and others say that having a 2nd vapor barrier between the insulation may cause moisture issues.

What if I put the old insulation on backwards, so the faced side is next to the duct? This way all of the insulation would be between the two faced sides... would this get around that moisture issue that some bring up (assuming it's a valid concern)?

Thanks for any input.



 
  #2  
Old 06-17-20, 01:27 AM
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So you are using the old and adding additional on top?

Are the ducts internal or external to the conditioned space?

I would not use mastic, the foil duct tapes are better;easier to install!
 
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  #3  
Old 06-17-20, 11:03 AM
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Yes, I was thinking about leaving the old on... like this:

All the ducts are in an unconditioned crawl space.

Someone raised the possibility of just replacing them with flex... which I suppose I could do, and it's about the same price as new insulation. I was under the impression that rigid would provide better performance though... do you have any thoughts on switching to flex instead? The system is 2.5T.

If the performance is the same or similar, then that would be a lot easier.
 
  #4  
Old 07-10-20, 02:11 AM
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Cost is always a concern, but what you have has very little R value, so I'd replace it. As for replacing the existing, clean round ducts with flex, be very careful to stretch it as tight as possible and use adjustable elbows instead of bending it. Resistance to flow through flex is huge if it's improperly installed.


 
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Old 07-14-20, 11:49 AM
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Thanks ferd42.

Yes, I ended up just throwing all of the old insulation out, and got a new roll of R6 ductwrap to use on everything. Wasn't that expensive... around $130.

I also stayed with rigid pipe and didn't use any flex on the supply side (except for a few feet at the unit to join into the main trunk).
 
  #6  
Old 07-19-20, 04:23 AM
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The wrap has to be loosely installed: Don't pull it tight "to make it look good"; it needs to look bad to preserve "Installed R Value".
 
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Old 07-25-20, 08:53 PM
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Was faced with same question on 30 year old duct work when original aluminum foil covered rock wool insulation developed many issues.

Added new cover using white plastic winter boat cover material, after sealing leaks in sheet metal ducts. Used electric heat gun, not propane touch.

Better than new, cheap fix and looks nice.
 
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  #8  
Old 07-26-20, 12:12 AM
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Code violation, assuming your boat cover doesn't have an HVAC-related UL mark.
 
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  #9  
Old 07-26-20, 05:09 PM
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Cover has UL labels. My son's Battery Fairy must have been there.
 
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  #10  
Old 07-26-20, 11:32 PM
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Great, wouldn't want you or your family at risk in the event of a fire.
 
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