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Discussion Starter · #1 ·
I am losing close to 9 degrees on an R6 flex duct run in my attic. I am not yet looking at doing any spray foam. is there a way to add insulation to flex duct? Can the vapor barrier be taken off, then wrap batt insulation with new paper vapor barrier or does it need a foil barrier? I suppose the other option would be to replace flex with pipe and wrap R13 around it.
 

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Our 30 ft. of 14" flex return in the attic was gaining about 4-6°F until i added another 2" layer of similar plastic wrapped insulation. The heat gain was reduced to around 0.6°F.


Did you take off the foil vapor barrier first? My thought is that it shouldn't be sandwhiched between two layers of insulation.
The original ( plastic ) vapor barrier is still there and has been for about 35 years.
 

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Thank you. So you basically used a new foil insulation blanket to wrap around the existing flex?? Did you use foil tape to tape the seams in the new insulation?
Plastic wrap over original plastic wrap. It's been so long since i've been in the attic i really don't recall what tape was used, sorry. I've since used aluminum tape of several projects and really like it, although it isn't nearly as convenient as that good old duck tape.
 

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Discussion Starter · #10 ·
My understanding is there is only a marginal difference from R6 to R8. I would rather try and wrap on an additional R6 or R8 insulation to what is already there which would give me R12-14.

What I am trying to understand is if I need to remove the original vapor barrier on the flex since the new additional insulation will have a barrier.
 

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2 vapor barriers on a return duct is not a problem.

2 vapor barriers on a supply duct can cause sweating and mold to form between the 2 barriers.

How long of a run is this flex duct. And what size room does t feed.
 
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