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hey guys
I am putting central air in my house and my Lennox furnace is 17 years old so while I am at it my hvac guy is going to put in a new furnace also.
my furnace is in a crawl space and to save some money I am going to insulate my metal ducts myself. my main trunk is metal then all the take offs are flex duct.
my hvac guy is recommending I use a double bubble wrap with an air space and I was thinking of using foil faced thermax in 1/2 or 3/4 and cutting it to size and foil tapping it together.

just wondering what any one thinks is a good way to insulate the ducts
like I said it is in a crawl space and the metal ducts do condensate in he humid summer

thanks nicko
 

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Sean
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It all really depends on access - bubble wrap will only get you an R1 or so & while having the air gap helps out on the radiant side being in the crawl space negates that

Sealing up all the leaks & then either pulling an insulation blanket over it or applying foam board is generally the easiest. Spray foam works great... as long as the ducts are clean, prepared properly & the family is willing to take a few days vacation at a hotel while the work goes on. (make sure they ventilate the area properly while spraying)

Another option is to bring the ducts into the conditioned envelope by either building walls around & insulating them or by going with a sealed crawl space - personally a sealed crawl would be my top choice of all the items listed above, just make sure they do it right
 

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i think your best bet is spray, it has been my experience that this prevents the condesate and that can lead to mold and all the other things moisture attracts bugs
 

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Pro
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flashheatingand said:
Actually, I have heard of somebody spraying foam on the ducts. Sounds intriguing. Also, There is a way to pull the flex installation jacket over the trunk line.
They are now an internal spray which is supposed to seal the ducts by as much as 60 % across the whole system.
 

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Sean
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They are now an internal spray which is supposed to seal the ducts by as much as 60 % across the whole system.
Great product but that just stops air leakage (up to a 1/4" as I recall) - it doesn't insulate. Quick note, be careful that anything that could allow spray getting into finished space is closed up good or you will have a big mess
 

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Thom
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Foam is a great insulator, it is also fantastic at burning. I wouldn't install it uncovered, protected from any possible flame.

If you do use foam, think about a foil faced polyisocyanurate. It doesn't support a flame like the others but it costs more. Still, you must foil tape all the joints and exposed foam edges.
 

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Dr Heat
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the industry stander is foil with 151 foil tape. I would use 1 inch foil if the duct is in a conditioned crawl if the crawl is ventilated 3 inch. Spray foam and bubble wrap do not meet the fsds requirement unless it is covered or buried.
 

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I would wrap it with r-6 foil blanket. You would need to staple, aluminum tape, sticky fab and mastic to seal it good. Otherwise, another option wold be to insulate it with duct board and aluminum tape.
 
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