Best way to insulate ductwork

I have a uninsulatable crawlspace with uninsulated metal ductwork running through it. I have been exploring the options for insulating the ductwork. At the current cost of gas not doing so seems immoral even though this is a rental unit.
The main duct is a rectangle, no I did not crawl under and measure it yet, and the branch runs are mostly 6" rounds.
As expressed by another poster in another thread I feel the bubble wrap foil is bubblecrap. The Hart and Cooley sleeves look like the best bet, with more work for the rounds. But what to use for the main duct and the boots?
There is an R6 product but you have to cut and fit. To really get the R6 it will need to be installed uncompressed. I am not crazy about the thought of dealing with that product in a crawlspace.
I have not found a semi-rigid product that could be applied to the exterior of the main duct. I am thinking something like is on the inside of insulated duct.
Any and all suggestions for the best way and payback for insulating the ductwork is appreciated.
Colbyt
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I'll repost what I said earlier in another thread:
Before insulating your ducts that run thru the crawl space you will want to seal them first -- duct mastic is considered best though I've used the foil tape to seal all the seams. I've used foil tape that has held up very well over ten years now. Don't use regular duct tape -- it doesn't last. Seal the seams of the heating ducts as well as the return air ducts.
I've used the fiberglass duct insulation by Manville that I picked up at Lowes.
After sealing my ducts in my crawl space and then insulating all the heating and air return ducts I noticed a big improvement in the air flow and the heat of the air from the registers as far as how much actual savings I'm now experiencing there are a lot of SWAG methods that others have talked about that you could look into but I do know I'm saving now for the life of the house and since I did the work myself my only costs were materials and time.
A friend of mine at work insulated his ducts using R-19 bats that he cut and wrapped around his ducts and held them on with wire and string. I just used the Manville duct fiberglass insulation that is a perfect fit for the 6" dia. ducts. For the ducts that ran down the center between my 12" floor joists I put an R-13 bat above the duct -- strips of R-13 on each side and another R-13 bat across the bottom which effectively filled the entire space between the Joists with the Ducts running down between them.
IMHO, sealing the duct joints was the most important step in the crawl space.
Another thing to consider is being sure to put down a good vapor barrier in the crawl space.

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For ductwork in the crawlspace I simply used the 24" paper faced fiberglass bats. Stapled to the floor joists.
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snipped-for-privacy@yahoo.com wrote:

insulate the crawl space and let the duct leaks warm the crawl and the hardwood floors? i do not know.
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