Payback on Duct Insullation

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We have used the Reflectix in several aplications and it seems to do what is claimed. Best bet is to put a spiral of 2" wide Reflectix down the pipe, to provide additional air space. Then cover that with a full layer of reflectix and overlap the seams 2". Then tape the seams with foil tape.
R-6 fiberglass duct wrap also works well, as long as it is is not compressed. R-6 ductwrap is approx. 2.5" thick. If it is applied tight to the duct, so that it "Looks Nice", it compresses the insulation and can reduce the R-factor to as little as R-2. Measure the circumference of the duct and add 14.5". Cut that much duct wrap and wrap it around the duct with a 2" overlap. Staple and tape the seams. Use only tape rated UL-181 B-FX. The 14.5" add-on stretch out is the amount recommended by NAIMA (North American Insulation Manufacturers Association.) for 2.5" thick duct wrap installed on round duct.. Different thicknesses have different recommended stretchouts. For 2.5" thick wrap on rectangular duct the recommended add on is 12.5".
Stretch
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On cost alone fiberglass wins.
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http://www.wapa.gov/es/calc.htm
this site has some interesting calculators. I have never used the heating one, but the a/c calculator has some remarkable numbers when you play around with duct insulation.
If you have not bought the R-6 then I would definitely think about something higher (according to the calculator).
Personally I would not use the stuff your installing. I would stick to fiberglass. I wrapped my fathers ducts in the garage. I used R-19 bats. Cut to length, lath to nail to the ceiling and tape the edges. He had a double garage, and the duct work was 2/3 of the width. Lowered the a/c bill $40 the first month. Heat was a little more. Sorry, it was a long time ago and I do not remember all of the numbers.
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