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
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
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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