On Mon, 22 Nov 2010 17:17:41 -0800 (PST), Ivan Vegvary
wrote:

My attic will be insulated on November 30 of this year. They will
blow in 24" of insulation.

From where? The top?

I have questions about the existing
ductwork. (Ductwork is 10 and 12 inch flexible insulated ducts)
Presently the ducts are laying within the ceiling joist cavity. When
they blow in the insulation, unless they take special care, it is
likely that the bottom quarters of the round ducts might end up with
little or no insulation. In many cases the ducts are 12" nominal and
fill up (because of the extra large outside diameter) most of the
14-1/2" 2"x6" joist cavity. Ergo, the insulation will not be blown
under the duct. However, there will still be a total of 24" of
insulation, approx. 10" of it covering the entire duct.

Question: Air (when not in motion) is a reasonable insulator so I
should leave things alone. Alternatively, it would take very little
work to simply place slats under the ductwork so they sit above the
ceiling joists, allowing them to fill the entire joist cavity.
Obviously, by raising the ducts six inches, I would be getting less
insulation on top of the ducts.

If insulation won't get under any part of the ducts now, why would it
get under the ducts then? Or do you mean that there will be a part
that is 6" high and 10 or 12 inches wide that will get insulation, and
the part just below the round duct won't? Oh, it all will. Then why
do think there will be no insulation undre the duct if you don't raise
the duct? There will be no insulation under the very bottom of the
duct, but that is touching the ceiling downstairs and will warm it a
little.

Does any of this matter? Am I wasting my time thinking about this?
BTW, my contractor is very cooperative and will do whatever I ask. He
will even raise the ductwork upon slats at no charge to me.

Does he have an opinion whether it's a good idea or not? Actually
there's a good chance he doesn't know. This is the kind of obcure
question that rarely comes up, for which there is little empirical
data, less of it in his possession, because after he does his job, he
doesn't stay there all winter to compare this year's figures with last
years.

Opinions greatly appreciated.

I don't think it would help, and might well be bad because of less
insulation abvoe the duct.

Thank you,

Ivan Vegvary