Attic insulation

My attic will be insulated on November 30 of this year. They will blow in 24" of insulation. 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.
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.
Opinions greatly appreciated.
Thank you,
Ivan Vegvary
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On 11/22/2010 8:17 PM, Ivan Vegvary wrote:

I'd say you want the ducts as close to the heated envelope as possible, the better to not be heating the attic. The insulation value of flexi-duct is close to nil- better than bare metal, but not by much. The heat they leak should more than make up for any air gap around them.
But I am not an HVAC expert, so what do I know?
--
aem sends...

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You're over thinking this. Imho it will be fine.
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On Mon, 22 Nov 2010 17:17:41 -0800 (PST), Ivan Vegvary

From where? The top?

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 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.

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

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Yes, from above.

The 12" ducts take up the whole joist cavity and the sides of the duct are touching the abutting ceiling joist. Therefore, no blown insulation will filter down into the two quarter-arcs on either side of the joint. However, as somebody else pointed out, the ducts should be closer to room temperature and not attic (well vented) temperature. This would argue for the ducts to stay close to the ceiling with lots of insulation on top even though some will be missing at the lower areas.
Ivan Vegvary
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On Mon, 22 Nov 2010 19:46:55 -0800 (PST), Ivan Vegvary

How much are they charging you to blow the insulation in. Total price, insulation and labor and whatever else.

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Ivan Vegvary wrote:

One thing to think about... I learned by accident that insulation is HEAVY. IN particular, cellulose insulation looks cost effective, but is heavier than fiberglass, BUT as it ages, it absorbs moisture and becomes MUCH heavier. Depending on the construction of your ceiling and how well it's attached, this may become an issue. I was planning a DIY cellulose installation. Glad I didn't.
I'd at least ask the question of your contractor.
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Good point! As it happens, this is an old house with weird construction. All of the ceilings have 1/2" plywood on the bottom covered with 3/8" drywall. Structurally very sturdy. While I certainly try to stand on joists only, I am able to stand on the ceiling and not break through, even at 230 lbs.
Thanks, Ivan Vegvary
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On Mon, 22 Nov 2010 19:49:27 -0800 (PST), Ivan Vegvary

As one who just stepped though his own ceiling, for the first time in 27 years and more than 100 trips to the attic, I'm jealous.

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I wouldn't worry about it. There will be very little airspace there, and if it is, air is a good insulator with another 12" or more of insulating material over the top of that. About the only thing you could do is shake your house really hard to get it to settle that last little bit, but that would damage a lot. I wouldn't worry about it. With 24", that's about as good as it gets.
Steve
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