Insulate furnace plenum in basement?

About two years ago we moved into a 35 year old house. The furnace was replace by the previous owners and is about 10 years old (92+ Rheem). The ductwork appears to be original (except for a few pieces around the furnace) A problem we've had last year (and this year too), is that when the weather cools down, the upstairs is about 5-7 degrees cooler than the downstairs. Partially closing the downstairs registers helps some, but I don't want to close too much and reduce the airflow through the furnace to a level that will cause problems. I'm looking for a solution to help the heat make it to the upstairs, without causing problems with the furnace. In the basement, above the furnace is the sheet metal plenum. There's a couple of short 6" ducts that run out of this to the rooms directly above. Off one side of the plenum, extending most of the basement, is a long (15 feet), rectangular duct, about 20" wide by 14" high; basically an extension of the plenum. The round ducts for the rest of the house feed out of this at various points along it's length. When the furnace is running, you can feel the heat radiating off of the sheet metal of the plenum "extension". To me this is heat that is being lost from going to the rest of the house. I was wondering if it would help to cover this with insulation, and if so, what type? Rigid foam sheets seem like they would be the easiest, but is that the best solution? Also, should I just insulate the long extension, or the plenum on top of the furnace itself?
Any suggestions would be appreciated.
Mike O.
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My father had exposed duct work in his garage. I used R-19 kraft faced batts. I use furring strips along the edges to hold the insulation in place. First month the electric bill dropped the cost of the parts. We were running the a/c at the time. Later when the heat turned on we noticed that the garage was colder but the furnace ran less. Insulate everything that you can that is exposed. Use at least R-8 for max effect.
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I insulated mine, it made a 2 degree increase upstairs. To get a 7 degree increase you need ductwork or possibly reduce downstairs airflow. But air temperature in the plenum has to be checked. To little air flow and the high temps will shorten furnace life . Get a pro out to look at your set up.
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Mike O. wrote:

...
I don't think insulating ducts is going to be the answer.
You have a distribution or second floor insulation problem.
In that house I would say you almost certainly have a distribution problem and the only way you are going to get that fixed is to have someone with professional knowledge see your home, measure as needed, and run the numbers.
You may be able to gain some by checking insulation. There may be non in the attic and you may be able to fix that. It should be fixed before doing the changes to the distribution system.
It is rather unusual that the upstairs would be too cool. I might also suggest that the distribution problem may be more of a return problem than supply.
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Joseph E. Meehan

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Joseph Meehan wrote:

We had the same problem in our home, I partially closed most of the registers over the course of a week to see if things changed. They did somewhat (though I do like the sleeping areas cooler). After I made sure that nothing was in the way of my return area (it's in a closet, the mrs put a laundry basket in front of it), the temps came close to balancing out.
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know I need more. It's on the list (which keeps growing..) to get done, but is not really an option at the moment. I'm just trying to maximize what I have now until I can get to some of the other things.
As far as the returns, I'm also wondering if that's an issue. There's two 16x8 grills upstairs, but probably twice or maybe three times that much intake area in the downstairs area. I've considered blocking some of the downstairs returns to force the system to pull more from the upstairs, but I want to be careful to not cut the airflow too much.
Mike O.
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Foam board is good. Fiberglass also good.
Sure, wrap it all up.
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Christopher A. Young
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On Sat, 23 Oct 2004 22:08:38 GMT, "Stormin Mormon"

Exposed foam in an occupied space is a really bad idea, and a code violation due to flammability. Why not use duct insulation made for the purpose or use rigid fiberglass insulation.
Dan
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Thanks. Good information.
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Christopher A. Young
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