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.