Need ideas for insulating around furnace chimney


Old 02-08-04, 02:49 PM
Thread Starter
Join Date: Nov 2002
Location: Trevor, WI
Posts: 97
Need ideas for insulating around furnace chimney

We have 2 bedrooms that our furnace chimney runs between. The bedrooms have had condensation drips at the wall/ceiling joint nearest the chimney. The house also has ice dam buildups right now, and I've just started trying to figure out the problems.
I was in the basement next to the furnace and felt cold drafty air coming down alongside the cinderblock furnace chimney. I'd imagine it's coming down from the attic, which means warm air can just as easily go up to the attic.
How should I seal around the chimney? Should I just stuff insulation up into the floor joists that surround it? Any better ways to do it.
This is just the beginning of sealing the house from the attic. I also have to seal my kitchen recessed lights, reroute my bathroom fan vent. Also am going to add a roof fan with a humidistat.

Thanks for any ideas,

Steve H.
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Old 02-13-04, 06:07 PM
Join Date: Nov 2001
Location: USA
Posts: 1,873
The framing (chase) around the chimney is spaced deliberately. The reason for it is the flue stack temperature. For oil heat, the temperature is usually around 500 degrees Fahrenheit. Today's more efficient natural gas heating units have a much lower flue stack temperature of around 250 degrees Fahrenheit. This temperature is usually measured at the flue connector to the lower end of the chimney. This means as the flue gases rise up through the chimney, the temperature begins to drop.

The reason you need to know this is if you have oil heat, it would be inadvisable for you to seal up this area. Because of the flue stack temperature would still be sufficiently high enough to cause a fire. If the material is directly against the chimney and was there for a prolonged period of time with that high of temperatures.

Even if you have natural gas heat, the material you choose is important, because of the possible expose over a long period of time. I would recommend using a high temperature foam insulation commonly used on steam pipes. You will not find it at Home Depot or Lowes. You will find it at a heating and plumbing supply store. I would also recommend sealing over the foam with a high temperature caulking, that you will find at Home Depot, Lowes or your local Heating and Plumbing Supply store.

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