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Old 11-13-2013, 08:06 PM   #1
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Sealing furnace flu


Hey guys,

I recently went in my upper attic (tri-level home) and added an additional layer of R30 unfaced fiberglass insulation over the existing R30. While up there I noticed that there is a large gap, approximately 2"-3" at it's widest point where the furnace flue enters the attic. I think I need to go back up there and seal it before the winter sets in. Before I climb up there though, I checked the furnace room as well and saw that there is another huge gap where the flue goes up through the ceiling. (see pics).
My question is what is the proper method of sealing these openings? I know it can get pretty hot and fiberglass insulation is not supposed to be used. Would Great Stuff Fireblock foam work for this situation? If I seal the bottom opening above the furnace room is it necessary to do the same in the attic? I hate climbing up into the attic and have been up there twice in the last month. I'm getting too old for this stuff. Thanks
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Last edited by Dave Sal; 11-13-2013 at 08:10 PM.
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Old 11-13-2013, 08:13 PM   #2
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Sealing furnace flu


that looks caps for what was used and called ducttile that is a very tight fit to the pipe going up ...don't think you need to do anything with it ....ben sr

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Old 11-13-2013, 09:45 PM   #3
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Sealing furnace flu


Thanks for the reply Ben Sr. You are correct in that the flue seems to be installed properly with the duct tile or whatever it is called. The builder apparently got that right, but my concern is the large hole (the chase?) that is allowing conditioned air to flow up the hole into the attic. I know that fiberglass insulation is a no no and was thinking of maybe stuffing it with Roxul. I'm thinking I should have posted this in the insulation sub-forum.
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Old 11-14-2013, 09:53 AM   #4
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Sealing furnace flu


flash it with metal and fire caulk it if it worries you.
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