Insulating Furnace Flue in Attic

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Old 12-07-07, 12:33 PM
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Insulating Furnace Flue in Attic

I have a huge icicle problem in the winter and it is not because I do not have enough insulation in my attic or roof vents. My boiler / furnace has the exhaust flue pipe going straight up thru the attic. And when boiler is on - is too hot to touch. I was wondering if I should insulate the furnace flue in the unconditioned attic only?

Boiler is in a utility room on the first floor. The Flue is a thin single wall light colored grey galvanized metal? all the way up to the roof. (Where it meets the roof it connects into a double wall hookup that extends 4 foot above the roof line)
Can I insulate the area in the attic only? This would amount to about 7 foot in length?
 
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Old 12-07-07, 07:10 PM
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Where do the icicles form that cause you the problem?

Ken
 
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Old 12-08-07, 06:16 AM
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The icicles mostly form on the eves only on the south side of house where the flue is. Ice also builds up in the one valley of the roof where the flue is five foot away, on the south side of the house.
It has snowed 27 inches in the past week. We have not had a temperature over 28 degrees and there has been no sun shine.
That part of the house has huge icicles now where the snow has melted. I have shoveled off the roof in that area numerous times last year. There is no ice build up on the majority of the shingles. Only in the valley and eve area.
 
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Old 12-08-07, 06:41 AM
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Here is a link I found that might help.

http://www.chimneylinerinc.com/chimn...sulationQA.htm

If your flue is HOT to the touch you probably can not use simple fiberglass insulation.

I am not a furnace professional but I did stay at a Holiday Inn

<edit>
Oh one thing you might consider is that you will still have HOT furnace exhaust leaving the roof and that will still melt some snow around the flue.
</edit>
 

Last edited by Pete OldNavy; 12-08-07 at 08:21 AM. Reason: Adding more content
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Old 12-08-07, 03:15 PM
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I'm not positive that your problem is caused by the furnace flue. Your description of the problem sounds like a classic case of poor insulation over the top of the exterior walls of the house. Out where it is difficult to get the proper amount of insulation. Nowadays a raised heel rafter and airblocks are the best way to eliminate the problem you are having. But nobody does that either unless they are forced to.

The flue probably contributes to the problem but you must be very careful about how you address it. I doubt that you have much over 350 degrees in that flue anyway and that isn't a whole lot of heat for that large area.

Ken
 
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Old 12-08-07, 03:26 PM
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Is this boiler room by the outside wall and the chase for the flue is near the outside wall and exits roof near eaves? If so, you indeed may have a lot of heat in that chase going up in the attic, within a few square feet of space, and where it enters the attic, it may have relatively dead air space at the low end of rafters and could be causing the melting, then icing up at the eaves.

What kind of insulation and what depth is it and how is it right around where the flue pipe pokes into the attic from below?
 
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Old 12-08-07, 07:40 PM
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That is another good point. Usually since there needs to be some clearance from combustibles, a chimney will have a chase around it that is not sealed very well. If house air gets pushed out there, you have even more warm air to worry about. You can seal around it with roof flashing and then use insulation board and foam to airseal it. Just keep anything but metal at least a couple of inches away from the chimney pipe. You can get the exact clearance from the manufacturers web site. Don't overlook anything up in the attic where heatloss can occur. Every thing you fix and airseal will help you in the long run.

Ken
 
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Old 12-09-07, 09:17 AM
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Let's assume there is no internal heat loss from poorly insulated area by the eves. Lets assume that there is no heat loss from the chase and it is sealed very well. The chimney flue is at least 8 feet from any wall.

Question is: Should I insulate the whole Boiler Flue in the attic that is in the vertical position and amounts to approx 7 feet of pipe? I am thinking a foil type wrap insulation. NO fiberglass.

This house was built in 1998 I can only assume that the rolled, paper backed double thick insulation which has been touching the flue pipe around the bottom base in the attic all of these years is a fire hazard and wish to find a better way to insulate there too! The chase is on the ceiling of the lower level. In the attic where the flue pokes thru from the utility room just has fire rated drywall right up to and touching the flue in the attic. and regular fiberglass rolled insulation on top of the drywall. or Don't fix something that is not broke?
 
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Old 12-09-07, 11:14 AM
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Another case of my reading posts too quick in the last couple days. Yes you did say 7 feet in your OP, and obviously at 7 feet, the pipe would not be too close to an outside wall hardly except under rare case of extreme high roof pitch house.

Can't you just tear off the insulation paper where it contacts it by the base?

Any idea of how hot that pipe gets when it is being run over long period of time, straight?

I take it you have not contacted any heating contractor that may even sell some very stuff you might be looking for.

You haven't had any condensation issues resulting from this? Some years back I had to go into a 4-plex attic and insulate an exact similar metal (bath fan) vertical pipes going through the ceiling, into attic and out roof (also about 7 feet high), and vertically wrap batting around them and tied it.
 
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