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How to seal off a fireplace chimney flue?

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How can I safely seal chimney flue that has an unvented gas fireplace insert.
asked in House Chimney by fjtharry (120 points)
edited by darekrudy
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1 Answer

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Hi Fjtharry,
Not long ago I answered similar quesiton, I believe you’ll find the answer by following this link: seal off chimney.
Let me now if you need more details. Just click "comment" below.
answered by darekrudy (21,730 points)
My chimney is still being used for the water heater so I guess the question I should be asking is can I just make the damper in the fireplace airtight by sealing off with the aluminum duct tape or do I need to also insulate behind the damper.   Also if I need to insulate behind the damper, can I cut the blue sheet foam and just lay it on the damper, I happen to have some I can use left over from when I insulated behind the attic door.  Thanks


I see you’re asking questions in two areas smiley,

You can use blue sheet foam for this purpose but if you are experiencing cold winters (freezing temperatures) and especially if your chimney is on an exterior wall, I would still recommend fiberglass. You can get a roll of unfaced fiberglass insulation for under $10, it will seal the flue better because it expands slightly.

If freezing temperature is not an issue you might be OK with what you have. Whatever you do, make sure that the water heater is not using the same chimney flue for venting and you’re not going to block it unintentionally.

Sorry about the posting in 2 places.  My chimney has 3 flues, one for the water heater and old furnace (I now have a new energy efficient furnace that is vented out the side of the house) one for the former wood burning fireplace in the basement that now has an unvented gas fireplace insert, and one for the former wood burning fireplace in the living room that also now has an unvented gas fireplace insert.  I want to close off the 2 flues for the fireplaces to stop the drafts from coming down the chimney and make the house more energy efficient as it is a 1950s cinder block stucco house.  I am in upstate NY so I do get many days of well below freezing temperatures but the chimney is not on an outside wall, it runs through the center of the family room in the basement, living room on the first floor, is walled in between 2 of the upstairs bedrooms and then the middle of a gable attic.  I am not concerned about the cost of the insulation, more of having all the extra material to throw out and am reluctant to handle fibergalss, although getting the fibergalss insulation behind the flue would probably be easier than the foam.  I am a novice at this sort of thing so unfortunately need more details than most people.  Do you still think the blue foam insulation will still be inadequate, I believe it is R8 rated.  Thanks


No problem, I get them all smiley
Honestly, I don’t know if the blue foam insulation will still be inadequate but this is the way I look at your project;
You are installing a fireplace gas insert directly beneath the damper. Even if the appliance has “zero” clearance installation requirements it will elevate surrounding it temperature which will be transferred onto the damper and beyond, the process is called stack effect - http://en.wikipedia.org/wiki/Stack_effect
Aluminum tape covering gaps around the damper and damper itself will not prevent warm air penetration, blue foam would probably be fine (you can even put 2-3 layers) but it might be difficult or even impossible to cut it so it fits perfectly, without any gaps into the opening above the damper.
The whole insulation process is to prevent condensation from forming inside the chimney and dripping onto your new appliance, so you don’t have to remove it and start the process all over again.
Alternatively to fiberglass you can use your blue foam and seal the gaps around the precut section with expending foam (be careful, it might get messy).
Ok, got it.  Just one more question, the fireplace inserts were installed 6 years ago, they can be pulled out to get at the damper in the chimney and there is about a 6 nch clearance between the top of the insert and the damper.  I have not seen any condensation dripping on to them, just wondering if closing off the flue is what will cause this condesation as from what I read and understood about the stacking, it is a normal occurrance in all chimneys.

Don't want to mess with that expanding foam, had to use it around the water and gas pipes on the exterior of the house and it was not pretty, so I will go with the fiberglass insulation and aluminum tape sealing of the damper.  Thanks


If it didn’t happen for the past 6 years you should be OK after sealing off the gaps and insulating damper to prevent drafts. Any condensation buildup / moisture inside the chimney can partially or completely evaporate for as long as the top remains open (screen and rain cap highly recommended). 
The chances of that happening improve dramatically in chimneys contained within the house, just like yours. So, even with the bottom of the chimney sealed off, with no draft induced by the pressure differential between the house interior and exterior, there still be some air movement from the lower portion of the chimney which is warmer because of the surrounding it conditioned home areas towards the colder top of the chimney. Just protect the top but leave it open and everything should be fine.
Happy New Year! smiley