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Jamie Buxton
01-16-2007, 4:12 PM
I have a big chimney freestanding in my living room. Where it goes through the ceiling, it is 3 feet by 6 feet. The attic is unheated. It seems to me that I have a huge thermal bridge going from my heated living room to the unheated attic. I'm considering wrapping the chimney with insulation in the attic. Does that make sense to anybody who knows more about this than I?

Rob Russell
01-16-2007, 4:57 PM
If you're trying to slow down heat transfer from the chimney to your attic, insulating it would make sense. I'd be curious as to how much that would help, though. Does the chimney feel really cold when you feel it up by your ceiling?

Dan Gill
01-16-2007, 5:46 PM
First a disclaimer: I am not a heating and cooling expert, nor do I play one on TV.

However, it seems to me that your chimney goes through your attic to the outside air. So insulating it in your attic would not stop any heat transfer. It would just mean that the heat transfer occurs in the outside air, not in the attic.

When you have the flue open, you will be transferring a lot of heat with the smoke to the outside. That's what a chimney is for. When the flue is closed, you'll be transferring less heat to the outside, but there will still be some transfer.

Like I said, I'm no expert.

Jamie Buxton
01-16-2007, 6:05 PM
Rob, the chimney feels cooler than the ceiling a few inches away.

Dan, my attic may be different from those in your area. There are big vents up there, so the temperature inside the attic is the same as the outside, save for whatever heat leaks through the insulation in the ceiling of the living space. This means that the attic is outside for thermal purposes. However, only a few inches lower down, the chimney is inside, and I'm trying to heat it. Masonry has almost no insulation value, so heat flows through the brick from the inside to the outside.
I think I don't have much air flowing up the flue when there's no fire. There's a damper in the flue, which is closed, and there are glass doors on fireplace openings, which are as close to sealed as I can make them.

Bill Lewis
01-17-2007, 8:29 AM
Jamie, to answer your question , yes the chimney is a thermal bridge. However it is not like you are thinking. It is the law of nature for all forms of energy to find their lowest state (entropy). So your chimney is not bringing in the cold, rather the mass of the chimney is absorbing the heat from the room. Masonry and stone are a great thermal mass, or energy absorbers. That's why your chimney feels colder than your ceiling. If your ceiling were constructed of thick concrete or stone, it too would feel cool to the touch, even with the insulation above.

So your chimney is absorbing the heat from the room and conducting it to the colder areas and then radiating it in to the attic space and the outside which are even colder than the chimney. By insulating the chimney in the attic section you could slow the heat radiating in to the attic space, but it would not stop the conduction thorough the chimney. This would only have a minimal effect on the heat loss of the room. Again, I am only referring about the conductive properties of the chimney. Any heat loss through the flue is by convection. Convective losses are relatively easy to stop which you have all ready done.

What you really are trying to accomplish is a thermal break to stop (or severely limit) the conductive heat transfer through the chimney mass to the outside. Think of slicing your chimney in the plane of the ceiling and inserting an insulating material. This would create a thermal break. Unfortunately there's no real economical way to do this, nor would the energy savings be worth the effort. This is particularly true in that you live in a relatively moderate climate. Consequently the houses in your area were not really designed or built with cold weather efficiencies in mind. Given that, there are probably other inefficiencies that are making your house feel colder as well.

Even though you have probably been experiencing the unusually colder than normal temperatures, this is a relatively temporary situation. Your best best is to light a fire in the fireplace, stoke it up to heat that thermal mass, and it will then radiate heat back in to the house, even after the fire has gone out.

Jamie Buxton
01-17-2007, 12:35 PM
Bill --
The masonry is, as you say, a huge thermal mass, and it does store heat. However, it does conduct heat from the inside to the outside of the house. It doesn't conduct perfectly -- that is, a piece 4 inches thick will conduct heat more easily than a piece 4 feet thick. I'm thinking that the distance from room temperature to attic temperature is now only a few inches, and that wrapping it in insulation in the attic makes that distance several feet at least. It seems to me that this should reduce the heat conducted up the masonry.

Bill Lewis
01-17-2007, 2:04 PM
...By insulating the chimney in the attic section you could slow the heat radiating in to the attic space, but it would not stop the conduction through the chimney. This would only have a minimal effect on the heat loss of the room...

Basically by insulating in the chimney in the attic space you are trying to keep the heat energy of the room in the chimney until it exits above the roof. Until the chimney warms to the same temperature of the room, it will still feel cold. So, yes, in theory it will slow the conductive process, but the reality is I doubt you'd see a comfort level change that would be worth the effort.

Jason Roehl
01-17-2007, 3:32 PM
The chimney will feel cool until it is much warmer than the room--because stone conducts heat (out of your hand) much better than air does. 70 degree air can feel cool (especially if dry), but so will 95 degree stone, but 95 degree air doesn't feel cool.