Is there any way to include a thermal break in a chimney?
Thinking out loud. What I’m thinking of is a brick chimney running through the middle of the house. Something mass-wise of the size of a fireplace chimney, but with a efficient wood stove mounted on it.
In kind of a simplified masonry heater, the wood stove sits mostly in the “fireplace”. It heats up the brick mass. Probably a single-wall metal duct exhaust out the stove and up the chimney.
This would seem to work for either a 1 story or two story (granted less heat on second), but the problem you run into is when the brick chimney transitions from heated space to unheated attic.
Ergo the question about thermal breaks. If you could thermal break the chimney at attic level, then the above-insulation portion of the chimney isn’t wicking away the heat of the below-insulation portion of the chimney.
What do ya think?
Replies
Are you trying to reduce conductive losses through the chimney itself? Don't worry. 20 feet of anything is good insulation.
Convective losses, however - you definitely benefit from closing it up tight when not in use.
Also note that in a wider warm chimney, dampered below, warm air will thermosiphon to the cold outside air above.
I thought about that when building my chimney, then gave it up as impossible. All that I could think of to do was to fill up the air space between the flues and the masonry sides with vermiculite. It was a lot of fun to dump the stuff in, didn't cost much, and stops the convective currents outside of the flues.
Andy
Andy Engel
Senior editor, Fine Woodworking magazine
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
"...it was a lot of fun to dump the stuff in..."Andy,You don't get out much, do you?Rich BeckmanAnother day, another tool.
With all the new fangled CMU's out, I'm surprised no one has created a block that has something mixed in it that would serve as a thermal break, while still providing the strength to use it in a chimney.
What if they mixed fiberglass in the conrete? Would that reduce the conduction?
As far as convection, you're always gonna lose some, but a chimney engineered to be used with a wood stove I would think could be made with a narrower flue than a standard fireplace chimney. Shoot, some wood stoves only require 4" flues.
Or maybe you just say the heck with it and convert from CMU/brick to double-wall stovepipe as it transitions from insulated space to non-insluated. You'd save that much more $$ on construction by not extending the CMU/brick out the roof.
jt8
You could just built the chimney out of Rasta (sp?) type of products.
I think "weigh-lite" blocks have vermiculite or pearlite mixed in.
Spheramid Enterprises Architectural Woodworks
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Adding FG wouldn't help. On its own, FG has all the R-value of window glass. It only works as insulation because it traps air pockets.Andy Engel
Senior editor, Fine Woodworking magazine
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
With an interior chimney, there is not much advantage to the thermal break you describe. Air-sealing the chase around the chimney (with sheet metal) is of great advantage, and puts you ahead of many.
Compared to what you must exhaust to maintain draft, the conductive losses are not significant. Actually, there is some disadvantage to a lot of thermal mass in the stack. Exhaust gases will not be buoyant until the stack heats up causing poor draft. Also, more creosote will form. That is why multi-wall SS flue has many advantages in this regard.