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Questions about Fireplaces, Masonry Stoves, and Chimneys
with replies by Thomas J. Elpel

Questions:

• How do I construct the inside of a chimney?

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How do I construct the inside of a chimney?

Tom,

I am building a log cabin fitted with a cobblestone chimney. I don't have any idea of how to construct the inside of the chimney. I know that I will lay stone atop fire brick but what do I need to do to construct the innards of the flue? Do you have any resources for me or any websites that might help me?

Thanks much,

Rob

Rob,

I've built three chimneys by three somewhat different methods, but the construction of the flue is the one thing all of them have in common. The chimney pictured here is the most conventional of the three. We helped my in-laws build this one in their log house. The chimeny is nearly forty feet tall from the basement to the top of the chimney above the roof. There is a wood stove in the basement, as well as the stove shown here on the main floor, before construction of the hearth. Each stove should have a separate chimney flue to draft properly, so there are two separate flues built into this chimney.

In a nutshell, the flue of the chimeny is made with ceramic flue tile. Ask at your local brickyard for ceramic chimney flue. It is available in several sizes. The ceramic flue tile expands and contracts as it heats and cools, so it should have space around it for that. The air space also helps to protect the rest of the chimney and the house from the heat of a chimney fire caused by soot build-up in the flue.

In a conventional chimney like this one, the chimney is built out of specialized cinderblocks that leave an air gap of about half an inch around the ceramic flue. The cinderblocks are mortarted together at the same time as the ceramic flue, so the inside and the outside of the chimney goes up layer by layer. Corregated metal tabs are mortared between the cinderblocks, such that they stick out of the chimney. Later, when you come back to finish the outside, the metal tabs are mortared into the joints between the stones. This chimney was faced with uncut fieldstones, mortared in place without forms.

One problem with the cinderblock method of chimney construction is that the chimney becomes very large by the time you lay up the ceramic flue tiles, cinderblocks, and the stone facing. A large chimney takes up more space and conducts more heat out of the house through the mortar, even when not in use.

However, you can eliminate the cinderblocks to mortar the stonework right around the flue, as long as you leave some kind of gap around the flue. (Local codes may vary.) One easy way to do that is to wrap the ceramic flue with fiberglass insulation as you go up. Then do your stone work up around that, being careful not to compress the fiberglass insulation too much. It is okay to compress the insulation somewhat, just so there is still some room for the flue to expand. The other two chimneys we built were both done with this technique. The difference between them was that I used slipforms to guide the stonework on one chimney, while the other one was mortared up with brick-like rocks without forms.

We carry a number of books that include building your own stone fireplace and chimney. Building with Stone by Charles McRaven covers the topic pretty well, but the fireplace designs are not energy efficient. The Stonebuilder's Primer by Charles Long covers the same kind of material, but in a completely different way, such that it might take both books to get a comprehensive outline of the process. My book, Living Homes: Integrated Design & Construction, barely covers chimney construction, but goes into depth on building energy-efficient masonry fireplaces. See also our selection of books on Masonry Stoves and Fireplaces.

Sincerely,

Thomas J. Elpel