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Community and Q&A

What kind of chimney for an energy-efficient home?

Tristan Roberts | Posted in Mechanicals on

The recent discussion on masonry heaters in super-insulated homes is great, and Martin’s “farewell to the chimney” post is a classic, but I have a question that I don’t think has yet been covered here.

I am designing a well-insulated home that will require a modest amount of Btu input that I want to provide with a modest woodstove. What is the best type of chimney to use, masonry or insulated stovepipe? (Or some other option I’m not specifying.)

A masonry chimney would have cheaper materials but higher labor cost, would take up more space and create more thermal and air barrier issues. The main reason I am considering it is for durability.

An insulated stovepipe (like Metalbestos) would be more expensive in materials, but very quick to install, and simpler to detail in terns of air barrier and moisture. I am inclined to go in this direction except for a nagging sense that masonry would be more durable over time.

I suppose however, that I’m likely to be lining a masonry chimney with stainless steel, and that can’t be any more or less durable than the stovepipe (?).

Anyway, I would appreciate input on how to best build a chimney for this application.

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Replies

  1. draginfly58 | | #1

    I think triple wall insulated stainless steel that is sized for the brand of wood stove that you choose would be the best choice. Cost and function.

  2. Dick Russell | | #2

    You've already stated the reasons for going with an insulated metal chimney (eg. Metalbestos or ICC) rather than a masonry chimney, and they are valid. It sounds like you are just concerned about the longevity of the metal chimney. I would be more concerned with the longevity of the masonry, particularly at the exterior, and I would be concerned as well about the longevity of the interface between masonry and wood framing with respect to building tightness against air and water leakage if that chimney is on an exterior wall.

    If you feel that in time either option might require replacement due to deterioration, which would be easier to replace, hmmm?

    In our new (superinsulated) house, in the interior finishing stages, I have a small woodstove, for both backup heat and because I like having a wood fire now and then. The stove is on the lower level (walkout basement), and the chimney pipe runs up through a small chase in the corner of a bedroom in the upper level, then through the attic and roof. The worst case scenario is I'd have to open that chase to get at the chimney to replace it. That may be when I'm long gone.

    Combustion air for the stove comes from an outside hood near floor level, through a 4" duct that has a four-foot loop up and back down and held close to the inner wall of the double frame structure, into the room just above floor level, and directly into the stove. I'm glad I put in that duct, because the blower door test showed the house to be exceedingly tight (0.65 ACH at 50Pa), and natural leakage might not be have been able to provide the necessary air flow. Even if it had been close, turning on the range hood or clothes dryer would have caused backdrafting if the stove took combustion air from the room rather than from an inlet duct. That is one reason I picked the particular stove; some stoves don't have provision for direct connection of a duct.

  3. Peter Powell, AIA | | #3

    Tristan, I basically agree with the comments above, but there is a possible compromise which I have designed in several houses. Depending on the location of your woodstove, consider building a masonry wall/chimney behind it which would include a clay or metal flue liner. Build this masonry chimney up to the ceiling or ridge and then switch to a Class A flue through the roof. This eliminates the thermal bridging problem of the masonry chimney and also provides some indirect heat storage in the masonry near the woodstove. If you want to build something even more sophisticated, the chimney can be designed to use the hollow cores of the block to serve as a return air chase, thus storing some of the excess heat which accumulates near the ridge or ceiling. I also would emphasize Dick's comment above that you need an outside combustion air supply and a woodstove equipped for it.

  4. draginfly58 | | #4

    I don't understand your recommendation AJ BUILDER.

  5. Michael Chandler | | #5

    I would argue for the metalbestos type flue over the triple wall. we used triple wall on our recent "affordable green" house and, while the price was right, the open chase created by that contributed, I think to the disappointing 2.05 ACH50 on our blower door test.

    When we do metalbestos I also like to start it at the ceiling with single wall or masonry running up the first section to save on the cost as well as to extract a little more heat from the fire.

    But the thing that I think makes a big difference in our hurricane area is that we flash the flue at the roof plane as if we were going to run it up with out a chase above the roof and then install a chase that is suspended just above the roof on spacers w/ treated double bottom plate securely bolted to the rafters and flashed into the shingles with a cricket and at the top as if it were a true chase but with drainage at the bottom only. The chase can be finished w/ stucco or veneer stone or even with 1/4" stucco-finish fiber cement board and elastomeric paint.

    The main reason we started doing this is to keep the roof from leaking if the chimney gets hit by a tree during a hurricane but it also keeps the roof plane intact for a sealed attic and allows us to foam the underside of the roof and hold the foam off the chimney without creating an air leak to the chase.

  6. Aj Builder, Upstate NY Zone 6a | | #6

    Another way we do it. Single wall pipe in front of stone faced wall that is well insulated. Then Metalbestos pipe as per code to exit envelope. I have also added high returns to bring warm air down to lowest level of the home though not the way Peter mentioned.

    We like the look of the stove pipe. The straight run is easy to clean. We put the wood stove on a large raised stone base. One positive besides mass for the large base is the safety factor it adds. Stove pipes that are not maintained can fall. Somehow I am hoping that if that were to ever happen, that the large platform may just give enough time to get all under control and avoid burning down the home.

    I am now a sprinkler system advocate too for added safety. Combining a sprinkler system with all future homes seem like the best code change coming in the history of codes. Would be nice if codes that important were retroactive.

    Interesting post Peter.

    Please note: anonymous, I have joined the ignore group after this last post to you, good luck and hope you come out of the closet to at least a nickname and add value to the GBA site instead of stirring things up.

  7. Aj Builder, Upstate NY Zone 6a | | #7

    Michael, great attention to detail. I like your methods. learned two things from you today. Time for an IPA.

    I would like to start a topic someday to discuss insulating valleys. Every home around my way has huge snow melt issues from the fact that valleys are loaded with solid wood, never a drop of insulation added. You seem like the type that would solve this if you lived in the Adirondacks. Any thoughts?

    My timber frame friends have this solved of course since they outsulate anyway. My only thought is to drop my beams down or totally out of the roof frame so that there is indeed insulation above the valley beams.

  8. Daniel Ernst | | #8

    Tristan,

    I don't think there is an answer to "what is the best chimney," unless we talk about context. Perhaps the question should be rephrased. I would propose that you ask "what is the most appropriate chimney in my particular situation?"

    Masonry chimneys have a long history---that stretches across the centuries. Stone, brick, and clay flue liners are all well known and well understood. But people have successfully used insulated metal chimneys for decades (but be sure to use UL-103HT listed components). When designed and built well, both provide useful, durable results. And there are newer technologies available: poured insulated concrete chimneys that surround stainless steel liners are a viable option. These are used in both new construction and in the renovation of older masonry chimneys.

    The answer to your question likely has more to do with your woodstove (i.e. size, combustion efficiency, location in the house), how you plan to use the woodstove, and your geographical location. Some points to consider:

    * Masonry chimneys complement masonry woodstoves. They help wring out the heat from the flue gases, provide additional mass for thermal storage.

    * Masonry technology is probably less appropriate for a small cast iron woodstove, especially if it is used intermittently. Masonry chimneys take longer to reach "operating temperature." Cold chimneys = condensation (in the form of creosote). So an insulated metal chimney may be a better choice, depending on your wood burning habits and stove design. This goes back to heat load. You say superinsulated, but until you nail down a Btu / hour rating, it's hard to say how much wood you should expect to burn in a given season.

    * Michael Chandler has to consider hurricane damage. Do you?

    * Other builders have to deal with seismic restrictions / concerns. Do you?

    * Depending on how you build a masonry chimney, it can be relined at some point in the future. But as you noted, insulated metal chimneys are modular and easy to assemble / replace. Which technology do you prefer?

    * Are you doing the work yourself, or hiring somebody? Do you have a competent mason, someone experienced with building chimneys?

    * What is your budget? Insulated metal appears to be expensive, but I'm guessing that when all is said and done, you would spend more for a masonry chimney.

    * Good design means you place the chimney within your thermal envelope---place it right in the middle of the house if possible. Sometimes this doesn't happen. Chimneys coupled to an exterior wall run colder. This would tip the scales toward an insulated metal chimney.

    * Aesthetics play a role for most people. Do you like the look of a masonry chimney, or do you prefer something more modern?

    If you've read through this site, and delved through other resources as well, then likely you have enough information to make an informed decision.

    One final comment: flue size is very important for good draft, efficient burning, and safety. Whichever system you choose, make sure the flue is sized properly for your woodstove.

  9. Tristan Roberts | | #9

    Thanks, everyone, including Anonymous (i think the issue is that there's another contributor to the GBA forums that got everyone's back up by posting some opinionated stuff without revealing their identity. AJ probably mistook them for you.)

    My inclination all along has been to go with a Metalbestos-type chimney, and nothing I've read here dissuades me from that. So far I've used Metalbestos in retrofit situations and I wanted to make sure I'm making the right choice for new construction.

    To answer some of the key questions raised, I would be using a modern efficient woodstove, properly sized flue, dedicated outside air, and middle of the house for best heat distribution. I'm in NE U.S., but Michael Chandlier gives me an interesting detail to think about -- thank you!

  10. Michael Chandler | | #10

    Tristan - Another advantage of middle of the house is exiting the roof near the ridge. if you do it close enough you can make the chase big enough that you don't need a cricket. I actually got a CO on my house six days before my wedding so the chimney chase here is a galvanized trash can wrapped in copper and stuck upside down on the ridge. with a copper roof jammed on top. Still working fine seven years later (marriage too) The plan was for that to be temporary and to go up and do a big horizontal veneer masonry chimney chase but we got off to other things.

    AJ on the valley thing can you work with over-framed valleys where you don't do a hip rafter at all but just lay a valley runner on the roof sheathing of your main roof and then rip off the excess sheathing and frame your hip rafters and ridge off the valley runner? Won't work with cathedral ceilings but for flat ceilings that's the fast and dirty way to get what you're asking for so long as you remember to rip off the orphaned excess sheathing. Mike Guertin had a pretty good description in FHB http://www.finehomebuilding.com/how-to/qa/tying-new-roof-to-old-one.aspx

  11. draginfly58 | | #11

    Your welcome Mr. Tristan.

  12. Aj Builder, Upstate NY Zone 6a | | #12

    Hey Michael, have designed and built many overlaid valleys mostly for additions.. I should have stipulated that the interior would be cathedral. I think I solved my problem as I asked it above as I see no other way to insulate the structural beam needed to hold up a cathedral style valley. Unless you have ideas for that situation. But thanks for trying.

    The reason that this issue is crossing my thoughts of late is that a few homes that I do work on that I did not build or design have major valleys built into cathedrals and the snow melt leading to ice damns is as bad as it gets. All of these homes were built in the last twenty years or so. Two of them were just built. We are building crap as an industry and it bugs the hell out of me.

    Also two of the homes have valleys that end over exterior doors. One of these has an out opening door that is rotted out in less than five years. Dopey design.

    Another two homes have their valleys end transitioning to shallow slope cold porch roofs. Water pooling enough to invite the neighbors for a swim.

    We sure can design and build crap but wow does it look nice and look at that granite counter top.

    Please people, somehow find competent builders before you build not after the icicles take over your home.

  13. Mike Strecker | | #13

    Hi Everyone, I'm planning to install a chimney similar to what Peter suggested "Build this masonry chimney up to the ceiling or ridge and then switch to a Class A flue through the roof. This eliminates the thermal bridging problem of the masonry chimney and also provides some indirect heat storage in the masonry near the woodstove."
    I plan to have 4" of rigid foam insulation on the roof top and cellulose inside. My question is how do you insulate around the metalbestos chimney pipe itself. I was hoping to use Rockwool Roxul since it is ratied to 2150 F, but found out it is not aprroved fot this even though it should work. Any suggestion or do you just seal with metal flashing and leave a 1-2 " space around the pipe through the ceiling. I see this as a major weak point in the thermal envelope if done this way. Thanks I realize this is an old post so hopefully someone can help.

  14. GBA Editor
    Martin Holladay | | #14

    Mike,
    It's important to follow the manufacturer's installation instructions strictly. Building codes and insurance regulations require you to follow manufacturer's instructions -- and safety demands it. If the manufacturer calls for a 1 to 2 inch gap, that's what you have to do -- even if it bugs you, and even if the detail incurs a thermal penalty.

    You can limit air leakage with metal flashing and high-temperature caulk.

  15. Jenny Belman | | #15

    Very insightful!!!!!! You can improve your home’s energy efficiency with relatively inexpensive ways like installing a better damper. Installing a wood burning insert inside your fireplace along with a flexible M-Flex stainless steel chimney liner can help you.

  16. Mike Strecker | | #16

    Thanks for the answer Martin - But has I was reading how to install cellulose insulation I came across this recomendation recently that is very similar to what I stated above.

    Q. Can you blow cellulose against a chimney?
    Hulstrunk: No, you need to keep cellulose away from chimneys and metal flues. A masonry chimney requires 2 inches of clearance. We advise installers to air seal the crack where the chimney penetrates into the attic, using metal flashing and high-temperature caulk. Then the installer should take a 3½-inch-thick Roxul batt — a mineral-wool batt — and wrap it around the chimney, securing it with a wire so it doesn’t fall off. Then you can blow cellulose right up against the mineral wool batt.
    We do the same thing with metal chimneys, although with B-vents the clearances get a little higher. National Fiber has published a technical bulletin listing the clearances.

    So is Roxul wrap of the chimney a acceptiple method to keep combustible clearance needed so that there is no air gap as the chimney exits throuhg the cellulose insulate rafter bay and rigid foam apllied on top of the roof deck? This was my original plan as stated before, In theory it should work and it appears it has been done based on the Halstrunk's comment, but is it allowed under code? Thanks again and sorry to jump in on this old post again.

  17. GBA Editor
    Martin Holladay | | #17

    Mike,
    Q. "Is Roxul wrap of the chimney a acceptiple method to keep combustible clearance needed so that there is no air gap as the chimney exits through the cellulose insulated rafter bay and rigid foam apllied on top of the roof deck?"

    A. The code specifies clearances between chimneys and wood framing (and other wood components). Bill Hulstrunk does not recommend that Roxul insulation be used to reduce code-required air gaps. He only advises that Roxul can be used as a buffer between a chimney and cellulose insulation.

  18. Mike Strecker | | #18

    Makes sense Thanks again for the quick response As usual I'll run it past the local inspector also to make sure.

  19. Expert Member
    Dana Dorsett | | #19

    The letter of the code regarding using stone wool in clearance gaps is a bit schizoid, and refers only to "combustibles", not just wood:

    http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_10_par061.htm

    " R1003.18 Chimney clearances.
    Any portion of a masonry chimney located in the interior of the building or within the exterior wall of the building shall have a minimum air space clearance to combustibles of 2 inches (51 mm). Chimneys located entirely outside the exterior walls of the building, including chimneys that pass through the soffit or cornice, shall have a minimum air space clearance of 1 inch (25 mm). The air space shall not be filled, except to provide fire blocking in accordance with Section R1003.19.

    Exceptions:

    1. Masonry chimneys equipped with a chimney lining system listed and labeled for use in chimneys in contact with combustibles in accordance with UL 1777 and installed in accordance with the manufacturer’s installation instructions are permitted to have combustible material in contact with their exterior surfaces.

    2. When masonry chimneys are constructed as part of masonry or concrete walls, combustible materials shall not be in contact with the masonry or concrete wall less than 12 inches (305 mm) from the inside surface of the nearest flue lining.

    3. Exposed combustible trim and the edges of sheathing materials, such as wood siding and flooring, shall be permitted to abut the masonry chimney side walls, in accordance with Figure R1003.18, provided such combustible trim or sheathing is a minimum of 12 inches (305 mm) from the inside surface of the nearest flue lining. Combustible material and trim shall not overlap the corners of the chimney by more than 1 inch (25 mm)."

    By the letter of the code, an R15 wrap of rock wool around a chimney as clearance to combustible foam or cellulose violates the "... The air space shall not be filled, except to provide fire blocking ..." phrase, and doesn't rise to meet any of the three exceptions.

    But there's a good technical argument that R15 rock wool provides better thermal isolation against ignition than either 1" or 2" of air gap.

    I'm not sure why this hasn't changed in the past three revs of the IRC, but unless some manufacturer is willing to step up and fire-test the assembly Bill Hullstrunk's recommendation will be a technical violation, albeit a (likely) safer way to build. Most (but not all) inspectors would give it pass.

  20. Fred Williams | | #20

    Dan Ernst, you say "And there are newer technologies available: poured insulated concrete chimneys that surround stainless steel liners are a viable option. These are used in both new construction and in the renovation of older masonry chimneys." Are you referring to a new cast in place reinforced concrete chimney with stainless steel flue liner(s)? I've seen lots of discussion about insulated grout/concrete poured around a stainless steel liner inside older masonry chimneys, but have found no precedent for a new stand alone cast in place concrete chimney. Since concrete and steel expand and contract at nearly the same rate, eliminating the masonry entirely seems like a beneficial concept (aesthetics aside). An all concrete chimney with stainless steel flue(s) would provide thermal mass (if located in the interior) and longevity both inside the flue and above the roof plane. I'd welcome any input.

  21. GBA Editor
    Martin Holladay | | #21

    Fred,
    I wouldn't spend much time focusing on poured concrete chimneys; they are rarely installed. Stick with more common options. Metal (stainless steel) chimneys are usually the way to go.

    Back in the 1930s, Helen and Scott Nearing were big advocates of homemade poured concrete chimneys. They built one for their sugar house, and bragged about how much safer their sugar house was than the sugar shacks built by their neighbors. You can read the full story in their classic book, The Maple Sugar Book.

    -- Martin Holladay

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