Does a flexible stainless steel chimney flue really need an insulation wrap?
I am preparing to install a stainless steel flexible liner inside an unlined masonry chimney on an exterior wall to provide draft for a wood stove. Every internet website selling the liners stresses the need for an insulation wrap to keep the vapors warm, improve the draft, and diminish creasote buildup. This has also been emphysized at GBA for similar reasons on multiple occasions. The available wrap liners are typically alkaline earth silicate (AES).
A rep for one of the large internet retailers has told me that the AES wrap is actually used to provide a firebreak zone between steel liner and masonry and does not really act to keep the vapor warmer in any meaningful way. Is anyone aware of any scientific studies that show the benefit of AES wrap to improve draft and decrease creasote buildup? It does not seem intuitive to me that a 1/2 “insulation wrap” could provide any meaningful change in flue gas temperature such that draft would be improved and creasote diminished. If in fact flue gas temperature is not meaningfully changed ( higher temperature) with these wraps then there is significant false advertizing being presented to us consumers. A firebreak is one thing but implying that draft is substantially improved with an AES wrap is something quite different. The wrap kits have a real cost to them but the benefit may be questionable.
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Replies
Timothy:
I read the same things and decided to insulate mine (there was only room for minimal insulation) when inserting a liner about 8 years ago. I do believe the insulation will reduce creosote by keeping the stack temperature higher, especially when starting a fire from a cold chimney, reducing the amount of time the creosote has to condense on the liner walls. The draft seems better more quickly when working from a cold chimney as well. I like the way my insulated SS liner performs over the previous clay liner only, based on my anecdotal woodturning experience. If someone had an outdoor chimney I would think the insulation would be even more valuable. But, you are right to be at least skeptical.
On an exterior chimney you will definitely benefit by insulating a flexible chimney. The warmer your flu, the better the draft, and the less chance of creasote build-up. Although some love the look of an exterior chimney, any knowledgeable mason will tell that an interior chimney is best. Sorry I didn't cite scientific data for the answer...I'll see if I can find the website that corroborates my info though.
Howard and Brad
The flue gas temperature can easily reach 500-600 degree F. What delta T change are we talking about that a 1/2 inch wrap can prevent from ocurring? Why don't we ever talk about R value with this AES wrap? I understand that the wrap is always used with the steel liners but I am beginning to think that there may be little or no science to support its usage when dealing with such large temperatures and small amount of insulation. $500, uninstalled ,for 30 feet of wrap multiplied by usage across the country is a lot of money.
Howard, your draft may have seemed better to you with the insulated liner but was this due to the insulation wrap or to the tighter,smaller, and more uniform surface of the liner? I am simply looking for any scientific evidence that these flue gas temperatures are actually substantively changed with the temperatures and amount of insulation wrap that we are talking about. This 1/2 inch insulation wrap sounds like it would be better labeled "miracle wrap".
Timothy, I've had three or four liners installed and no one ever offered me any wrap. A regional thing?
Malcolm, every dealer I spoke with in my area, Pittsburgh, pushed the necessity of a wrap for draft purposes. I am surprised that your experience has been otherwise. Other than the typical retail marketing verbiage I can find no technical information supporting the use of these wraps. I am hoping that someone can direct me to some technical information supporting the use of insulation wrap in this setting.
Timothy, I'm interested to hear what you can find out. There's a chance the installers I used were idiots.
I agree that sellers of SS liner, flexible or rigid, present insulation as a "definitely should do". There may be no real study/science available, -someone would have to have done it, but I'd be interested to hear anything scientific here. I realize various factors could be at play in my experience, -it's always difficult to really isolate for one variable, and is why I shared my observations as anecdotal. But absent some info that it it's not worth doing I would probably do it again if I ever needed another liner.
In my area a minimum of 1" of loose rock wool insulation is required by code between old tile flue liners and a retrofit stainless liner. This limits the chances of a hot flue liner igniting any old creosote buildup or birds nests etc that might otherwise be found (or end up after the fact) in the old flue. I'm not sure a half-inch fiberglass wrap of the flue liner would cut the mustard with the local inspectors- I doubt it, but maybe.
Howard
I agree with your thinking but there are situations where the space is so limited that only the liner can be installed and there is no room for wrap insulation. Should major restoration be done simply to install the wrap? I have also been told that not infrequently the passage is so tight that the wrap is shredded although the homeowner is not informed. The firebreak attribute may be enough to warrant wrap usage but personally I put more value on enhancing draft as the risk of chimney fire should be very low with the liner alone assuming the masonry does not have major structural damage and the liner is clean. If the wrap really does not improve draft then it should not be marketed that it does. I am hoping that one of the knowledgable GBA pros will weigh in on this discussion and let us know what scientific data exists to show that 1/2 inch AES wrap improves draft.
Dana
If I am understanding you correctly your code requirement is predicated on the wrap or rock wool, or whatever, acting as a firebreak and not as a draft enhancer which is what the insulation is marketed as. Absent code requirement for firebreak do you know of any data showing that insulation wrap, or poured vermiculate, will actually keep flue gases warm enough to improve draft and decrease creosote?
Timothy:
While I still think (until/unless scientifically informed otherwise) that insulation is advisable, I don't think it is critical. On my own chimney, 6" rigid liner barely fit into 8" clay liners (the inside diameter is more like 6.5 to 7"), I poured in what vermiculite I could, mainly filling the corners, so there is not much. I still felt it was a considerable improvement to the chimney overall. I'm glad I don't have to deal with the codes Dana does. While they may be well meaning, THAT is the case where major restoration could well be needed to fit a dubious 1" of insulation. Folks would be encouraged to not make the improvement due to cost. Many (or most) chimneys would not fit a new liner of appropriate size and 1" of insulation.
When we lined my son's chimney more recently, -his furnace needed a 7" flue and we didn't want to reduce, I found a mason who broke out the clay liner with a rotating tool driven by a drill. This went very smoothly and was $400. We then had room for half inch wrap on flexible liner.
But there are few easy/cheap fixes where chimneys are concerned.
Howard
I did find a bit of information that may shed light on the value of insulating a steel liner. There is a graph in the product literature for TherMix insulation that shows data from an experiment where flu gas was maintained at 1000F and temperature was recorded 1 inch from a tile liner and was found to be about 225 F after 4 hours compared to 160 F 1 inch from a single wall steel liner surrounded by 1 inch of TherMix. Assuming that 1/2 inch of insulation wrap is equivalent to 1 inch of TherMix (it might actually be less effective) we are talking about a 55 F difference in temperature of the flu gas. Is this enough temperature increase to improve draft or diminish creasote buildup ? I have no idea. My suspicion is that the science has not been done to show a difference. As is so often true industry comes up with a product line, practitioners within that industry pick it up, and the consumer pays for it not knowing whether there is value to the product or not. Unless data is presented that refutes the validity of the product it continues to be used since it financially benefits the industry and the practitioners. Fire safety may be enhanced with the liner insulation but improved draft and diminished creasote buildup may be non existent. I would be curious to hear Martin's view on this issue.
Timothy:
I wouldn't think that temperature difference would be meaningful. When I did mine I wanted to add more insulation, but space did not allow. I otherwise agree entirely. Sellers can make the general claim that insulation is valuable, and there is nothing to render them unable to make this claim with a semi-straight face.
I started looking for this. They key question on temperature is does the exhaust path including its walls maintain a temp high enough to not allow creosote deposition. Creosote deposits around 250 F. https://www.uky.edu/bae/sites/www.uky.edu.bae/files/AEES-36_0.PDF
I found this pdf of a Finnish doctoral dissertation on metal chimneys and fires. Lots of interesting info in here. Apparently flue gas is often much higher than the manufacturers estimation on a stove, and apparetnly mineral wool has an organic fraction that can combust. https://trepo.tuni.fi/bitstream/handle/10024/116985/978-952-03-1245-9.pdf?sequence=2&isAllowed=y
I'd like to understand more about the Thermix study to think much more on this. A 65 F difference 1" outside of a clay liner could be quite significant (it is after all a 27% difference in the external air temp, which doesn't include the radiant fraction). I'm not an engineer but have access to the equations to figure out how this may translate to internal flue temp, it may or may not be worth drawing up a picture and making a model of this, surprised no one has done so already on here or elsewhere. It's simple enough to do.
That seems to be the key question for most settings, is will you prevent creosote buildup by not dropping below 250 F on the flue liner wall given a large range of variation in flue gas temps caused by variation (in more or less descending magnitude):
fuel type and 'dryness',
stove type and combustion efficiency (secondary combustion, catalytic, etc...)
length of liner
air space in the chimney outside of the liner
temp external to the chimney and extent above roof line, exposure within dwelling and dwelling temp
quality of the masonry
another thing to remember is creosote will reignite after being deposited and burns around 1000F, so preventing any buildup whatsoever, even during starting, will likely increase the longevity of the liner even if you are still keeping it relatively clean from long term buildup by preventing surface combustion, pitting, and associated corrosion.
With what info I have I think a liner for ~$180 (25ft), some tape ($20), clamps ($30), and stainless mesh (~50), or ~$280 all together, seems somewhat reasonable to protect ~$600 worth of liner kit ... but then again it is a 50% cost increase.
Hard to say, colder climates where the temp difference is extreme and people are using older style stoves this probably makes the most sense. I see in the EU, lightweight insulated masonry blocks surrounding solid stove pipe is much more common, and will likely significantly outperform 1/2" of rockwool in terms of maintaining the flue wall temps above 250 F.
I'm waiting on a few article requests and will share what I find out. If anyone has seen better studies on internal temp variation in a range of conditions in insulated vs uninsulated flue liners I would love to see them as well.
I have a similar dilemma and question. I am thinking of re-routing a boiler exhaust from an existing terra cotta tile-lined brick chimney (running through the interior of the house) to a new Class-A insulated stainless steel flue (also running through the interior of the house). My thinking is that the insulation will help retain higher flue gas temperatures, improve draft, and reduce condensate inside the flue. The representative of the boiler manufacturer however said that he recommends installing a flexible metal liner inside the existing terra cotta tile-lined chimney and leaving the resulting ring of airspace between the metal liner and terra cotta tile as insulation around the flue. He claims the air space coupled with the terra cotta and brick will insulate better than the Class-A insulated metal flue, especially at the top where the chimney penetrates the roof. I asked if pouring Thermix or perlite insulation into the airspace between the metal liner and terra cotta tile would be beneficial. He thought that it would, but he seemed uncertain and offered no corroboration for his recommendations. I'm skeptical that an irregularly shaped residual air space between a round metal flue liner (5" diameter) and square terra cotta tile (roughly 7" x 7" inside) could insulate as well as 1" of continuous ceramic insulation around a Class-A pipe, but I realize I have no basis for making a qualified comparison. Does anyone?