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Green Building Bible, Fourth Edition
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    • CommentAuthorKenny_M
    • CommentTimeFeb 10th 2018
    Hi all,
    Sorry if I have too many disparate posts going on at the moment but thought this needed to be on a different thread. As mentioned on other threads story is that chimney is being rebuilt at the moment by a stonemason. The stone is thin in places, around where the flue passes along the gable wall. A proposal has been made to:

    1. Drop liners down the flues for the lower two fireplaces, needed for one which is going to be used, just in case wanted in the future for the other (I'm working on the premise that I won't want fires upstairs) and these are already closed off.
    2. Seal off at top of fireplace builders opening
    3. Pour a mixture of X (lime mortar/vermiculate or leca etc?) into the stack from the top which will set and bind everything together, as well as providing insulation for the woodburner flue.

    There are plenty of examples on the web of loose fill being poured into flues around liners for insulation reasons, but not so much about doing it with a wet mix for structural reasons, which is why I am asking if anyone has any experience of this? Just trying to evaluate the pro's and con's.

    Secondly, if is a good idea in principle then what should the mix be? As an old building I am presuming that a breathable/flexible lime mortar mix would be better than pure cement. I also understand that perlite, vermiculite and leca are commonly used, from what I have read leca is standing out as the better option, and is used with lime mortar to make limecrete for floors, but I haven't seen anything about limecrete being used in chimneys.

    Has anyone had any experience of filling a chimney flue in this way?

    • CommentAuthordb8000
    • CommentTimeFeb 10th 2018
    No experience of what you suggest. We back filled ours with Leca.

    The problem with a lime mix that will set is that the liners don’t last forever. You wouldn’t be able to replace it would you?
    • CommentAuthorKenny_M
    • CommentTimeFeb 10th 2018
    I mentioned that to him, but I guess his take on it was that some of the better liners have a 30 year expected lifespan, and I suppose if the liner failed there would still be a clay/mortar set flue behind this.
    • CommentAuthorgyrogear
    • CommentTimeFeb 10th 2018
    Depending on amount of access (and I suspect that since it is "being rebuilt", that access is OK...), then my suggestion would be to build an internal hard flue around the liner, comprising hollow masonry units, then fill the space between the HMU and the existing stone flue, with sloppy mortar etc. Then you have the option of replacing the liner in the future.

    • CommentAuthorKenny_M
    • CommentTimeFeb 10th 2018
    Thanks gg, that would work at the top where the stack is being rebuilt, but I am hoping this rebuild doesn't go down so far that I would be able to put a hard flue around it, as that would get really expensive. I suppose I could go for a large diameter liner as a sort of former, that would allow a narrower liner to be passed down in the future should it be required. However, I am not sure how necessary this is, given the lifespan of quality liners, and the fact that with a fill around the liner there would be a hard liner there to back it up should it fail?
    • CommentAuthorgyrogear
    • CommentTimeFeb 10th 2018
    Posted By: Kenny_MI suppose I could go for a large diameter liner as a sort of former,

    great idea !

    But bear in mind that if / when the inner liner "goes", your backup solution would not necessarily work OK, because with modern stoves, and moreover going forwards in time, improved specifications, the volume of the liner actually becomes critical to stove performance...

    • CommentAuthorKenny_M
    • CommentTimeFeb 11th 2018
    Yes, I don't think there is going to be a perfect solution here. I am working on the premise that if I fit a good quality twin wall liner, and fill around it with say a lime mortar/leca mix, then if the inner and outer steel liner fails, there will still be in effect a hard clay/lime mortar flue on the outside of that, and the original flue on the outside of that. It will be hard going for me to burn through all of that in the foreseeable future, and if it does it won't be the end of the world to not have a stove! I think the main thing is the integrity of the flue and stack from a structural point of view.
    • CommentAuthorgyrogear
    • CommentTimeFeb 11th 2018
    I hear you !

    Without labouring the point, it might also be worthwhile to install a thinnish (80mm ?) secondary conduit alongside the liner, to pick up "extraneous heat" by the thermosiphon principle: when the stove is running, open the bottom of the air pipe, and you get hot air delivered further up the chimney, for warming upstairs for example.

    I am having to do this on a retrofit basis, and it is a bit of a head-ache ! so if you get chance now, then here is the heads-up, LOL

    • CommentAuthorJamster
    • CommentTimeFeb 13th 2018
    I've seen this done with a balloon type set up to create a chimney void with lightweight leca-type mix around it. You can then drop a liner down that if you want but you have the smooth relatively narrow and straight chimney in place rather than a rough edged masonary one. Not sure how strong the lightweight mix is structurally though?
    • CommentAuthortony
    • CommentTimeFeb 13th 2018
    It is very strong, often reinforces the whole chimney structure
    Posted By: JamsterI've seen this done with a balloon type set up to create a chimney void with lightweight leca-type mix around it

    I've heard of this some time back - I googled inflated tube chimney liner and found


    doubtless there are others
    • CommentAuthorKenny_M
    • CommentTimeFeb 14th 2018
    Thanks gyro - that's a great idea. I think my only worry about this would be that if those two pipes got too close to each other on the way up the stack, and over time one managed to start to leak into the other, I could be delivering CO to a bedroom. I'd be keener to implement a version of that idea if I could find a way of spreading the heat to other parts of downstairs, but with all the solid walls that would be more hassle than its worth!

    Thanks Jamster/Tony/Peter, I did see something like this before, but I suppose one problem is where to get the inflatable former, and in the absence of this, maybe a flexible liner is the next best thing. Having seen the rough quote now, I can see he plans to use 8 inch liners. This surprised me at first because the stove I was looking at takes a 5 inch liner, and most take 6 inch, so one upside of this would be that another liner could be slid down inside it, maybe.

    Another potential issue is that there are at least two bends. In the links Peter posted, they actually have to break into the wall to support the liner at the bends. I don't think this is part of the masons plan, I think it is just to drop the liner in and then pour the mixture on the outside. This would result in the liner sitting against the bends and that section of the liner remaining uninsulated, and that part of the flue unsealed by the mortar mix. For me that then becomes a weak point for condensation and if it corroded and a leak developed, it would be right into into the original masonry. I've told him to hold off on this work for the moment anyway, while I have a think about it, and finish everything else first.

    Incidentally, I got this back from Dupre minerals about the mixing:

    "Composed of a mixture of 4-10mm uncoated Leca® clay aggregate and NHL5 mixed at 3:1 (Aggregate : NHL) and then gently compacted. Compaction reduces the mix by approx. 10%. The drying times of the mix varies due to humidity, using for example Hanson NHL curing time is appx 24 - 48 hours. In certain warm conditions, it may be necessary to lightly mist the mix so that it does not dry out too quickly.

    I have also spoken with our technical team in regards to using Lime Motar with Micafil Vermiculite (this product is appx x3 times lighter than Leca) and they cannot see an issue with this at all for your property. I have attached further information on using Micafil for your chimney. You would need to try to keep the mixture as dry as possible (but enough for it to bind together). Use the Lime Mortar in the place of cement on the attached document for the correct ratios."
    Posted By: Kenny_MAnother potential issue is that there are at least two bends.

    If you have 2 bends how do you propose getting the liners in, especially the second one? or are you proposing flexible liners?

    Posted By: Kenny_Mbut I suppose one problem is where to get the inflatable former

    As far as I know about this type of job is that the company have the liners, come and do the job and take the liners away afterwards. It is not a DIY effort.

    You should know how many bends, where they are and the diameters of the chimney before you start discussing solutions!
    • CommentAuthorKenny_M
    • CommentTimeFeb 14th 2018
    Peter, it would be flexible liners, I had thought this was clear, but re-reading my original post I could see how it might not have been. There will be two bends, where it bends out away from the centre of the stack, and where it bends in again before it enters the fireplace. I've never seen a straight up and down chimney flue before in an old building, so again I am probably making an incorrect presumption there that it was clear that there would be two bends.

    The companies mentioned are too far away from me, and the stonemason would be doing this so its not really a DIY job.The main reason I brought it up was because it was a proposal made to me by the stonemason and i'm not entirely convinced by the details or the lack of previous examples of this being done using flexible liners as the former, or the mix used etc.
    If, as per your post above, the contractor is using 200mm (8 inch) liners, you would have the opportunity to insert a new 150 liner if the 200 one failed in future. You'd struggle to get any guarantee of all-round insulation, though, so in terms of insulation to reduce creosote build-up, you'd be limited to what little you could get down the gap between liners and whatever was used around the 200 liner.
    • CommentAuthorKenny_M
    • CommentTimeFeb 14th 2018
    Thanks Nick, as I said above there is no perfect solution here. If I fit large diameter liners there will be little insulation on the outside, smaller diameter then the wont be able to be replaced. In all cases it is going to be difficult to avoid the liner touching a wall at some point. A narrow diameter liner specced to the stove and wrapped with chimwrap or something like that is probably the most uniform solution in terms of insulation, but does nothing to help the original flue in structural terms. I will ponder on this and make no sudden moves!
    Going back to your original post
    Posted By: Kenny_MThe stone is thin in places, around where the flue passes along the gable wall.

    If the chimney is being rebuilt why can't the stonemason build the thinner parts a bit thicker? And then just use a normal liner that can be replaced as necessary. Who is unhappy with the thickness of the stone work of the chimney and how long has it been standing and in use in this state?

    If you want to put in a liner down the chimney and fill around it, I don't see how you will be able to fill past the first bend without breaking in to the chimney to do the filling.
    • CommentAuthorKenny_M
    • CommentTimeFeb 14th 2018
    Peter, these are good points, and its this lack of detail and too many unanswered questions that has stopped me agreeing to it as of yet. Its the stonemason that has suggested this. He is rebuilding the stack, but the point was that looking down the flue some of the feathers are broken away further down etc, and his suggestion was that by filling the chimney everything would be bound together. I am presuming that the mix would have to be loose enough to be poured down and reach the bottom of the flue, in the same way that loose insulation would be poured down, but so far I am not completely convinced of any of this, or even that there are no risks associated. This was the reason that I posted about this and some of the points that have been made are just reinforcing my concerns.

    In theory if the steel liner is completely surrounded by a solid leca and mortar mix, then it should never need replaced as there will be a hard clay liner created on the outside of the flexible steel liner, but that would require the liner to be somehow positioned central to the original flue all the way down, and to be sure that the poured mix completely surrounded the liner, and that it wasn't porous or cracked in places, and I can't say that I am convinced at the moment that this is achievable in practice.

    My original plan was just to drop down a singe liner, sized for the one fireplace/stove I intend to use. I haven't been home during the day this week to inspect the flue, or to discuss this any further with the mason, and unless I am convinced about the solutions to the above then I'll be going back to the original plan.
    What do you mean by 'some of the feathers are broken away further down' - what are feathers?
    Any indication of how long they have been broken away? and otherwise how thick is the stonework in the area of the broken feathers
    • CommentAuthorKenny_M
    • CommentTimeFeb 15th 2018
    Most of the flue is sandstone. The feathers are the single brick dividers at the top that separate the flues where they meet at around roof/stack level. Might be a Scottish term, there seem to be a few of these that are different in relation to buildings, and other stuff! :)

    How thick, I don't know to be honest, I haven't been up this week as its been dark by the time I got home. I don't have a pic of the inside, but below is the outside. Its been patched with cement in the past, and it had been patched quite far down. The last time I went up with them the cement hadn't been taken down too far, but I got some good news yesterday over the phone that they have found solid stone to build off nearer than the top than they first thought. I need to catch up with the Mason again in person to get a proper update.

    • CommentAuthorgyrogear
    • CommentTimeFeb 15th 2018
    further to my previous contribution viz.

    Posted By: gyrogearmy suggestion would be to build an internal hard flue around the liner, comprising hollow masonry units, then fill the space between the HMU and the existing stone flue, with sloppy mortar etc.

    I have checked on another (French) site and got the following feedback to a similar question:

    "Filling the space between the liner and the stone flue will prevent ventilation. Since the liner itself is already an insulant, it is recommended not to install anything, in order to provide a ventilation gap to remove not just any condensation forming on the outer wall of the liner, but also any gases forming (via leakage of the liner etc.). In terms of insulation, dry air is a good enough insulant especially if it exists as just a thin film".

    This advice seems to support my earlier suggestion, namely : sleeve the liner inside a hard (masonry) liner - leaving an air gap between the two.

    Then fill the gap between the outer hard liner, and the inside of the stone flue, with whatever you can, such as sloppy lime mortar etc.


    • CommentAuthorKenny_M
    • CommentTimeFeb 15th 2018
    Thanks gg, but not sure how I could achieve this. I maybe haven't been clear enough that this is an old house, the flue curves twice to reach the fireplace, and its only the stack at roof height that is being rebuilt. There is no way to build a hard liner around the flexible liner as there is no access to the flue to do this.
    Posted By: Kenny_MThanks gg, but not sure how I could achieve this. I maybe haven't been clear enough that this is an old house, the flue curves twice to reach the fireplace, and its only the stack at roof height that is being rebuilt. There is no way to build a hard liner around the flexible liner as there is no access to the flue to do this.

    The only way that I know to fill in around a liner is to break into the chimney at the bends.

    Is the chimney built onto the wall (back of chimney is the front of the wall) or is it built into the wall? If it is built into the wall it may be easier to break into the bends from the outside.

    From the photo above - the wall obviously needs a re-render, but what is that apparent black hole(?) in the centre of the picture?

    If it were mine I would be seriously be considering extending the roof by 1 tile (or slate) and putting on EWI.
    If it was me I would not do it. On our house we had a very large damp patch on an upstairs bedroom wall behind an inbuilt bookcase. Ripped out the bookcase and took out over a tonne of damp sand. Looked up the hole created and could see the sky. Further investigation by removing plaster revealed a magnificent original stone fireplace. Turns out when dodgy builder installed the stone fireplace downstairs he reused the original chimney upstairs but closed it off by filling the chimney around the liner with sand. It was ok until the pointing on the chimney failed. Now closed off with a proper blanking plate and the damps all gone.
    • CommentAuthorKenny_M
    • CommentTimeFeb 16th 2018
    Posted By: Peter_in_Hungary
    what is that apparent black hole(?) in the centre of the picture?

    If it were mine I would be seriously be considering extending the roof by 1 tile (or slate) and putting on EWI.

    The apparent hole is just that, a hole. This pic is after the cement render started to be stripped, and this was the section that the mason was concerned with, but as they worked further down it was a bit more solid. That section of the wall is being rebuilt at the moment, and the chimney is in the process of being rebuilt now.

    I've been over the EWI thing in another thread, its not that simple, and I would need a planning application as I am in a conservation area. I'd like to consider EWI of some form in the future, but it will be a huge expense on a house like this, and as I can never do the front of the house it would be an incomplete solution. CWI and glazing on the extension, and a new boiler, are the priorities for the pennies at the moment.
    • CommentAuthorKenny_M
    • CommentTimeFeb 28th 2018
    Thought I would upload the guide that Dupre sent me about using micafill vermiculite to fill chimneys, in case anyone is interested in this. I had originally asked them about leca, but they recommended vermiculite as it is much lighter.

    As a recap, the proposal here came from the stone mason doing my chimney, to fit flexible liners in the flues, and pour a lime mortar/vermiculite mix down from the top of the stack. This would form a weak, light limecrete around the inside of the flue, fill any gaps between the stone and hold it all together. With the age of it there is a loose stone, and sand coming away and dropping down the flue over time, and the limecrete should hold it all together and prevent anything else coming away. Until they train small monkeys for this sort of work, obviously you can't crawl up the flue and repoint/reline it so the only way to reline it and repair other than taking the stones out of the wall all the way up and down 4 flues which would be a massive, expensive task.

    As pointed out in some of the comments above, its far from a flawless plan, but to not do it is not flawless either as the flue could continue to crumble from inside without anything to hold it together. My biggest reservation to be honest would be the fill, and i'm not sure what's to stop the mixture sticking half way down on a bend or narrow section the rest all backing up and setting behind it.

    Anyway, i'll have to make a decision soon but just thought i'd throw up the PDF in case anyone else is interested.
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