Flue: to fill or not to fill

[SparkyTris]

Old stone-built house, damp gable. ground floor + 1st floor, then small loft void. suspected primary cause of damp is chronically failed chimney/cap. I am in middle of replacing chimney/cap. Which is to say I've removed the failed chimney & concrete cap element as far as the top of the stone stack. Have temporarily enclosed this with a ventilated box to stop rain entering directly the exposed top surface stonework.
old Stonework gable has been rendered 1997 with modern cement wetdash, which appears to be sound in all places. I suspect that this "good" render is holding moisture into the stone gable, which is now (hopefully) very slowly drying out at a trickle, via the internal wall and a little bit through the un-capped (but weatherboxed) chimney top section

Purists will scream at this point and say that the gable should have been rendered in lime plaster so that the stonework can self-ventilate. I mostly agree with this, and suspect that the sound modern render is preventing drying. We are where we are, the render is all "good" so its going to stay until it goes boss at which point I will remove it & rethink.

upon removing the failed chimney/cap I confirmed that both flues are original which is to say bare stonework. (no fireclay liner)
I cleared debris from and thoroughly swept both flues.

Whilst we have good access up there (decent scaffolding ringin the chimney stack), I was minded to install provision for multifuel stoves in ground floor and first floor. that is, install stainless flexi flue liner. Bit OTT perhaps on the first floor (presently bedroom but who knows what in future). I do not intend to install a stove in first floor at this point, only to make provision for the possibility of one. I may install a stove on ground floor.

my local supplier of chimney technology talked me through the way they "usually" do a stove chimney. This involves filling the void around the installed flexi flue liner with dry micafill (vermiculite).

I dont like that concept (micafill filler) in this case because:

1. I have to assume that the old stonework gable end is always going to be more or less damp. (obviously, hopefully less damp after this intervention). It needs all the ventilation it can get, and depriving the voidspace of airflow by filling it seems like bad news.

2. it seems like a fundamental change to that part of the stone building, and we don't know how it might respomd. this seems a woolly argument: it is, I mean Im not expecting it to explode etc but I just dont like the idea of it. cant put my finger on why. maybe to do with 1 above. I think its because I know that vermiculite acts a bit like soil ie, it soaks up water and don't let it go easy. (vermiculite is good for growing plants in)

not using micafill will mean:

1. flexi liner is free to vibrate along its height (more likely to fail with a hole)

2. I should provide ventilation of the void top and bottom (otherwise I end up with an enclosed weather system occurring within said void.

I note that the very helpful booklet from Schiedel that came with the flexi liner tube mentioned finishing techniques using micafill filler AND ventilating an un-filled void. I take it that both options are recognised by building standards then.

I would be very interested in comments on the benefits or otherwise conferred by vermiculite as a filler in this situation.

The reason for this intervention is primarily to fix a damp gable probably caused by a leaking chimney, (other sources of water ingress are possible). The building is an anachronism, and the real cause of the persistent damp is that the building is no longer operated within its "design envelope" which (presumably) included having an agricultural matriarch driving a stove 24/7/365. Installing flue liners with a view to multifuel stoves is very much a "while we're up there" job, but the more I think of it, the more I think it would be a good idea to follow through and install stove on g/f if for no other reason than to provide direct heat to this damp gable. waste wood & decent draught/draw is plentiful.

 

[Scally-Ho]

"I would be very interested in comments on the benefits or otherwise conferred by vermiculite as a filler in this situation"

In my day, a flexible flue liner was never installed without pouring granulated vermiculite between the liner and flue way before the final top flue terminal plate was installed.

 

[SparkyTris]

way before the final top

I don't doubt that it (poured vermiculite filler) is frequently used, but I'm trying to work out if it's a good thing or not.
you say "way before", what do you mean - do you mean to imply that you pour the verm then allow a period of time for drying, or you pour the verm, then allow a period of time for the rain to get in?
strikes me it might be a bit of a "cavity wall insulation" thing, which I believe was in vogue for a period of time, and it was done quite a bit, and now it is often being removed in favour of enhanced ventilation.

what is the point of vermiculite filler? arguments I have heard so far are:

1, insulates the flue, thereby theoretically improving the draw (the rocket-stove effect) - nb that an air jacket is also a very good thermal insulation
2., also by insulating the flue, prevents condensation forming on the flexi flue, which likely exacerbates corrosion. however, if there is damp in the vermiculite, condensation will still form on the flexi. if there is damp in the stonework, then it will soak up into the verm.
2. supports & braces the flexi flue within the chimney.

 

[mrrusty]

I can certainly see the sense in keeping the chimney space ventilated if the stone work is trapped behind an impervious render. I would vent at the bottom to outside, not inside, because if you vent inside you risk introducing warm humid air to cold internal surfaces and promoting condensation.

However, there is another challenge. Almost certainly there will be salts in the chimney caused by the reactions of soot, combustion gases, water and lime. The salts are hygroscopic and will suck moisture out of the air to grow crystals. In some cases they can pull so much moisture they actually liquefy. In this case I can see the sense of backfilling with vermiculite to restrict the introduction of water vapour to the salts.

Personally, I would vent to outside and promote ventilation. This is the way the original cavity walls were constructed with airbricks top and bottom to keep the cavity ventilated and the walls dry. Cavity walls were originally introduced, not for insulation, but to separate the damper outer wall from the timber joist ends to stop them rotting off.

 

[ree]

OP,
Pics would help?
Forget about vermiculite - its messy to install, and problematic.
Besides some installers dont know how to prepare the vermiculite.
Modern double wall SS liners are all the insulation you need.
SS Liners dont need supporting inside the flue, & will typically last, without corroding, for a long time.
150mm diameter liners will work best for you.

Definitely dont go installing external air bricks to vent the flue(s).
Installing air bricks to vent flues with liners doesn't make sense.

You presumably dont have cavity walls but FWIW never go installing any ventilation such as air bricks to vent any cavity walls - thats a big no no.
Sealed cavities are insulating cavities - what would be the purpose of introducing maybe freezing air into an insulating space?
Among other requirements the Bldg Regs require sealed cavity walls to prevent loss of insulation.

Rendering a chimney stack with a lime or a cement mix is typically done to cover failing brickwork - ie. a rendered stack is a bad idea.

Cavity walls were introduced in the early/mid 19thC to prevent damp walls causing health issues.

 

[ree]

Edit:
but it seems that it took another seventy years or so before cavity walls were introduced into working class homes - that was after millions of them had already been built without cavity walls, & so mostly poor people suffered the TB consequences.

 

[mrrusty]

If anyone has any interest in early cavity walls it's all here from historic England https://historicengland.org.uk/imag...arly-cavity-walls/heag083-early-cavity-walls/ including on the first page reference to airbricks to provide drying ventilation and on page 3 reference to the two leaves being separated to protect the internal woodwork and also allows for the evaporation of condensation.

I don't make this stuff up - it's all referenced.

 

[StephenStephen]

Just to throw something else in here:
We had a steel flue installed on the advice of the sweep.
The burner had worked better without it.
I think it's worth weighing up the pros and cons of not installing a flue, particularly if you've stonework which needs drying out. A flue is often not a requirement, though it is often presented as one.

 

[ree]

Poster #8,
Perhaps you would like to read the Bldg Regs Approved Document J -the section regarding flues.

 

[ree]

Poster #7,
There are other references regarding early cavity walls.
The Historic England document is referring to older buildings but even then it notes how sealed cavities can provide a "warm space".
As my above: Bldg Regs require & have required sealed cavities.

 

[StephenStephen]

Poster #8,
Perhaps you would like to read the Bldg Regs Approved Document J -the section regarding flues.

Nope, not really, but thanks for the offer.

 

[ree]

*********

 

[mrrusty]

It isn't a BR to line a chimney but is usually recommended. If the flue is sound, more heat goes in to the building and less out the top without a liner, but I agree it is potentially dangerous if not properly checked. https://www.hetas.co.uk/do-i-need-my-chimney-lined/

 

[stuart45]

All the HETAS fitters I know will always fit a liner. Although technically they don't have to, it means being certain the flue in in good condition. Doing a sealed smoke test would mean that the regs used to allow for a certain amount of smoke to escape, but the amount was left to the skill of the operative. That's really not a good reg, so it's much easier to just line it. Manufacturers state that their appliances run more efficiently with the correct sized liner as well.

 

[StephenStephen]

Poster #11,
In that case you are not concerned about having given potentially dangerous advice to the OP & any interested readers - but you are not concerned about the consequences.

I will gently point out that you have no idea whatsoever about what I am or am not concerned about - I would respectfully suggest that you have literally no experience of the inner workings of my mind.
My suggestion would be to consider apologising for your insult, and be disciplined in future in avoiding straying into areas which are irrelevant to the post, and which are also outside of your knowledge and experience.

To return closer to the topic, thank you to mrrusty and stuart45 for a more nuanced and accurate take on the side issue of liner or not, than either ree, or I as an amateur, can offer.

Sorry I've nothing useful to offer regarding the core question of whether to fill round the flue if you fit one.