woodburners & insulation fo chimney

[MetManMark]

All

We have decided to install 2 woodburners in our house. These will go in the living room and the room behind which will be our office. These rooms are at the gable end of the house. One of the chimneys protrudes from the house (ie external) the other is internal (ie the chimney is within the house).All of the installers have said that the chimney needs to be lined (which is fine). However, some have said that it needs to be insulated with a 'chim-wrap' (essentially a celotex style wrapper) or through the addition of vermiculite beads, others have said that no insulation is required. We will run the stoves pretty hot to make sure that all the gases exit the chimney and don't condense. The chimneys are about 9m high.

The chap that seemed most knowledgeable and works closely with woodwarm stoves (our preferred option) suggests the vermiculite - which we don't really like the idea of. I thought that filling the chimney with stuff isn't a good idea as it should be allowed to breathe. Thoughts?

Thanks in advance
Mark

 

[discobarry]

I'm no expert but our house is a 1650 farmers cottage with two stove on an external stack and we just have ours lined and this is fine? You worry about the use of vermiculite but you forget your stoves are full of it, i used it for heat protection at my last home behind the stove as it was pretty close to the wall and it was fine.

 

[MetManMark]

It is more that I am not keen on filling the chimney with small beads. Surely this will limit air flow within the chimney and stop any moisture escaping? We intend to lime render the house next year and wasn't it to breathe.
Mark

 

[DJH]

In the days before I became a 'real' plumber all solid fuel back boilers I installed always had a weak mix of lime and vermiculite and any new flues were also surrounded with the same mix. As far as I'm concerned it's a tried and tested method which both increases the efficiency of the flue by insulating it and also helps stabilise the existing brick/masonry.

Hope this helps

Doug

Just noticed your last post Mark. Mixing lime with the vermiculite will bind the beads together and will be as breathable as lime mortar is.

 

[MdB]

I looked into this - a lot...

You have found chimwrap and vermiculite. Two other options are leca and rockwool.

Chimwrap is really rather expensive and rather thin.
Vermiculite can turn to mush if it gets wet - I think. Didn't look too much into this.
Leca is used in limecrete and won't give you issues with breathability.
Rockwool is a bit cheaper but adds considerably to the diameter and isn't the easiest to install into a tight space.

We ended up using rockwool for our stove chimney as we had the chimney open so it was easy to fit.

For our open fire I filled the gap around our concrete liner with leca + lime. This is a build control requirement.

 

[Zebra]

Vermiculite is breathable,is it not? However, if your chimney is large, you might need quite a lot, and so the rockwool sleeves might work out cheaper. As they wrap around the flue and don't come into contact with the chimney, they won't have any effect on breathability of your chimney.

 

[skier-hughes]

I found that insulating the liner in one of our chimneys helped immensely with getting a good fast draw when lighting the fire, suppose the flue just warms up quicker.
The other fire is so larger any fire burning in it creates enough heat to get it going well.
We used rockwool, it was (or appeared to be) really easy to do.

 

[worms]

We have a woodburner and an external chimney, we lined the chimney but did not insulate it. Big mistake! It sucks! Literally!

When it is really cold, as you light the fire, the smoke is drawn into the room until the stove and then the chimney warm up. This isn't great but is bearable if you are prepared to set a very small fire of very dry kindling in it first and wait for the flue to warm up before putting bigger logs on it.

However inevitably, you come home from a weekend away, to a freezing cold house, want heat quickly, put a match to all manner of combustible rubbish that people have helpfully put into the stove and fill the house with a horrible, acrid, carcinogenic smoke, setting off the smoke alarms and having to open all the doors and windows until the air is breathable again!

Definitely insulate the external chimney!

 

[CliffordPope]

We have a woodburner inside a very big inglenook chimney opening, tapering to about 2' square internal at the top.

I just fixed rockwool sleeving round the pipe and shoved it up the hole one section at a time until it stuck out of the top, clamped on a suspended cowl, and lowered it back until it was hanging on the pot/cowl.

The inglenook is boarded at the bottom, where a metal plate helps to take the weight of the flue.
I can't see any point in trying to fill the considerable gap between the flue and the chimney - it's about 8' by 6'.
The insulation doesn't get hot, that's its job.

 

[AMc]

Depending on the size of the chimney and the construction of the register plate (if any) filling the void will reduce convection currents in the stack which naturally move heat from where you want it to the outside world - heat rising, chimney stacks being exposed on all sides for the last couple of feet etc. This is particularly important if the room is otherwise heated and the burner is only used occasionally.

When we had our floor insulated and underfloor heating installed in a small room with a large but rarely used wood burner it was hard to get the room warm without lighting the burner. Lighting the burner overheats the room because its much too big
I spoke with my installer and he recommended a register plate across the 4' x2' opening. It made a 1-2C difference in the room temperature in a few minutes. It also significantly reduced the draught coming under the door into the room from the hall outside.

I had a black steel plate fitted for about £150 all inc. I chose steel over fireboard on the recommendation that if I wanted the chimney filled with vermiculite it would be a simple job in the future. In practice that hasn't been necessary but if we replace the old oversize burner with a more appropriate appliance I probably would do that too.

 

[88v8]

We will run the stoves pretty hot to make sure that all the gases exit the chimney and don't condense.Mark

A rockwool sleeve on the flue liner will be fine. If it's good enough for thatch - which it is - it's good enough for anything.
And it gives uniform insulation all way down, unlike vermiculite or other loose fill around a flexy liner which will vary in thickness according to where the liner (uncontrollably) snakes as it goes down, sometimes resting against the side of the chimney so there's no insulation at all.

Fit a flue gas thermometer to the flue pipe of each stove. You can't tell by just looking at the fire how hot it is. If you run it too hot, the liner will suffer, and the stove baffle plate and body will warp.

Ideally, the stove should be towards the front of the fireplace so it throws the heat into the room. Don't know what make you have in mind, we have a Clearview and an Esse, I would buy another Clearview tomorrow, wouldn't have another Esse if you gave it me.

On a general note, as has been commented, installing the register plate makes a big difference to the heat loss from the room. Negatively, it also radically reduces the ventilation of the room and perhaps of the house. The flue of an open fire, lit or not, moves a lot of air, and that will cease altogether when you instal the stove. To maintain a degree of ventilation/breathing, leave at least the stove damper open when not in use.

Ivor

 

[Flyfisher]

We have a woodburner inside a very big inglenook chimney opening, tapering to about 2' square internal at the top.

I just fixed rockwool sleeving round the pipe and shoved it up the hole one section at a time until it stuck out of the top, clamped on a suspended cowl, and lowered it back until it was hanging on the pot/cowl.

The inglenook is boarded at the bottom, where a metal plate helps to take the weight of the flue.
I can't see any point in trying to fill the considerable gap between the flue and the chimney - it's about 8' by 6'.
The insulation doesn't get hot, that's its job.

That's good to read, because that's my plan for our large inglenook/chimney. I've already installed the liner (when the top of the chimney was rebuilt) and propose to add rockwool insulating sleeves a section at a time and puch them up the liner. Not sure how easy it will be but I can use step ladder to reach over half way up the chimney anyway, so that should help!

I guess it all depends on the size of the void between the liners and the chimney brickwork, but we'd need a lorry-load of vermiculite to fill our stack.

 

[DJH]

Nobody has mentioned this method of forming a new flue inside an existing chimney which I've seen used on quite a lot of occasions on historic properties I've worked on where removing and destroying historic fabric to access the chimneys has been judged unsuitable. This method involves sending an inflatable tube down through the chimney where it is sealed at the fire opening and then inflated. A pumice mix ( I'm not exactly sure of the hardening material) is then pumped from the chimney down alongside the inflated tube filling the void between it and the structure.

I know that the first think everyone will think is that there could be places where the tube touches the masonry and the mix will not surround the pipe at these places but it was thought that this was a small price to pay rather than have any demolition work which would have to take place to insert flue pipes. I seem to recall that a camera was also sent down the chimney both before the works were carried out. Not too sure of the costs of using this method but to maintain the integrity of the structure it was thought the best option and maybe cheaper in some cases and, of course, quicker and an added bonus that the stack is reinforced at the same time.

Doug

 

[GRB]

Has anybody used the Furanflex system for chimney lining? We are thinking about it but it would be good to know what others think. From what I can gather it is a flat tube (of a composite material) that is put down the flue, inflated and then steamed for a bit. The steaming hardens the composite material and hey presto your chimney is lined. All seems to good too be true...

chimneycare.co.uk/FuranFlex-Chimney-Liner.html

Thanks

 

[plasticpigeon]

Vermiculite is an expanded rock, I'm sure it won't turn to mush if it gets wet!!Like others, I'd have thought vermiculite or other insulation would be a good thing.

 

[DJH]

I've also in my time removed quite a few solid fuel back boilers to replace them with gas fired units and can't recall any time when the vermiculite mix behind the boilers had deteriorated. As Plastic says vermiculite is a good and proven option.

Wood burning stoves were coming into more general use just about when I decided to concentrate on leadwork and I remember hearing about the expensive cost of twin walled stainless steel liners which I believe, are required for solid fuel stoves. The wrap around, or clip on insulation, seems to be very good option especially where access can be gained if the fire opening is large enough to get in to install it in which case it would be too big to fill with a loose fill.

Doug

PS Anyone got snow over there? Now's the time not to get up on a roof

 

[worms]

To maintain a degree of ventilation/breathing, leave at least the stove damper open when not in use.

Very wise advice, but with an uninsulated, external chimney you do need to shut the damper during really cold weather or the cold air coming down the chimney and out through the unlit stove is reminiscent of that scene in "The Day After Tomorrow" when the eye of the weather pattern is directly overhead and the supercooled air rushes down and freezes everything it touches!

 

[Penners]

The choice of liner insulation is up to you, but I strongly recommend you to insulate.

Combustion gases are full of acidic tars. If the flue liner is not insulated, there is a strong risk of the gases cooling as they rise up the liner, to the point where the tars start to condense out. Stainless steel or not, it doesn't take long before these tar deposits can cause severe corrosion of the flue liner, and its ultimate failure.

So regardless of whether you insulate with a chimney-full of vermiculite granules or with some sort of wrap around the flue liner, the important thing is simply to insulate.

 

[Andrew Bodenham]

Mark,

Although the regulations do not specifically mention insulation of retro-fit flue liners the manufactuers of such liners do recommend that insulation is used. I cannot think of any good technical reason why insulating around a liner would be detrimental, but I can think of several reasons why it's a good idea. In my opinion installers who recommend that insualtion is not needed are probably just trying to get job done as quickly and easily as possible, rather than trying to achieve good performance and improved longevity. Sure, the thing will work without insulation, but the liner will be more likely to suffer from condensation, which I think will accelerate corrosion and ultimate failure. As others have mentioned, insulation aids rapid flue heat-up from cold, which helps when starting the stove.

I recently installed a new liner using the 'chim-wrap' system obtained from a stovesonline. This is a fibreglass quilt contained in a tough voven fibreglass jacket which straps around the liner pre-installation. I used this because there was not sufficient clearance in the flue for the more bulky rigid jacket type insulation. Installation was very easy, and so far performance has been very good. I like the idea that when the liner eventually fails the jacket will act to contain the flue gasses, and the whole assembly will be relatively easy to remove for replacement. I've always felt that using cement-bound backfill such as LECA or vermiculite ignores the fact that one day the liner will require replacement.

 

[MdB]

I cannot think of any good technical reason why insulating around a liner would be detrimental

I believe there is the potential of the liner getting too hot which will reduce its life. The bottom metre should be uninsulated and made from stove pipe because of this if I remember the regs correctly.