Insulating a double skinned chimney

[GUMPY]

I am thinking of buying a double skinned chimney rather than making my own.

https://www.midlandchandlers.co.uk/products/double-skin-chimney-plain-18-x-6-ch-002-m

Would it be a sensible thing to fill the void between the skins  in the chimney with fire rated insulated foam?

https://www.screwfix.com/p/no-nonsense-fire-rated-expanding-foam-hand-held-750ml/83897

Or am I mad?

 

[OldGoat]

17 minutes ago, Loddon said:

I am thinking of buying a double skinned chimney rather than making my own.

https://www.midlandchandlers.co.uk/products/double-skin-chimney-plain-18-x-6-ch-002-m

Would it be a sensible thing to fill the void between the skins  in the chimney with fire rated insulated foam?

https://www.screwfix.com/p/no-nonsense-fire-rated-expanding-foam-hand-held-750ml/83897

Or am I mad?

 

1). Yes

2).   MC say there's a drip feature to prevent condensation.

If you fill the void with gunk there'll be nowhere for the condensation to go - and you'll drown....

 

"Black powder coated chimney with no decorative bands or hooks. Double skin 'lined' version to allow condensation to run back into the main flue tube for burning off. Designed to fit over 6" (152mm) roof collars. Length: 18" (457mm) Diameter: 6 1/8" (156mm). "

[Tony Brooks]

18 minutes ago, Loddon said:

I am thinking of buying a double skinned chimney rather than making my own.

https://www.midlandchandlers.co.uk/products/double-skin-chimney-plain-18-x-6-ch-002-m

Would it be a sensible thing to fill the void between the skins  in the chimney with fire rated insulated foam?

https://www.screwfix.com/p/no-nonsense-fire-rated-expanding-foam-hand-held-750ml/83897

Or am I mad?

 

No you are not mad. It makes a great difference to the amount of tar leaking out bteween the outer chimney and the roof plate. However its best to make sure the inner actually enters the top of the flue or is absolutely rigidly aligned dead over the flue so any drips go back down the flue rather than drip onto the roof plate. I used ordinary expanding foam but but a ring of screwed up kitchen foil around the bottom. Absolutely no signs of the foam burning or melting.

3 minutes ago, OldGoat said:

1). Yes

2).   MC say there's a drip feature to prevent condensation.

If you fill the void with gunk there'll be nowhere for the condensation to go - and you'll drown....

 

"Black powder coated chimney with no decorative bands or hooks. Double skin 'lined' version to allow condensation to run back into the main flue tube for burning off. Designed to fit over 6" (152mm) roof collars. Length: 18" (457mm) Diameter: 6 1/8" (156mm). "

 

I trust the red bit is all a joke. By filling the void you will prevent fumes getting into the gap which in turn prevents them condensing on the inner surface of the outer chimney. Foam (or lightly screwed up kitchen foil) will also help insulate the inner chimney so it runs hotter which again helps minimise condensation.

[GUMPY]

I have checked,  as my flue is not central in the collar I had to bend the chimney  liner slightly off centre, one way round the chimney doesn't sit down as it should since it is resting on the flue, the other it goes on another 3-4cm so it appears that the inner is inside the flue.

 

Off to screwfix it is then.

 

[Tony Brooks]

24 minutes ago, Loddon said:

I have checked,  as my flue is not central in the collar I had to bend the chimney  liner slightly off centre, one way round the chimney doesn't sit down as it should since it is resting on the flue, the other it goes on another 3-4cm so it appears that the inner is inside the flue.

 

Off to screwfix it is then.

 

Once you have done the foam and its set a quick look down the chimney when you fit it will  show if it's on correctly - or paint a couple of witness marks on the chimney and roof plate.

[BeerOnTheCut]

Didn't want to start another thread so I'll add on here.....

 

Just had a new lined chimney made, was advised to use Rockwool type stuff but that comes in huge rolls, I would end up throwing away 95% of it which is a waste.

 

Can you buy anything smaller or is the expanding foam the next best thing ?

[David Mack]

If you give your location, someone here near you may have some offcuts of loft insulation they could let you have.

[Tony Brooks]

20 minutes ago, BeerOnTheCut said:

Didn't want to start another thread so I'll add on here.....

 

Just had a new lined chimney made, was advised to use Rockwool type stuff but that comes in huge rolls, I would end up throwing away 95% of it which is a waste.

 

Can you buy anything smaller or is the expanding foam the next best thing ?

 

I used spray foam capped at the lower end with aluminium foil to protect the  foam from direct heat.  I have heard that loosely scrumpled aluminium foil also works.

[blackrose]

On 29/09/2020 at 10:23, Loddon said:

 

Would it be a sensible thing to fill the void between the skins  in the chimney with fire rated insulated foam?

https://www.screwfix.com/p/no-nonsense-fire-rated-expanding-foam-hand-held-750ml/83897

Or am I mad?

 

 

I would say no to that part. This is a fire-rated PU foam not a fire-proof foam. This means that the ignition time is retarded compared to standard expanding PU foam, but it can still burn.

 

While I think it's probably unlikely to actually catch fire in situ in your chimney, I do think there's a fair chance that the PU foam could melt and drip down the flue where it could catch fire or alternatively ooze out all over your roof! I don't know what foam the flue manufacturers use in their insulated flues but I suspect it's a more fire resistant grade than Screwfix no-nonsense. 

 

Anyway, if i does work and doesn't melt, I think we'd all be interested to know if the new double-skinned insulated chimney improves the performance of the stove.

Edited September 26, 2021 by blackrose

[GUMPY]

12 minutes ago, blackrose said:

Anyway, if i does work and doesn't melt, I think we'd all be interested to know if the new double-skinned insulated chimney improves the performance of the stove.

It worked fine on the occasions I used it last winter, as to performance it's no better than my previous single chimney with a rolled up litho plate stuffed down the centre hole and no insulation. It is however more visually pleasing.

Edited September 26, 2021 by Loddon

[Ray T]

I’ve wrapped the inner skin with exhaust bandage, then wound thin wire round that to keep it in place, finally filled with fire retardant foam.

Never had tar running outside the chimney or any other issue.

Edited September 26, 2021 by Ray T

[BeerOnTheCut]

On 26/09/2021 at 12:17, David Mack said:

If you give your location, someone here near you may have some offcuts of loft insulation they could let you have.

Currently around the Great Ouse river network.

 

Cheers

[ditchcrawler]

Would this negate the advantages of a double skinned flue as I would have thought it would cool the flue gases as they leave the stove https://www.recoheat.co.uk/flue-pipes-how-it-works

[Ex Brummie]

I have a piece of 4" st/steel topped with a swedish cowl. I use a standard 6" N/boat chimney outside of this to shield the 4" pipe with no lining. As they are separate parts, I know the inner pipe is inside the collar. I don't have a problem with tar as it is a flue for a Bubble stove, but a pad around the bottom of the outer chimney would work in the same way as with a single chimney. 

[jonesthenuke]

I would certainly not use spray foam, fire rated or not. At a high enough temperature the foam will either melt or in the worst case ignite. It would be much better to use glass or ceramic fibre insulation as it is fire proof rather than fire retardant. Our chimney interspace is insulated with rockwool style insulation. Another option is the woven glass fibre tape used to wrap exhaust pipework, but it would be fiddly to fit it.

[waterworks]

On 29/09/2020 at 10:23, Loddon said:

I am thinking of buying a double skinned chimney rather than making my own.

https://www.midlandchandlers.co.uk/products/double-skin-chimney-plain-18-x-6-ch-002-m

Would it be a sensible thing to fill the void between the skins  in the chimney with fire rated insulated foam?

https://www.screwfix.com/p/no-nonsense-fire-rated-expanding-foam-hand-held-750ml/83897

Or am I mad?

 

It would be a very, very bad idea. 

 

Use Mineral wool if you must, what difference it will make is arguable. 

[GUMPY]

17 minutes ago, waterworks said:

It would be a very, very bad idea. 

 

Use Mineral wool if you must, what difference it will make is arguable. 

I did it with https://www.screwfix.com/p/no-nonsense-fire-rated-expanding-foam-hand-held-750ml/83897

Two years ago, no charing, no shrinkage, no melting.  The fire isn't on 24/7 from November to March  but when I am there its on all the time.

I also lengthened the inner so it sticks six inches into the flue pipe. The sprayfoam  makes a huge difference to the usability of the chimney holding everything rigid which mineral wool wouldn't. 

 

What is the reasoning that makes it a bad idea? 

 

Edited December 12, 2022 by Loddon

[waterworks]

On 12/12/2022 at 17:13, Loddon said:

I did it with https://www.screwfix.com/p/no-nonsense-fire-rated-expanding-foam-hand-held-750ml/83897

Two years ago, no charing, no shrinkage, no melting.  The fire isn't on 24/7 from November to March  but when I am there its on all the time.

I also lengthened the inner so it sticks six inches into the flue pipe. The sprayfoam  makes a huge difference to the usability of the chimney holding everything rigid which mineral wool wouldn't. 

 

What is the reasoning that makes it a bad idea? 

 

That foam isnt fire proof its just meant to withstand a fire for a specific length of time in an emergency. 

 

Like I said glass fiber lagging or mineral wool would be used in any industrial application to insulate boiler flues or exhausts. 

 

My reasoning is if your going to buy a product then you might as well buy one designed for the job and is 100% fireproof. 

[Tony Brooks]

On 12/12/2022 at 17:13, Loddon said:

I did it with https://www.screwfix.com/p/no-nonsense-fire-rated-expanding-foam-hand-held-750ml/83897

Two years ago, no charing, no shrinkage, no melting.  The fire isn't on 24/7 from November to March  but when I am there its on all the time.

I also lengthened the inner so it sticks six inches into the flue pipe. The sprayfoam  makes a huge difference to the usability of the chimney holding everything rigid which mineral wool wouldn't. 

 

What is the reasoning that makes it a bad idea? 

 

 

Whilst not saying it is not the right think for the job I did mine with ordinary expanding foam and it never cause a problem. I did cover the bottom with aluminum foil to ensure it did not stick to the roof collar whist curing and it stuck. My inner only extended a few mm into the flue. There was never any signs of melting or burning, but then I never had the stove run away. I concede it may be different with an insulated flue or a large stove running hard.

[Sea Dog]

On 12/12/2022 at 17:13, Loddon said:

What is the reasoning that makes it a bad idea? 

I'd be worried about the expansion of the foam deforming the chimney. You can get low expansion foam, but I'm not sure whether it can also be temperature or fire resistant. Looks a lot like those using ordinary expanding foam haven't had deformation issues though, doesn't it. Given the collective experience, I'd be inclined to go with the fire rated stuff whilst bearing in mind the  expansion when pulling the trigger.

[Chris Lowe]

On 26/09/2021 at 12:34, Tony Brooks said:

I used spray foam capped at the lower end with aluminium foil to protect the  foam from direct heat.  I have heard that loosely scrumpled aluminium foil also works.

This would probably do the job, it's fire rated canister insulation

 

[GUMPY]

1 hour ago, Sea Dog said:

I'd be worried about the expansion of the foam deforming the chimney. You can get low expansion foam, but I'm not sure whether it can also be temperature or fire resistant. Looks a lot like those using ordinary expanding foam haven't had deformation issues though, doesn't it. Given the collective experience, I'd be inclined to go with the fire rated stuff whilst bearing in mind the  expansion when pulling the trigger.

Expansion is not a problem provided you don't overfill, I did mine in 2 parts leaving room at the open end for expansion and then cut back any excess.

Fire rated stuff is still OK after 2 years of use. To be honest I can put my hand on a single skin chimney when my stove is in use so its not that hot.

Obviously if I accidently  left the stove open and  it ran away that would be a different temperature and I've only done that once in 30 years.

 

[waterworks]

I did notice a reduction in draw on my stove during the few minus zero nights we just had, would I bother to insulated the chimney for a few days every other year? I suspect it's not worth the effort. 

[LadyG]

1 hour ago, waterworks said:

I did notice a reduction invisited my  draw on my stove during the few minus zero nights we just had, would I bother to insulated the chimney for a few days every other year? I suspect it's not worth the effort. 

I just kept feeding mine with nice dry coals  it was really a grand episode,

No birds visited my bow deck which is well provisioned, I have five Canada type geese hanging around , skating.

Edited December 22, 2022 by LadyG

[blackrose]

About 15 years ago I saw a metre long length of lengthways slit 6" dia glass wool pipe insulator sitting in a skip, so I took it and as an experiment put it around my big 6" single skin vitreous enamel chimney over a few cold clear nights, with some tape holding it around the chimney.

 

It made no perceptible difference to the stove draw or anything else and convinced me not to bother buying a double wall insulated chimney. 

 

Different stove/flue/chimney combinations will behave differently of course, but my experiment tallied with what one flue supplier told me over the phone, which was that insulated chimneys worked best and were essential over longer lengths such as up the side of a building, but over such a short distance as a boat chimney an insulated flue was unlikely to make much difference to stove draw because the flue gases would have no time to cool, condense and slow down anyway.