Is vermiculite poured in neat?

[epsilonGreedy]

I have a noddy question about filling a chimney with vermiculite. The chimney reaches 7m high and will contain standard concrete/pumice flue section joined with fire cement.

 

Should the vermiculite thermal insulation be be poured into the flue chimney cavity loose straight from the bag or should it be mixed with cement and water so that it sets in the chimney and helps brace the flue?

 

I understand loose fill is correct for metal chimney liners but a very knowledgeable lady at Clearview stoves in Ludlow looked surprised when she discovered I was intending the fill my chimney with loose vermiculite.

[Declan52]

Mix it 6/1 with cement and just enough water to get it damp so it will set.

[Vijay]

but won't that make the chimney unusable in the future?

[epsilonGreedy]

Just now, Declan52 said:

Mix it 6/1 with cement and just enough water to get it damp so it will set.

 

 

Thanks. Is it usual to churn the mix in a conventional cement mixer?

 

I have been told the flue cavity should be filled progressively as the chimney goes up, so I am assuming this involves mixing small quantities just enough to fill a vertical meter at a time.

[PeterW]

Check the Dunbrik website but when I did the last one we used 600mm sections and glued them together with the adhesive (lip down) and then as we went we used an 8:1 mix of vermiculite and OPC mixed in a barrow with a splash of water. 
 

The trick is to ensure your throat / support is built first and has properly gone off, put the next length of liner and then brace that, fill with the insulation mix and leave to set for a couple of days (a good job for a Friday) then you are inserting liners into a solid base and they don’t move. 

[epsilonGreedy]

2 minutes ago, Vijay said:

but won't that make the chimney unusable in the future?

 

 

I hope not!

 

I should clarify the vermiculite is poured into the cavity between the masonry box that forms the inner chimney and the flue section. The inner flue remains open.

 

I think the vermiculite helps protect the chimney from high temperatures of the flue and it also allows the flue to get up to operating temperature quickly which promotes a healthy air draw up the chimney from the fire place. In addition to that I now appreciate the hard set vermiculite fill also helps physically brace the flue within the larger chimney structure.

[epsilonGreedy]

2 minutes ago, PeterW said:

Check the Dunbrik website but when I did the last one we used 600mm sections and glued them together with the adhesive (lip down) and then as we went we used an 8:1 mix of vermiculite and OPC mixed in a barrow with a splash of water. 
 

The trick is to ensure your throat / support is built first and has properly gone off, put the next length of liner and then brace that, fill with the insulation mix and leave to set for a couple of days (a good job for a Friday) then you are inserting liners into a solid base and they don’t move. 

 

 

Ok. My local BM supplied a base lintel concrete plate from the Torterra Red Bank range which simplifies starting the flue I hope. One downside is this positions the flue a bit too far out from the inner block wall hence we will need some bends to curve the flue in at ceiling joist level otherwise the chimney breast intrusion into the bedroom above is excessive.

 

https://www.forterra.co.uk/chimneys-roofing-flue-systems/chimney-air-brick-ducts-flue-product-selector?cat=175mm-internal-diameter-circular-flues

 

After visiting ClearView and handling their preferred flue parts made by an outfit with a German sounding name starting with "S" I am concerned the pumice content of the Forterra parts is quite low.

[epsilonGreedy]

I found this installation diagram on the ClearView web site which suggested a 12 to 1 mix. Not sure what Leca is, a brand name for vermiculite?

 

Quote

Insulation: 12:1 Leca and cement mix Min. thickness 50mm

 

https://www.clearviewstoves.com/useruploads/downloads/Pg10. 07 drawing Pumice Liners.pdf

 

[MikeGrahamT21]

13 minutes ago, epsilonGreedy said:

I found this installation diagram on the ClearView web site which suggested a 12 to 1 mix. Not sure what Leca is, a brand name for vermiculite?

 

 

https://www.clearviewstoves.com/useruploads/downloads/Pg10. 07 drawing Pumice Liners.pdf

 

Leca, in gardening terms at least anyway, tends to be a form of clay, which usually comes in pellets, tends to be used in landscaping to soak excess water up

[PeterW]

33 minutes ago, epsilonGreedy said:

 

Ok. My local BM supplied a base lintel concrete plate from the Torterra Red Bank range which simplifies starting the flue I hope. One downside is this positions the flue a bit too far out from the inner block wall hence we will need some bends to curve the flue in at ceiling joist level otherwise the chimney breast intrusion into the bedroom above is excessive.

 

https://www.forterra.co.uk/chimneys-roofing-flue-systems/chimney-air-brick-ducts-flue-product-selector?cat=175mm-internal-diameter-circular-flues

 

After visiting ClearView and handling their preferred flue parts made by an outfit with a German sounding name starting with "S" I am concerned the pumice content of the Forterra parts is quite low.

if you’ve not already purchased, go for a 200mm internal diameter flue. 
 

Bends are fine - use 45 or 22.5 degree not 90, and make them nice and slow and no more than two 45 in the whole of the run. 
 

I wouldn’t use Leca- it’s an expensive option for this sort of thing and vermiculite works fine. 

[PeterW]

And if it is an RM78, just inset it jnto your block work courses so it will move back by 100mm then build with concrete bricks to make back up to a full block. 

[epsilonGreedy]

48 minutes ago, PeterW said:

If you’ve not already purchased, go for a 200mm internal diameter flue. 
 

 

Oh too late, I had previously phoned my now regular BM and they quoted for their standard 175mm off the shelf flue parts. I actually have two working chimneys to build and have enough flue sections onsite to build one. I could revise my plans and fit a larger 8" flue in the larger fireplace if there is a good reason to do so?

 

The smaller fireplace is being built to accommodate a 5kw wood burner stove with a 6" flue outlet. The other fireplace will have a larger 8kw stove ( yes I know far too much for a new build, it is an aesthetic choice for the room size) and according to the Clearview brochure this also has a 6" outlet. 

 

Your post prompted me to review chimney flue sizing and this following site suggests matching chimney flue size to stove flue size or the chimney flue can step up a bit in size but not too much. The same site also goes on to say the fireplace opening should be x10 to x12 the flue size though I do not anticipate ever operating an open fireplace.

 

Quote

 

Venting a Masonry Fireplace

When a home is built with a masonry fireplace, there is a huge range of possible sizes. It’s important to have a builder who knows how a fireplace works and makes sure the fireplace opening and the flue size are the right match to create excellent venting. In general, the flue can be 1/10th to 1/12th as large as the opening.”

 

 

https://www.mychimney.com/blog/chimney-fireplace-sizing/

[Jeremy Harris]

1 hour ago, epsilonGreedy said:

I found this installation diagram on the ClearView web site which suggested a 12 to 1 mix. Not sure what Leca is, a brand name for vermiculite?

 

 

https://www.clearviewstoves.com/useruploads/downloads/Pg10. 07 drawing Pumice Liners.pdf

 

 

Leca can be used as a loose pour insulation medium, that is pretty heat resistant.  It's fired clay beads: https://www.leca.co.uk/main-navigation/domestic/chimney-flue-fireplace-backfill/

 

 

[epsilonGreedy]

1 hour ago, PeterW said:

And if it is an RM78, just inset it jnto your block work courses so it will move back by 100mm then build with concrete bricks to make back up to a full block. 

 

 

Yes an RM78. I just looked at the product spec 78Kg... no wonder I struggled to carry that across the site.

 

I must admit getting the design of the chimney right is taxing me and my lead brickie.

 

I have already demolished the first 2m high chimney breast because it intruded too far into the room, my error. A timely technical chat with Clearview Stoves clarified that fitting our favourite stove design with its 14kw output was absolutely nuts. Apparently the quoted variable ranges of wood burning stoves should not be relied on, they achieve their optimum cleanest burn at maximum rated output.

 

Now we are designing for a smaller stove and ClearView confirmed the external air draw kit does not require additional fireplace depth, we have been able to knock 200mm off the chimney breast depth. After viewing the many working stove displays at ClearView's fabulous showroom in Ludlow we also discovered that a wood burning stove looks quite attractive when set 50mm forward of the chimney breast face, so that is another 50mm clipped off the chimney breast.

 

The main design challenge we have is that my external chimneys, which stand 1.9m proud of the hipped roof, have an old fashioned wide look. We have to settle on the external chimney dimensions now because it simplifies the chimney build if the flue base dimensions continue straight up through the roof line. I have dry built a small demo chimney in facing bricks and 5 bricks by 3 seems about right.

[Vijay]

4 hours ago, epsilonGreedy said:

 

I hope not!

 

I should clarify the vermiculite is poured into the cavity between the masonry box that forms the inner chimney and the flue section. The inner flue remains open.

 

I think the vermiculite helps protect the chimney from high temperatures of the flue and it also allows the flue to get up to operating temperature quickly which promotes a healthy air draw up the chimney from the fire place. In addition to that I now appreciate the hard set vermiculite fill also helps physically brace the flue within the larger chimney structure.

 

My bad, been a long week and didn't read your post properly, I understood that you were filling in an empty chimney. Sorry.

[epsilonGreedy]

44 minutes ago, Vijay said:

My bad, been a long week and didn't read your post properly, I understood that you were filling in an empty chimney. Sorry.

 

 

Given some of the strong views on his subject expressed here I have at times wondered if I should just plonk 3 false chimneys on the completed house. However earlier this week we drove nearly 4 hours across country to visit a specialist stove showroom and when sitting in the kitchen exhibition room while one of the staff sketched out a design I could hear the background sound of the stove drawing air and was convinced again I want a working wood burner.

 

In practice I doubt it will be lit more than 30 days year. 

[ProDave]

Just to add my experience.  Our previous house the shell was built buy a builder and the dry poured vermiculste into the gap between the liner and the masonry.

 

Present house does not have a chimney but instead twin wall flue pipe.

 

I hope you are fitting a room sealed stove with ducted combustion air in from outside via a pipe, not from the room?

[Jeremy Harris]

2 minutes ago, ProDave said:

I hope you are fitting a room sealed stove with ducted combustion air in from outside via a pipe, not from the room?

 

Be daft not to.  Both the house we had in Scotland, that was built around 1990, and our last house, built around 1982, had an external air feed to the fireplace as standard, just to reduce heat loss.  Should be standard, I'd have thought, on any modern installation.

[ProDave]

2 minutes ago, Jeremy Harris said:

 

Be daft not to.  Both the house we had in Scotland, that was built around 1990, and our last house, built around 1982, had an external air feed to the fireplace as standard, just to reduce heat loss.  Should be standard, I'd have thought, on any modern installation.

I don't just mean an air feed to the fireplace, I mean a pipe to take that to the stove.

 

My 1930-s house was terrible. NO provision for air to the open fireplace.  On the few occasions that I used the open fire when I first moved in, I found the easiest route that air could take to get IN to the house to replace combustion air that went up the chimney, was by drawing it DOWN the upstairs chimney and fireplace, then down the stairs and under the living room door.  BIG problem with that, on a still day it drew SMOKE down the upstairs chimney.

 

 

[epsilonGreedy]

7 minutes ago, ProDave said:

I hope you are fitting a room sealed stove with ducted combustion air in from outside via a pipe, not from the room?

 

 

Yup all part of the plan. The sitting room wood burner will draw air horizontally through the cavity wall because there is a subwall mid way down the room that bisects the fireplace where the stove will sit hence through the floor is not viable. Clearview manufacture custom air ducts for their stoves to be supplied externally.

 

The Snug stove is more problematic because the fireplace backs up to the rear hallway. I hope when I scrape back the builders crud on the floor I will find that the beam spacing permits me to core drill a 75mm hole down into the suspended floor void which will provide a fresh air supply.

 

I thought the smaller 5kw stove could draw air from the room without an air brick but apparently that is not allowed in a new build. 

[epsilonGreedy]

16 minutes ago, ProDave said:

Just to add my experience.  Our previous house the shell was built buy a builder and the dry poured vermiculste into the gap between the liner and the masonry.

 

 

Is it standard practice not to run floor insulation under the hearth?

 

We have allowed for a 2" to 3" thick hearth plus say 65mm of floor flow screed across the whole room, even so I do not want to worry about insulation sheets below the fireplace melting. 

[Jeremy Harris]

21 minutes ago, ProDave said:

I don't just mean an air feed to the fireplace, I mean a pipe to take that to the stove.

 

My 1930-s house was terrible. NO provision for air to the open fireplace.  On the few occasions that I used the open fire when I first moved in, I found the easiest route that air could take to get IN to the house to replace combustion air that went up the chimney, was by drawing it DOWN the upstairs chimney and fireplace, then down the stairs and under the living room door.  BIG problem with that, on a still day it drew SMOKE down the upstairs chimney.

 

 

 

 

This is what we had in both houses, although I sealed it off in the last house.  Both houses were originally fitted with baxi back burners, with a room-sealed air intake below floor level, via a duct leading to a big airbrick outside.

[ProDave]

29 minutes ago, epsilonGreedy said:

 

Is it standard practice not to run floor insulation under the hearth?

 

We have allowed for a 2" to 3" thick hearth plus say 65mm of floor flow screed across the whole room, even so I do not want to worry about insulation sheets below the fireplace melting. 

My stove sits on a 30mm hearth on top of the insulated floor.  But it is a stove certified by the manufacturer to not exceed 100 degrees at the floor so can actually sit on just a 12mm superimposed hearth.

[ProDave]

37 minutes ago, epsilonGreedy said:

I thought the smaller 5kw stove could draw air from the room without an air brick but apparently that is not allowed in a new build. 

My stove is rated at 4.5kW so it could legally not have any air intake, but to do that would be plain stupid as it would massively imbalance the mvhr when it was lit.

[epsilonGreedy]

5 minutes ago, ProDave said:

My stove sits on a 30mm hearth on top of the insulated floor.  But it is a stove certified by the manufacturer to not exceed 100 degrees at the floor so can actually sit on just a 12mm superimposed hearth.

 

 

Ok good to hear this is an option otherwise a fireplace on a solid floor slab would form a large cold bridge.

Edited March 13, 2020 by epsilonGreedy
Removed cross post Q about kWs