Insulating chimneys with 2 flues

We have a number of these but only use 1 fireplace.

Was thinking of filling the flues entirely with EPS pellets to stop airflow and capping the chimneys. Would this work?

Regarding the fireplace we use, the chimney stack will have one dormant flue and one active. Could there by heat transfer between the flues which could melt the EPS?

Many thanks,

B.

I would really welcome some advice on this if someone could help.......

Yes there would be heat transfer.
You need 200mm of blockwork between EPS and a flue- which you are unlikely to have. (It is included on the standard Agrement Cert for this).
If the flue was old, it is furthermore quite likely that there are holes and cracks in it - meaning that whatever thickness there is, is even less.

Rockwool would be a non-combustible alternative.

However I wouldn't recommend filling a chimney - the chimney extends beyond the roof, and relies on ventilation/heat to keep it dry.
Cap the chimney in lead, provide a few holes in the lead (punched through from below - so that the punched hole sticks up like a small volcano). Stuff a black plastic bag with newspaper up the chimney at the fireplace. There should be 1 or 2 lengths of pipe stuck through the plastic bag as well - to allow air from the room get into the chimney.

Do not use EPS.

From

http://en.wikipedia.org/wiki/Polystyrene

"Polystyrene foams are produced using blowing agents that form bubbles and expand the foam. In expanded polystyrene, these are usually hydrocarbons such as pentane, which may pose a flammability hazard in manufacturing or storage of newly manufactured material..."

Most chimneys are constructed minimally in newer houses and are filled around with material of variable quality in some older houses.

Heat transfer is likely to occur, with the possibility of a serious chimney fire occurring.

Consider removing the chimney entirely, taking engineer's advice beforehand.

ONQ.

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All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.

Thanks Superman and ONQ.

Removing the chimneys wouldnt be an option as there are too many of them....

If the chimney was capped and allowed to dry out, I am surprised that ventilation would still be required. Why is this and where could the moisture get in, particularly if it was filled with rockwool as suggested?

Am I right in considering that the loss of heat, through the chimney wall as opposed to up the fireplace, would be similar to an uninsulated solid brick external wall?

If we put in a stove with a flue up the chimney, would this reduce the risk of heat transfer/fire?

Thanks for your help.

B.

Moisture is in the air at any temperature above zero degrees.
A family produces on average 14 litres of water vapour a day.

Rockwool is not proof against moisture transference or build up.
Vapour check the warm side and vent the cold side is the method.
Without a fire venting up through it a chimney is a massive cold bridge.

Yes, its like an uninsulated wall - its intended to vent heat, not conserve it.

No, the risk or combustion of the plastic pellets would remain and rate of heat transfer would remain the same.
The amount of heat to transfer might reduce depending on the rating of the appliance.
The greatest risk will probably be closest to the appliance.

You're welcome.

ONQ.

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All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.

Thanks ONQ.

We were thinking of externally insulating the house but I think now that this would be insufficient as the extenal insulation would be ineffective around the 3 chimneys stacks which are on the external wall - if each stack is 4-5 feet in width, this adds up to a considerable area.

Presume external insulation and drylining the walls on the 6 rooms with chimneys would be daft.....

Hi buyingabroad,

You're very welcome, but don't be disheartened.
Everything you do to insulate the house will be a step in the right direction.

You must try not to create cold spots or ignore cold bridges or places where moisture laden air might gather.
If the stacks are externally insulated this unusual measure might provide a very useful upgrade to the house as a whole.

Personally I would be more inclined to seal up the chimney from the room side and ventilate the room another way.
Thus warm moist air is checked from entering, but any air that migrates into the chimney void gets vented away.

I would still ventilate the outlet, but I would seal the ope to restrict warm moist air getting into the chimney flue.
I would also try to ensure that my attic / roof insulation continued to meet or to overlap with the flue insulation.

If this is a period dwelling especially if it is listed, you will need to take specialist advice on how to approach the project
Its very important that you work within the limitations of the technology of the building or to know how to go beyond them.

Even if the dwelling is is neither period nor listed, you should appoint someone to look at the issues and work out the details.
This person should be appointed to conduct limited inspections and make a photographic record at all stages of the new work.

Empirical studies of both new and retro-fit insulation show substantial differences between well-done and poorly fitted work.
Tight fitting of insulation panel to each other with all gaps sealed appears to be very important to the over all performance.


ONQ.

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All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.

The moisture comes in from the exposed blockwork externally and vertically down from the top of the chimney.
What can occur in old chimneys which are not used is that the moisture comes down the flue and brings tar deposits to the chimney breast in the rooms below - it is rather difficult to deal with as a problem (requiring the removal chimney and all contaminated material).

Hi Superman,

I was assuming that the perforated cap you suggested will keep out the moisture from the top and that the external insulation would protect the brickwork.
In other words, external sources are removed, leaving the normal migration routes routes through materials and jointing from warm rooms to interstitial spaces.
In relation to your comments on tar, which I confess I knew little about, I did a little research and discovered this Guide to Curing Chimney Problems which states -

A.10. PARTIALLY BLOCKED CHIMNEY POT
When smoke flows through a flue, the particles of soot are particularly attracted to cold surfaces such as the inside of a chimney pot. Usually most of the soot is removed by the sweep’s brush, but if the smoke is high in tar content the deposits may resist everything but a hammer and chisel. In districts where housecoal with a high tar content is commonly used, chimney pots should be as short as possible (but not less than 150mm) and well protected by steep flaunching; it is also advisable for the pot to have parallel sides see A.9 with an outlet not less than 200mm (8in) diameter or larger if the flue is larger, preferably salt glazed internally.

TEST A sweep’s brush usually reveals the obstructions.

REMEDY If the trouble is recurrent, replace the pot with a shorter one of a larger size, or use smokeless fuel. The ‘dovecote’ top shown in Fig.10 is sometimes used to protect large flue outlets against wind and rain; it is not, however, intended to replace the recommended square or circular termination, which should be built into the brickwork so that only 50mm (2in) or so projects above the stack. The combined area of the four openings should equal at least twice the area of the flue.

Click to expand...

ONQ.

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All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.

onq said:

Hi Superman,

I was assuming that the perforated cap you suggested will keep out the moisture from the top and that the external insulation would protect the brickwork.

Click to expand...

It will depend on how good the dpc on the chimney is - in any case the chimney is an exposed large block which will allow a certain amount of water to penterate the flue (from the vertical sides of the chimney as well as the remaining top surface - and from there come down the chimney.
This is why even when a chimney is blocked up (and not fully removed and/or roofed over), ventilation needs to be provided to flue.

onq said:

In relation to your comments on tar, which I confess I knew little about, I did a little research and discovered this

Click to expand...

The soot deposits over time in a chimney where no fires are used (which dries out the flue) will become wet from moisture coming down the chimney. That moisture migrates to the chimney breast - due to the higher temperature in the rooms. This brings the soot and tar with it - contaminating the breast.
It is an issue that arises with 19th century and earlier houses - porous breasts in brick and a large number of fireplaces no longer used.

Thanks guys. I will take a few photos and provide a link to them to show you what I am on about.

Superman said:

It will depend on how good the dpc on the chimney is - in any case the chimney is an exposed large block which will allow a certain amount of water to penterate the flue (from the vertical sides of the chimney as well as the remaining top surface - and from there come down the chimney.
This is why even when a chimney is blocked up (and not fully removed and/or roofed over), ventilation needs to be provided to flue.

Click to expand...

Accepted on the ventilation, but the point of external insulation is that the walls are no longer exposed to the elements, unlike your description of the chimney above.

They are wrapped in the external insulation and this in turn is clad or in myst Irish solutions I have seen, covered in mesh and rendered over with a "water proof" render.

(I know, I know )

I agree that the difficulty arises in the chimney/roof interface and the continuation in section of the line of insulation, depending on how the attic is insulated

- flat quilt
- "follow the line of the roof"
- part follow the roof and part vertical and horizontal

Thank you for clarifying the Tar issue, Superman.


ONQ.

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All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.

onq said:

They are wrapped in the external insulation and this in turn is clad or in myst Irish solutions I have seen, covered in mesh and rendered over with a "water proof" render.

Click to expand...

I would think that the external insulation would only rise to soffit level and therefore the chimney block above would be permeable

Hi buyingabroad,

I'm not sure what you mean by soffit - I presume you mean below the roof slates.

If you leave the projection of the chimney stack exposed above the level of the roof slates, then Superman's advices in posts No. 3 and No. 9 above apply - moisture can enter through the chimney.

My advices concern a chimney which is totally externally insulated and rendered out side of this to make it weather resistant.

A chimney that forms part of a gable has an entire wall that forms part of the flue enclosure and all of this would be insulated.

I'm not sure that this has ever been done for a masonry chimney stack.


ONQ.

[broken link removed]

All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.

I'm back again on this one.

How does filling the chimney with blown in insulation compare with cavity wall insulation? Moisture would also penetrate the cavity wall in much the same way as is being described for the exposed chimney stack. If it works for cavity then I feel it should work for the chimney also??

there is a blown mineral wool product on the market, why not contact the manufacturers (not an installer) and ask about its use in such a location, explain you'll be properly capping it at both ends etc.

[broken link removed]

I'm still not convinced you'll be able to seal the chimney sufficiently to not have moisture penetration

lowCO2design said:

there is a blown mineral wool product on the market, why not contact the manufacturers (not an installer) and ask about its use in such a location, explain you'll be properly capping it at both ends etc.

[broken link removed]

I'm still not convinced you'll be able to seal the chimney sufficiently to not have moisture penetration

Click to expand...

Thanks, good suggestion.