Retrofit non-combustible insulation sleeve on flue?

[MarcelHoldinga]

Does anyone have any advise on retrofitting non-combustible insulation around the flue for a wood burner?

It appears that when ours was installed, the installer left the 50mm gap to combustibles around the flue and didn't fill it any n/c insulation material.

The result is that there is a lot of cold air blowing through the roof void and into the "warm" loft space, which in turn leads to cold air blowing through the light fittings in the ceilings of rooms below the loft.

Everyone I've spoken to about this has said "Oh, that's just a necessary evil that you'll have to put up with", but having seen this thread, I'm wondering if anything can be done to seal up the 50mm gap around the sarking board and the layers below it, so we don't have the north-easterlies blowing nearly as strongly inside as they are outside (not to mention the heat loss)...

 

The flue goes straight up through the vaulted ceiling/roof - the angle of the roof is around 20 degrees.

 

Thanks in advance for any ideas!

[Cpd]

I have seen padded white blankets designed to wrap around flue pipes , something like this https://uk.rs-online.com/web/p/thermal-insulation/8405526/?cm_mmc=UK-PLA-DS3A-_-google-_-PLA_UK_EN_Engineering_Materials_%26_Industrial_Hardware_Whoop-_-Thermal+Insulation_Whoop-_-8405526&matchtype=&pla-343435779033&gclid=Cj0KCQjwjo2JBhCRARIsAFG667Vu0lvuZRt9HiXJb4CohprQ8GSTDgNCXSjozpNzVsJGBjuPQu-9pAUaAl4YEALw_wcB&gclsrc=aw.ds

 

just a quick internet grab but gives you a starting point. 

 

[ProDave]

I bought an insulated sleeve as part of my kit from flue-pipes.com but they don't seem to sell it any more, but at least this gives an idea what you are looking for.

 

It comes as 2 halves with a band to strap them together.  I fitted the sleeve to the pipe before putting it through the roof.

 

[saveasteading]

I have an uninsulated steel flue rising through an entirely masonry structure, which was closed off with mortar. The concern was therefore not to burn anything in the wardrobe it passed through. I made a box-out of plasterboard, using metal stud for framing, then surrounded it with 50mm rockwool batt then stuffed the box it hard full with more rockwool. 

As it easily accessible I have often checked the temperature of the boxing and it is never more than slightly warm, even though about 1.5m from the stove at full belt.. Therefore it seems that you could stuff the gap with rockwool or similar, as long as it is the fireproof grade.

To be sure, you could get the rockwool material that is sold as fire blanket. It has wire mesh on it, which may help or hinder.

 

Please research further though, as all circumstances are different.

 

There are fire-block expanding foams, but I do not think they are suitable. They are for on-off fires to prevent spread for a limited time, not for regular heat. 

[Iceverge]

On 23/08/2021 at 18:21, MarcelHoldinga said:

Does anyone have any advise on retrofitting non-combustible insulation around the flue for a wood burner?

It appears that when ours was installed, the installer left the 50mm gap to combustibles around the flue and didn't fill it any n/c insulation material.

The result is that there is a lot of cold air blowing through the roof void and into the "warm" loft space, which in turn leads to cold air blowing through the light fittings in the ceilings of rooms below the loft.

Everyone I've spoken to about this has said "Oh, that's just a necessary evil that you'll have to put up with", but having seen this thread, I'm wondering if anything can be done to seal up the 50mm gap around the sarking board and the layers below it, so we don't have the north-easterlies blowing nearly as strongly inside as they are outside (not to mention the heat loss)...

 

The flue goes straight up through the vaulted ceiling/roof - the angle of the roof is around 20 degrees.

 

Thanks in advance for any ideas!

 

It sounds primarily like an airtightness issue. I assume it is an insulated twin wall flue. If so you can get an airtight grommet to join to your airtightness layer. 

 

[MarcelHoldinga]

Thanks all,

 

We've not fired the stove yet, so I don't know how hot the flue gets, but it sounds like it should have had something fitted to prevent air blowing through the gap.

The flue is an insulated (I hope) twin wall, so an airtight grommet might be an option, though it's all fitted. I'll have a look what my chances are of stuffing the gaps around the flue with fireproof rockwool.

 

[Mr Punter]

I would just tightly stuff in some Rockwool.  You could use some chicken wire to hold it in place.  Where is the airtightness layer?  Is there a ceiling up there?

[jamieled]

I bought something similar to @ProDave though not quite as sophisticated. It is a rockwool sleeve with a foil outer. Found it in one of the online stove shops. This was the back end of last year.

[AliG]

You can get a flue collar that goes around the flue without removing it, something like this.

 

I would fill the hole with rockwool for insulation and tape around the collar with heat resistant aluminium tape for air tightness.

 

According to Google you can also just fill the hole with fire rated expanding foam, but this feels like a bit of a bodge.

 

https://www.glowing-embers.co.uk/ChimneyAndFlues/TwinWallInsulatedFlueSystems/ShieldmasterTwinWallFluePipe/8200mmShieldmasterTwinWallFluePipe/8200mm30-45degFinishingPlate-ForTwinWallFluePipe

 

[ProDave]

I wish I had known about the air tight grommet mentioned above, but would be stupidly difficult to retro fit.

[saveasteading]

Before committing to grommets, expanding foam etc, please check that it is suitable for a permanent closure for a very hot flue pipe. This will not be the same as a fire blocking material that only has to last for one or two hours. For example the intumescent filler tapes  in doors, when a flame is on it it expands and looks like a cigarette end. It fills the small space and stops flames going through but has no strength then falls apart like ash.

Similarly the red sealing grommets, as put around vents on industrial roofs, are for very hot fumes, but best check how hot.... a wood burner gets hotter than  a gas or oil burner, and also can have chimney fires.

[ProDave]

4 minutes ago, saveasteading said:

Before committing to grommets, expanding foam etc, please check that it is suitable for a permanent closure for a very hot flue pipe. This will not be the same as a fire blocking material that only has to last for one or two hours. For example the intumescent filler tapes  in doors, when a flame is on it it expands and looks like a cigarette end. It fills the small space and stops flames going through but has no strength then falls apart like ash.

Similarly the red sealing grommets, as put around vents on industrial roofs, are for very hot fumes, but best check how hot.... a wood burner gets hotter than  a gas or oil burner, and also can have chimney fires.

In my case the insulated sleeve that I used was made and supplied by the same manufacturer as the twin wall insulated flue, so one should be able to trust that a product sold for this use should be safe indefinitely for use in contact with that flue.

 

In practice, in my case, the twin wall flue at the roof exit, approximately 7 metres above the stove, gets barely tepid even when the stove has been burning for some hours,  but in other installations that may be very different, e.g. you might have a very much larger stove than mine.

[saveasteading]

41 minutes ago, ProDave said:

the insulated sleeve that I used

No doubts at all about that.

I was just worried that some of the tubes of stuff, and pipe sealing rings, readily available seems to be described as fireproof.  Have known builders think that can be used round flues when it is only intended as a fire emergency barrier.....2 hours life maximum.