Fire-rated foam around twin wall flue?

[geraldthehamster]

A twin wall flue will penetrate a flat roof about 1.7m above a wood burning stove.

The warm roof comprises OSB - Celotex - OSB - EPDM.

Can I fill the 60mm gap that must be left around the twin wall flue, with fire rated foam, for example this?

https://www.firesealsdirect.co.uk/fire-rated-expanding-foam.html

I have heard of this being done, but I don't know what kind of foam was used, or whether it's acceptable.

Cheers
Richard

 

[JohnD]

that one appears to be a pink intumescent foam which, if your house catches fire, chars and swells to prevent passage of flame or smoke, e.g. into the next room or through a ceiling or doorframe. I use it round my fire door linings, the instructions say the gap should be quite small, I forget the size.

It's not the same as the stuff needed to seal a hot flue.

I remember there are special high-temperature sealants, my BiL asked on here a while ago (he has a vintage range with a steel flue in a "historic" home)

edit

here
https://www.diynot.com/diy/threads/sealing-a-flue-slate.179025/#post-1226091

 

[geraldthehamster]

Thanks. Yes, there are sealants and I've used them, but not to fill a 60mm gap.

Flue suppliers list foams, but don't say what their application should be in connection with a stove installation

https://www.directflues.co.uk/index...category_id=47&option=com_virtuemart&Itemid=7

 

[JohnD]

maybe you should use some kind of collar to take up the gap, then you'll only need to seal the joint.

 

[tomfe]

Fill it with rockwool.

 

[^woody^]

You can use an intumescent foam. Strictly, manufacturers don't recommend these foams for gaps greater then 30-40mm nowadays, but that's for compartmentation purposes. In this situation that would not be an issue.

But as Tom says, rockwool would be better - and if you fill most of the void with it, that will help with keeping potential heat transfer at bay to near-by timber.

Ether way, the problem to mask the gap at ceiling height will remain, but if you ar fitting a cover plate or collar then that will deal with that and you could then use an intumescent mastic between the plate and ceiling.

 

[geraldthehamster]

Thanks chaps. So an intumescent foam counts as non-combustible for regs purposes? I don't suppose it will ever get hot enough to become tumescent.

What about the one that Direct Flues are selling among their sealants? I'm struggling to think what other use they could be selling it for, but it's not intumescent.

https://www.directflues.co.uk/index...category_id=47&option=com_virtuemart&Itemid=7

There's no issue with masking the gap from view, as the whole thing is having a dummy chimney breast built around it.

Yes, I'm considering rockwool, with a plate below to hold it in. Hardie, perhaps. Foam would have the advantage of helping hold everything rigid.

Cheers
Richard

 

[lostinthelight]

maybe you should use some kind of collar to take up the gap, then you'll only need to seal the joint.

I am with you on this one as collars are sold for this purpose. I have always left an air gap around the outer skin to dissipate heat evenly so there is no localised hot spot... just my view!
litl

 

[rssteve]

As it's not a ventilated roof space you should be using a ventilated firestop to stop heat build up around the flue. The only company I know which allow the gap to be filled when using solid flue is dinak and they do a special setup using the dinsulator.

 

[geraldthehamster]

Where is the ventilated firestop going to ventilate to?
https://www.directflues.co.uk/index...category_id=31&option=com_virtuemart&Itemid=7

 

[JohnD]

hot enough to become tumescent.

about 100C IIRC

In normal use it is stiff, but slightly spongy and slightly flexible (does not crack or come loose when doors are slammed).

 

[geraldthehamster]

I've found this on a supplier's website, in relation to passing a flue through a wall. If they're right, it indicates that it's OK for fire rated foam to be in contact with a (twin wall) flue.

"The Wall Sleeve is used to protect the chimney in the wall passage when a 90deg flue system passes to the outside wall. The gap between the flue and the wall sleeve can be filled with fire rated expanding foam."

https://www.chimneyandflue.co.uk/Straight-Wall-Sleeve-5"