Hi - your guy sounds like an idiot. The thickness of your chimney is irrelevant. Insulation will a) reduce heat loss and b) ensure that more even temps are maintained, thus prolonging the life of your flue. Is he HETAS registered? Check out their site, plus the Solid Fuel Association.

Don't cheapskate.

Why on earth would you go to the trouble to "suck it and see" and then maybe end up lining the chimney later (when it's half knackered)? If you've got labour in to do the rest of the work, why not just get it all done at once? The liner materials are fairly minor compared to the labour. The heat from a stove can easily crack and knacker an older unlined chimney. A liner will prevent this. Like I said - why skimp?

I'm not trying to cut corners. It was the installer that told me it wasn't necessary, especially as the chimney is internal. He is insisting on an air vent though which tends to suggest he follows guidelines.
He's done dozens of installs and has never had an issue with not insulating.

I'm actually a bit confused about this. If I insulate does it not mean that more of the heat is going straight out the chimney. As the chimney is internal, will I not get the benefit from whatever heat goes into/through the chimney stack and back into the house?

I could understand a case for insulating the bottom 200mm or so to stop heat loss into the chimney from the register plate. Would this make sense?

I meant to point out again that the chimney is quite broad. It's also 8m in length so filling with vermiculite would take a fair few bags (maybe around 1500L). Not exactly a cost effective or ecofriendly option and not great if I ever need to replace the flue.

You're right about problems if you need to reline the flue. We had a liner installed for a SF Rayburn over 15 years ago which was insulated with vermiculite. Started having problems and it turned out the builders had put the liner in upside down with no cap so rain and wet soot was leaking into the vermiculite. When it was replaced, apart from the huge quantity of loose vermiculite the stuff that had got wet turned into a cement like substance and the installers had a terrible job creating enough of a passageway to get the new liner in. Needless to say I haven't had the other three liners insulated!

Perhaps this is the answer.. see "Rockwool sleeves" on this page..
http://www.stovesonline.co.uk/wood_burning_stoves/Flexible-Flue-Liners.html

Quote: Insulating flexible flue liners in wide chimneys has always been a problem because of the large amount of leca needed to backfill the chimney. Now we carry rockwool sleeves, by Rocklap, that just clip around the liner as it goes down the chimney. The rockwool sleeves are strong lengths of pre-formed insulation with a one piece, factory applied foil facing with an integral self-adhesive lap.

I've no idea if it's any good. I'd be tempted to put more tape around the outside or perhap some stainless steel wire ties.

We have 4 woodstoves and a woodfired cooker. The cooker and one stove are (seperately!) flued via twinwall insulated flue systems (Selkirk/Negarra). One stove is flued via a 'Thermocrete' insulated pumice concrete chimney. One stove is flued via a register plate into a clay liner internal chimney. The final stove is flued via a flexible SS liner that is insulated with Rockwool and vermiculite.

The SS liner with vermiculite insulation is the best performer - far less soot/deposits and better draw.

As a proportion of the overall cost of lining flues, the insulation aspect, although not negligable, is money well spent. As per the many good posts above, the insulated flue will...

improve draw, speed up the lighting process and help achieve the burn phase much quicker, saving you time and wood.
prolong the life of your stove and the stainless steel liner.
reduce heat losses from the building 24 hours a day whether the stove is lit or not.
In extreme circumstances it could save your life as your noxious gases are less likely to fall back down the flue after the burn phases.

As for problems getting back to the liner in future..start the pour with a mix of vermiculite and lime or cement to create a plug at the bottom. dry vermiculite, then another mix at the top to plug and bond before the pots go on..

Our (Mid Wales) chimney breasts are 10 metres, one with originally 5 flues, the other with 4. The vermiculite at just over a tenner per 100 litre bag from any builders merchants amounts to around 500 quid on this big building with 4 stoves, one rayburn and a log boiler. Most chimney breasts would not take more than 1000 litres=100 pound or so..

Mixed bunch of advice there fidhw! And neelpeel's installer may have been an eejut fresh out of college, or a guy with years of experience who used his eyes and accumulated knowledge. In principle we line every time, but I wouldn't call his advice dud unless I'd had chance to see the chimney for myself. He may have a point.... horses for courses.

A few observations: On occasions, as somebody pointed out above, a warm, dry (pref lime mortar constructed) chimney with a correctly used stove, sans lining, will deteriorate at a negligible rate (all chimneys deteriorate, btw, even volcanic pumice, it's just a matter of time and chemistry...), especially a warm, internal chimney where condensate - and hence sulphur attack - is less likely.

If you are advised to line - the most likely scenario - pumice is viewed as a superior material in many quarters, backfilled with weak leca (or vermiculite)/opc mix. DO beware damp ingress after any type of lining. A flue with a 'static' water ingress problem that wasn't noticeable previously due to the large surface area capable of keeping damp at bay, aided by a large updraft,, may suddenly be a real dripping problem post lining, water tracking down the lining through the excellent water-retentive properties of vermiculite. No surprise that nurserymen use it for that reason!

Clay not so good and prone to thermal shock. Cast-in-situ (perlite) very good in some circumstances, especially where structure is suspect. Stainless (flexi) is a variable commodity, both in material quality (we have taken many dozen 304 and 316 flexis out, tho the latest 904 is apparently better...time will tell), installation technique (big variable here) and in possible damage to the windings (look up chimney after lining!). Think ten years longevity if going the flexi route - you may get twenty, but unlikely much longer, no matter what the guarantee says....