Hi all

I've got a wood burner installed in a builder's opening. Single wall flue into a flexible chimney liner. The thin steel closure plate is higher than the lintel in front - i.e. you can't see it when stood in front of the stove. I therefore often wonder how much heat gets "trapped" in that pocket of space behind the lintel, and subsequently escapes through the closure plate into the chimney void.

So I've been thinking about insulating above the closure plate using mineral wool and wondered what people think to the idea? I've not come across much info of people doing that. Seems sensible to me but perhaps I'm overlooking something.

I did mention it to the chimney sweep when he was last here and he was against the idea. Not sure how much I trusted his opinions though (on that and other things!). One of his concerns was the mineral wool combusting. I told him that in my limited knowledge it's not combustible but he wasn't having any of it!

Any thoughts?

Many thanks

Providing it doesn’t have any recycled plastics content

Your sweep is right on at least two counts..

Almost all mineral wool on the market contains all sorts of stuff you would not expect. Some of it combustible, others just break down into nasties when they are heated. If you do go down this route you need the Rockwool product used in the marine environment, for some reason that stuff is ok, but it costs a fortune..
Check data sheets to be sure.

I know this goes against the grain of what most people on this forum are aiming for, but insulation in the chimney cavity, ie outside the liner, but inside the breast, is liable to cause serious problems with damp down the line. Unless you happen to live in Spain or somewhere equally dry where water ingress into chimneys is unlikely.

We never seal closure plates fully and also make sure there is an airflow path outside the liner out of the pot. Permanent trickle ventilation through the entire stack.

As Phil says above, burn good seasoned wood and burn it hot and your liner will last well beyond its warranty. Liners absolutely do no need insulated to last a long time, they need to be installed properly with attention to detail. A good stove well installed with a good user will last longer than you think. Seal it up and introduce some moisture and it might not last til next Christmas..

An annual sweep by a qualified person is often a requirement of your household insurance.. so the advice from your sweep is good, he is making his annual sweep easier.

Detailing of the liner at the top of the stack is more important than anything else. We regularly replace other installers liners that have literally been thrown in through lack of care, knowledge, and often a safe working at heights setup. Record so far is a liner that lasted less than one year.

We used mineral wool industrial pipe insulation sleeves that slide over the liner- aside from dust still look the same as when they were fitted in 2006