Wood stove: Insulating above the closure plate, is it common? Good idea?

[Oxbow16]

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

 

 

 

Edited December 14, 2023 by Oxbow16

[JohnMo]

Mineral wool is not combustible, not so certain of glass wool.  Rockwool is certainly not combustible and is used in twin wall flues.

 

I would be less concerned about not how much heat is trapped, but more concerned about how much heat escapes as it's a huge thermal bridge if uninsulated.

 

We used a twin wall flue, but used rockwool around it in the roof to keep the insulation at the same value as the rest of the roof.

[bassanclan]

Usually insulate around flues etc with vermiculite

[Oxbow16]

8 minutes ago, JohnMo said:

I would be less concerned about not how much heat is trapped, but more concerned about how much heat escapes

 

Yes, sorry, that's what I meant.  I've amended the post to make it a bit clearer.  

 

 

7 minutes ago, bassanclan said:

Usually insulate around flues etc with vermiculite

 

The stove has only been in 2-3 years, so not looking to do anything too major.  I know heat will be lost from the flexible flue liner and accept that.  At this stage I'm just thinking about ways to stop the heat on the room side of the closure plate escaping into the chimney void.  A lot of heat naturally comes up from the stove top (and the single wall flue) and does a good job of heating the closure plate which gets pretty hot to the touch.  But I'm guessing with no insulation on the other side of the plate, the heat will transfer and be lost.  Would insulation on the other side of the plate help prevent some of that?  

 

Cheers

[Alan Ambrose]

I can't quite visualise your set-up - a drawing and/or photo would help.

 

Like JohnMo, I insulated around the twinwall in the roof with mineral wool - making sure that I was observing the distance to combustibles limits (e.g. for the roof membrane). BC signed off on it.

 

I don't know exactly how much of a thermal bridge I have left. I guess I could figure it out, but it strikes me that if you have a wood burner and a flue, you're prepared to take the hit on the thermal bridge.

[Oxbow16]

Thanks for the reply   

 

Here's a diagram as it would be from the side if the fireplace wall wasn't there.... 

 

I should add that from what I remember the space around the flexi flue liner is not all that much at all.  It's a narrow space.  Perhaps that's why vermiculite wasn't used?  Not sure...  

 

Cheers

 

Edited December 14, 2023 by Oxbow16

[Post and beam]

I have been thinking about just this issue recently. Amazing what you find on this forum. My proposed fireplace is huge, probably 2 square metres so the heat loss could also be huge i think.  I have been thinking concrete backer boards at the low side with PIR above. Surely the heat wont be enough for it to be an issue then, for the PIR.

 

This has just become a can of worms.

[Cpd]

I had a large open fireplace and used 150mm of kingspan around it and then 100mm of cement block on the face to protect the insulation, I then used high density rock wool above the closure plate ( my closure plate is actually 50mm concrete slab) it all works very well.