Can I insulate under concrete hearth?

[Homerdoh]

I'm preparing the old fireplace for a wood burner and was just wondering if it would be ok / beneficial to fit 100m celotex under 100mm concrete slab?
It's only a small area as pics show, my only concern I guess is the heat going down through the slab and affecting the insulation!

Any advice appreciated

Untitled

• Homerdoh
• 18 Oct 2014
• 1

Untitled

• Homerdoh
• 18 Oct 2014
• 1

 

[DIYWell]

Writing as someone who had a stove put in a couple of years ago, nah, not worth it.

In the summer you won't notice a cold spot, and in the winter you'll have the stove going.

You'll need to build up the subhearth to a height that'll keep your hearth level. I'm guessing you're either planning to extend outwards to form a brick hearth, or fill the entire base with a hearth and back piece.

What I'd do it work out what kW output stove you need to heat the room. If it's a large room, you might want to bring an exrternal air duct through though the back hearth to the stove. Also knowing the kW you're aiming for then you can judge the physical size and how that will fit the chamber. Perhaps that'll need altering to take the stove too.

Easiest option is to get a stove installer in to get a rough quote and pick their brains about what prep you can do before they start.

 

[Homerdoh]

Ok thanks, I was planning on filling in the hole with concrete and using that as the hearth and then laying tiles to finish.
The bricks need another 2 courses laid to bring hearth above floor level and that will leave the hole at about 350mm deep, is this too deep for a single pour of concrete? This is why I thought adding some insulation first to make up some of the depth.

Thanks

 

[Norcon]

Heat rises. What ever goes down will come back up.

 

[oldbutnotdead]

Doing a similar job here (old fireplace sort of came apart in my hands, pics to follow).
Building regs say for a solid hearth nothing flammable underneath unless you've got at least 50mm air gap below the (minimum 125mm thick) solid hearth or the solid is at least 250mm thick.
Also (annoyingly) they say vents within the hearth area only on specialist advice (HETAS presumably- I was planning to duct through the hearth with flexi metal tube to avoid room draughts). Might take the other bit of advice in this thread and get a stove bod in to have a look.

 

[Homerdoh]

Fill it with concrete it is then!

I'm toying with the idea of losing my chimney stack from loft floor level upwards and just having a steel tube out through the roof, is this a good idea? How hot will the tube be and will it be ok just fixed to the loft floor joists and roof rafters? Also what's the situation with regard to loft/roof insulation around or near a stove flue?

 

[Burnerman]

Just a point Homer, but before you pour any concrete into your hearth, lay down a polythene vapour barrier.....original hearths were straight on to earth and they just wick the water up.
Flue pipes get hot as hell......you need the twin wall stainless stuff that locks together if the flue is internal.
John

 

[DIYWell]

Just a point Homer, but before you pour any concrete into your hearth, lay down a polythene vapour barrier.....original hearths were straight on to earth and they just wick the water up.
Flue pipes get hot as hell......you need the twin wall stainless stuff that locks together if the flue is internal.
John

The bottom of my flu pipe sits at about 250C during normal but it can hit 400C when I run a hot fire once in a while to burn out the deposits.

 

[DIYWell]

I'm preparing the old fireplace for a wood burner and was just wondering if it would be ok / beneficial to fit 100m celotex under 100mm concrete slab?
It's only a small area as pics show, my only concern I guess is the heat going down through the slab and affecting the insulation!

Any advice appreciated

Untitled

• Homerdoh
• 18 Oct 2014
• 1

Untitled

• Homerdoh
• 18 Oct 2014
• 1

Out of curiosity, I felt around under my stove this evening. Flu pipe is at 250C, hearth directly below the stove is cold to the touch. Thinking about it, it does have an ashbox at the bottom to insulate it from the fire.

So, hearth under my stove cold, but if I ripped it out and fitted a fireplace, then it would soon get very hot. Best not to insulate then

 

[DIYWell]

Doing a similar job here (old fireplace sort of came apart in my hands, pics to follow).
Building regs say for a solid hearth nothing flammable underneath unless you've got at least 50mm air gap below the (minimum 125mm thick) solid hearth or the solid is at least 250mm thick.
Also (annoyingly) they say vents within the hearth area only on specialist advice (HETAS presumably- I was planning to duct through the hearth with flexi metal tube to avoid room draughts). Might take the other bit of advice in this thread and get a stove bod in to have a look.

Yep, I spoke to a HETAS fella pre install. He was happy for me to run ducting from the outside wall, specified size and material, but I had to leave the through the hearth part to him so he could be confident it was fitted properly. Probably didn't save me much, but made me feel all macho cutting metal ducting and core drilling through a wall.