Expansion gap between concrete/vermicrete hearth and outer brick wall

[Jek]

Hi everyone,

First post from me. I've recently started a cob build in southwest Scotland.

I've built a 1x1x1m open topped brick base and have filled it 3/4 full with rubble and sand.

My plan is to pour a 50mm thick concrete base on top of the rubble fill, then above this a 100mm thick 5:1 vermicrete insulating layer, before laying the fire bricks on top. The firebricks will lie approximately flush with the top row of bricks on my base.

My question is...do I need to leave an expansion gap around the edge of the concrete base or the vermicrete insulation layer? I am concerned about thermal expansion of these causing my brickwork base to crack.

I attach a photo of the build so far.

Cheers,
Jek

[Toomulla]

I would be more concerned with heat loss from your description of the construction in my opinion the fire brick floor should finish in line with the dome and be enclosed within the insulation which will accomodate any expansion anyway