DIY Doctor UK DIY Forums

Hi,
I cant get hold of 300mm trench blocks for love nor money. (2 courses needed). My question is if I build 6 courses of engineering brick for outer leaf and dense concrete on inner leaf. I'd rather not fill the cavity which is below ground with concrete (cost), can I not just use another dense concrete block in the middle?
So brick/block/block. ?

1x strip of 300mm DPC across the top and build inner and outer leaf as per normal with 100mm cavity above that.

Talking of cavity, in 2 minds over what to use.
I need the best soundproofing as well as insulation.

Looking at either Rockwool full fill cavity 100mm or PIR T&G cavity 50\60\70mm

Rockwool cant transmit moisture but leaves no airgap but better sound insulation. The building will be a mini recording studio, so that's why I wanted to use the Rockwool but unsure. Suppose the best is PIR in the cavity with Rockwool on an internal leaf with insulated plasterboard but leaves less room space.

The whole building will be block brick as external is been wood bat clad with cedar and render around the back walls facing the fence.

Any advice, please? or hints and tips?

Thanks

Im going run the dpm up to inner brick
On the first course of internal block drop a cavity tray down to external dpc. Just worries me having thr dpc along brick and tray down to it. Slipage. Long as both layers are mortored should be ok.
Seen dpc then tray onto of it. Thats what confuses me. Why cant tray act as external dpc too