We are new to all this and trying to design a house extension using natural and breathable materials whilst keeping a sensible approach to affordability. We are in the north of scotland where deliveries can be expensive. Someone mentioned to me using foamglass blocks around the perimeter of the inside wall for cold bridging, someone else that this is an expensive option. The current plan is for leca base with concrete/ underfloor heating and plinth to hempcrete walls. anyone else have any ideas regarding alternatives to foamglass ?

Kate,

If you are looking for 'PH'-levels of insulation, U-values <=0.15W/m2k then yes, worry about the cold-bridging. (Marmox etc..)
If not, it won't be significant, don't worry.

Good luck..