Sandwich elements are widely used in the building envelope, in walls and foundations in particular. The thickness of sandwich elements is increasing as the demand for reduced heat loss from the building envelope is required. The building industry is searching for means and alternative materials to reduce the volume of the building envelope, but at the same time obtain the same thermal performance. Sandwich element constructions might be suitable for highly effective insulation materials as VIPs (Vacuum Insulation Panels). The possibilities of optimizing the thermal performance and by the same time decreasing the thickness and reducing the volume of aggregated clay sandwich construction block systems with VIPs has been investigated. Numerical simulations with heat conduction models and also CFD-models have been performed in order to study the optimal design of the block, the influence of thermal bridges and the influence of vertical and horizontal joints on the thermal performance of a wall. The work has resulted in an optimal design for a prototype block which has been produced and general knowledge about the influence of convection in vertical joints. The simulations show that for vertical joints less than 3 mm in width there will be no significant heat transport by convection. The numerical simulations also show that an U-value of 0,08 W/m2K can be achieved for such a system, with a thickness of the block being 300 mm. The work was carried out in the framework of the Norwegian centre for Zero Emission Buildings (ZEB).