This pair of Passive Houses will have a sub foundation insulation; A standard method used in Germany but one of the first times it has been used in the UK.
The topsoil is scrapped away and a shallow trench is dug for a small retaining wall.
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Before the sub foundation insulation is placed a dwarf retaining wall is built
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This is not load bearing but is purely to support the insulation
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600mm of Extruded polystyrene with a density of 45Kg/M3 and a thermal conductivity of 0.032W/(m.K) is placed around the perimeter of the wall.
![](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEi4B7O4pYVwGjD8scIeLwtUSTZfdjsKp8yXkXI6nQgn2D7StmQbkJh-O65iHklEv4NA_ZJje5iR3dOO0h_PH2gAs-0Gx2U20xHuCXW25aLL9S7yRHNL4NwNMDGqeb3LUODZOuzdgPwYynHi/s200/7%25268+Perim+Ins+19.JPG)
The hardcore is then sand blinded to produce a flat level base ready to receive the insulation.
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Slabs of insulation are then placed carefully over the expanse of the foundation
The foundation insulation is then increased in thickness laid over previous layers crossing the junctions of the preceding layer to improve the load bearing capacity. A total thickness of 300mm is finally placed
Finally an impervious membrane is laid over the polystyrene which acts as a gas barrier and protects the insulation from damage when the concrete is poured