The main four processes involved are illustrated below: 

found 01 found 02 found 03 found 04
The system consists of clearing the top soil to 600mm below finished floor level, then covering with compacted hardcore. On top of this expanded polystyrene blocks are laid around the perimeter, followed by polystyrene sheets inside the building. On top of the polystyrene, steel mesh is installed along with under floor heating pipes if required.

Once everything is in place the slab can be poured which forms your finished floor screed.


The passivhaus version provides a thermally bridge free foundation, or the less costly version a very efficient solution.

The drawings below shows the thermally bridge free passivhaus foundation version which has the outside of the timber sole plate supported by a thermally separated ring beam to the main slab so there is minimal thermal bridging through the sole plate / slab interface. Alternatively, a solid sole plate can be used which can overhang the polystyrene for a passivhaus solution:

edge detail olof no labelsl 2015 02 05soleplate

Slab advantages:

  • Concrete in foundation is all warm side of the insulation
  • Quick to install once the site is prepared
  • Simple / elegant
  • Various options varying in price, down to passivhaus thermally bridge free options
  • Specialist process
  • Surface power float finished ready to be tiled / carpeted – no screed


  • Expensive, particularly the passivhaus version
  • Not suitable for certain ground types – shrinkable clay for example