This is the LECA (Lightweight Expanded Clay Aggregate).  It is formed by firing liquid droplets of clay into a 1000 degree C kiln, which causes the liquid to turn to steam expanding the clay into "malteser" type of fired clay balls.  The balls are honeycombed and easily crushed.  An alternative to this is 'recycled foam glass' which looks like crushed concrete blocks.  We would have liked to use this product as it was recycled but it was going to work out  about double the cost - £145 a cubic metre rather than £75.


LECA or Recycled Foam Glass act as both the damp proof membrane and the insulation for the floor.  They both prevent moisture from being wicked up from the ground below and they allow moisture to drain through.  This is a critical part of the breathing floor concept and different to common current practice, which relies on a damp proof membrane (quite often a polythene sheet) blocking moisture from being drawn up into the floor.  However, this does not allow any moisture to get out either.  Foam glass or LECA above 120mm in thickness is certified to resist the wicking of moisture.  It is also insulating but does require a reasonable depth to give you an acceptable U-Value.  We have 380mm and this is achieving a U-Value of 0.2W/m2/K, which is not exceptional (although the U-Value required to meet Building Regulation is now worked out wholistically, a value of 0.25W/m2/K would be acceptable for a ground floor).  Ideally we would have like to have bettered 0.2W/m2/K (the lower the number the better).


There is a new product, Hempcrete, which we have been told about by the Limecrete Company which is an alternative to Limecrete but can achieve much better U-Values.  We will investigate more about this!