Our Touchwood strip foundation detail uses conventional building methods to create a thermally bridge free foundation system – so can be built using your local diligent ground worker.
The simple concept removes the thermal bridging from outside by having 150mm polystyrene between the block work skins, followed by 300mm of insulation underneath the slab so that cold bridging to ground is also minimised.
There is no need for a screed layer – the slab is powerfloat finished ready for your floor finish – meaning all the concrete within the slab acts as thermal mass to the home. The detailing is critical – particularly the unconventional work sequence which will be different to a ground workers usual method of working.This can be a thermal bridge free solution suitable for passivhaus.
A fully insulated foundation raft can be a very simple solution which uses less concrete and entails less digging and spoil removal than conventional foundations.
The main limitation is the soil type encountered at the site - many sites are shrinkable clay where there is a possibility of heave (expansion or shrinkage of the clay) and the soil may not have sufficient bearing capacity.
To ascertain the type of soil a survey should be carried out and the bearing capacity calculated - if there is a risk of any contamination this can be checked at the same time. If the soil is suitable the soil is scraped off to a depth of around 500mm, a layer of aggregate compacted across the whole area and carefully levelled. The pre-formed polystyrene (EPS) formers are then installed around the perimeter and infilled with EPS sheets to a thickness of 300mm. If any underfloor heating is required this is laid on top and reinforcing steel laid on top. The 100mm concrete is then laid on top, this is power floated to a smooth surface as this is your finished floor which can be directly carpeted / have timber flooring laid on top, or can even be polished to a mirror finish though this can be expensive.